Journal articles on the topic 'Earch convection'
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1
Liu, Zijing, Min Min, Jun Li, Fenglin Sun, Di Di, Yufei Ai, Zhenglong Li, et al. "Local Severe Storm Tracking and Warning in Pre-Convection Stage from the New Generation Geostationary Weather Satellite Measurements." Remote Sensing 11, no. 4 (February 13, 2019): 383. http://dx.doi.org/10.3390/rs11040383.
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Accurate and prior identification of local severe storm systems in pre-convection environments using geostationary satellite imagery measurements is a challenging task. Methodologies for “convective initiation” identification have already been developed and explored for operational nowcasting applications; however, warning of such convective systems using the new generation of geostationary satellite imagery measurements in pre-convection environments is still not well studied. In this investigation, the Random Forest (RF) machine learning algorithm is used to develop a predictive statistical model for tracking and identifying three different types of convective storm systems (weak, medium, and severe) over East Asia by combining spatially-temporally collocated Himawari-8 (H08) measurements and Numerical Weather Prediction (NWP) forecast data. The Global Precipitation Measurement (GPM) gridded product is used as a benchmark to train the predictive models based on a sample-balance technique which can adjust or balance the samples of three different convection types to avoid over-fitting any type of dataset. Variables such as brightness temperatures (BTs) from H08 water vapor absorption bands (6.2 μm, 6.9 μm and 7.3 μm) and Total Precipitable Water (TPW) from NWP show relatively high ranks in the predictive model training. These sensitive variables are closely associated with convectively dominated precipitation areas, indicating the importance of predictors from both H08 and NWP data. The final optimal RF model is achieved with an accuracy of 0.79 for classification of all convective storm systems, while the Probability of Detection (POD) of this model for severe and medium convections can reach 0.66 and 0.70, respectively. Two typical sudden convective storm cases in the warm season of 2018 tracked by this algorithm are described, and results indicate that the H08 and NWP based statistical model using the RF algorithm is capable of capturing local burst convective storm systems about 1–2 h earlier than the outbreak of heavy rainfall.
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Zheng, Zhang, Liu, Liu, and Che. "A Study of Vertical Structures and Microphysical Characteristics of Different Convective Cloud–Precipitation Types Using Ka-Band Millimeter Wave Radar Measurements." Remote Sensing 11, no. 15 (August 1, 2019): 1810. http://dx.doi.org/10.3390/rs11151810.
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Millimeter wave cloud radar (MMCR) is one of the primary instruments employed to observe cloud–precipitation. With appropriate data processing, measurements of the Doppler spectra, spectral moments, and retrievals can be used to study the physical processes of cloud–precipitation. This study mainly analyzed the vertical structures and microphysical characteristics of different kinds of convective cloud–precipitation in South China during the pre-flood season using a vertical pointing Ka-band MMCR. Four kinds of convection, namely, multi-cell, isolated-cell, convective–stratiform mixed, and warm-cell convection, are discussed herein. The results show that the multi-cell and convective–stratiform mixed convections had similar vertical structures, and experienced nearly the same microphysical processes in terms of particle phase change, particle size distribution, hydrometeor growth, and breaking. A forward pattern was proposed to specifically characterize the vertical structure and provide radar spectra models reflecting the different microphysical and dynamic features and variations in different parts of the cloud body. Vertical air motion played key roles in the microphysical processes of the isolated- and warm-cell convections, and deeply affected the ground rainfall properties. Stronger, thicker, and slanted updrafts caused heavier showers with stronger rain rates and groups of larger raindrops. The microphysical parameters for the warm-cell cloud–precipitation were retrieved from the radar data and further compared with the ground-measured results from a disdrometer. The comparisons indicated that the radar retrievals were basically reliable; however, the radar signal weakening caused biases to some extent, especially for the particle number concentration. Note that the differences in sensitivity and detectable height of the two instruments also contributed to the compared deviation.
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Huang, Yipeng, Murong Zhang, Yuchun Zhao, Ben Jong-Dao Jou, Hui Zheng, Changrong Luo, and Dehua Chen. "Inter-Zone Differences of Convective Development in a Convection Outbreak Event over Southeastern Coast of China: An Observational Analysis." Remote Sensing 14, no. 1 (December 29, 2021): 131. http://dx.doi.org/10.3390/rs14010131.
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Among the densely-populated coastal areas of China, the southeastern coast has received less attention in convective development despite having been suffering from significantly increasing thunderstorm activities. The convective complexity under such a region with extremely complex underlying and convective conditions deserves in-depth observational surveys. This present study examined a high-impact convection outbreak event with over 40 hail reports in the southeastern coast of China on 6 May 2020 by focusing on contrasting the convective development (from convective initiation to supercell occurrences) among three adjacent convection-active zones (north (N), middle (M), and south (S)). The areas from N to S featured overall flatter terrain, higher levels of free convection, lower relative humidity, larger convective inhibition, more convective available potential energy, and greater vertical wind shears. With these mesoscale environmental variations, distinct inter-zone differences in the convective development were observed with the region’s surveillance radar network and the Himawari-8 geostationary satellite. Convection initiated in succession from N to S and began with more warm-rain processes in N and M and more ice-phase processes in S. The subsequent convection underwent more vigorous vertical growth from N to S. The extremely deep convection in S was characterized by the considerably strong precipitation above the freezing level, echo tops of up to 18 km, and a great amount of deep (even overshooting) and thick convective clouds with significant cloud-top glaciation. Horizontal anvil expansion in convective clouds was uniquely apparent over S. From N to S, more pronounced mesocyclone and weak-echo region signatures indicated high risks of severe supercell hailstorms. These results demonstrate the strong linkage between the occurrence likelihood of severe convection and associated weather (such as supercells and hailstones) and the early-stage convective development that can be well-captured by high-resolution observations and may facilitate fine-scale convection nowcasting.
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Lai, Anwei, Jinzhong Min, Jidong Gao, Hedi Ma, Chunguang Cui, Yanjiao Xiao, and Zhibin Wang. "Assimilation of Radar Data, Pseudo Water Vapor, and Potential Temperature in a 3DVAR Framework for Improving Precipitation Forecast of Severe Weather Events." Atmosphere 11, no. 2 (February 9, 2020): 182. http://dx.doi.org/10.3390/atmos11020182.
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An improved approach to derive pseudo water vapor mass mixing ratio and in- cloud potential temperature was developed in this paper to better initialize numerical weather prediction (NWP) and build convective-scale predictions of severe weather events. The process included several steps. The first was to identify areas of deep moist convection, utilizing Vertically Integrated Liquid water (VIL) derived from a mosaicked 3D radar reflectivity field. Then, pseudo- water vapor and pseudo- in- cloud potential temperature observations were derived based on the VIL. For potential temperature, the latent heat initialization for stratiform cloud and moist adiabatic initialization for deep moist convection were used based on a cloud analysis method. The third step was to assimilate the derived pseudo- water vapor and potential temperature observations, together with radar radial velocity and reflectivity into a convective-scale NWP model during data assimilation cycles spanning several hours. Finally, 3-h forecasts were launched each hour during the data assimilation period. The effects of radar data and pseudo- observation assimilation on the prediction of rainfall associated with convective systems surrounding the Meiyu front in 2018 were explored using two real cases. Two sets of experiments, each including several experiments in each real case, were designed to compare the effects of assimilation radar and pseudo- observations on the ensuing forecasts. Relative to the control experiment without data assimilation and radar experiment, the analyses and forecasts of convections were found to be improved for the two Meiyu front cases after pseudo- water vapor and potential temperature information was assimilated.
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Palotai, Csaba, Shawn Brueshaber, Ramanakumar Sankar, and Kunio Sayanagi. "Moist Convection in the Giant Planet Atmospheres." Remote Sensing 15, no. 1 (December 30, 2022): 219. http://dx.doi.org/10.3390/rs15010219.
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The outer planets of our Solar System display a myriad of interesting cloud features, of different colors and sizes. The differences between the types of observed clouds suggest a complex interplay between the dynamics and chemistry at play in these atmospheres. Particularly, the stark difference between the banded structures of Jupiter and Saturn vs. the sporadic clouds on the ice giants highlights the varieties in dynamic, chemical and thermal processes that shape these atmospheres. Since the early explorations of these planets by spacecrafts, such as Voyager and Voyager 2, there are many outstanding questions about the long-term stability of the observed features. One hypothesis is that the internal heat generated during the formation of these planets is transported to the upper atmosphere through latent heat release from convecting clouds (i.e., moist convection). In this review, we present evidence of moist convective activity in the gas giant atmospheres of our Solar System from remote sensing data, both from ground- and space-based observations. We detail the processes that drive moist convective activity, both in terms of the dynamics as well as the microphysical processes that shape the resulting clouds. Finally, we also discuss the effects of moist convection on shaping the large-scale dynamics (such as jet structures on these planets).
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Nakagawa, Takashi, and Shun-ichiro Karato. "Influence of realistic rheological properties on the style of mantle convection: roles of dynamic friction and depth-dependence of rheological properties." Geophysical Journal International 226, no. 3 (May 11, 2021): 1986–96. http://dx.doi.org/10.1093/gji/ggab197.
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SUMMARY In order to generate plate tectonics, the near surface layer should not be too strong, but the causes for not-so-strong near surface layer remains unclear. We conduct mantle convection modelling in the spherical geometry to investigate the influence of the strength of the near surface layer. We explore a range of friction coefficients including the static high friction coefficient (∼0.6) as well as the reduced friction coefficients by fast fault motion in earthquakes. When the friction coefficient is low enough (<0.03), the surface layer is yielded by the convective stress, and the style of mantle convection appears the mobile-lid mode (plate tectonics style of convection). This style is relevant for the Earth where fault motion is unstable because of the low surface temperature. In contrast, for a high friction coefficient, the surface layer is too strong, generating the stagnant-lid mode. This case corresponds to Venus where fault motion is stable because of high surface temperature. Our calculations show that, in plate tectonic style of convection, the mantle convection is likely to be more vigorous, inducing the high convective stress that helps the operation of plate tectonics. In contrast, when stagnant-lid mode of convection appears, the convective vigor is likely to be low, inducing the low convective stress. Therefore, in each case, the interplay between the surface strength and convective stress tends to maintain the same mode of convection in a self-consistent way. We also investigate the relationship between mantle temperature and heat flux for two different modes of convection upon a change in friction coefficient. We found that the heat flow associated with mobile lid convection caused by low friction is less sensitive to the mantle temperature compared to a conventional mantle convection model, where the heat flow is highly sensitive to mantle temperature. This provides a possible mechanism to solve the thermal runaway paradox.
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Anders, Evan H., Adam S. Jermyn, Daniel Lecoanet, J. R. Fuentes, Lydia Korre, Benjamin P. Brown, and Jeffrey S. Oishi. "Convective Boundary Mixing Processes." Research Notes of the AAS 6, no. 2 (February 28, 2022): 41. http://dx.doi.org/10.3847/2515-5172/ac5892.
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Abstract Convective motions extend beyond the nominal boundaries of a convection zone. These motions mix fluid through multiple mechanisms collectively called “convective boundary mixing.” In this note, we discuss three distinct fluid dynamical processes: convective overshoot, entrainment, and penetrative convection. We describe the structure of a convective boundary that these processes create. To resolve discrepancies between models and observations, the stellar astrophysics community should distinguish between these processes and parameterize each of them separately in 1D evolutionary models.
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Arango-Reyes, Karen, Marco Barranco-Jiménez, Gonzalo Ares de Parga-Álvarez, and Fernando Angulo-Brown. "A Simple Thermodynamic Model of the Internal Convective Zone of the Earth." Entropy 20, no. 12 (December 18, 2018): 985. http://dx.doi.org/10.3390/e20120985.
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As it is well known both atmospheric and mantle convection are very complex phenomena. The dynamical description of these processes is a very difficult task involving complicated 2-D or 3-D mathematical models. However, a first approximation to these phenomena can be by means of simplified thermodynamic models where the restriction imposed by the laws of thermodynamics play an important role. An example of this approach is the model proposed by Gordon and Zarmi in 1989 to emulate the convective cells of the atmospheric air by using finite-time thermodynamics (FTT). In the present article we use the FTT Gordon-Zarmi model to coarsely describe the convection in the Earth’s mantle. Our results permit the existence of two layers of convective cells along the mantle. Besides the model reasonably reproduce the temperatures of the main discontinuities in the mantle, such as the 410 km-discontinuity, the Repetti transition zone and the so-called D-Layer.
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Richardson, Mark T., Brian H. Kahn, and Peter Kalmus. "Trajectory enhancement of low-earth orbiter thermodynamic retrievals to predict convection: a simulation experiment." Atmospheric Chemistry and Physics 23, no. 13 (July 13, 2023): 7699–717. http://dx.doi.org/10.5194/acp-23-7699-2023.
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Abstract. The 3-D fields of temperature (T) and specific humidity (q) retrieved by instruments such as the Atmospheric Infrared Sounder (AIRS) are predictive of convection, but convection often triggers during the multi-hour gaps between satellite overpasses. Here we fill the hours after AIRS overpasses by treating AIRS retrievals as air parcels which are moved adiabatically along numerical weather prediction (NWP) wind trajectories. The approach is tested in a simulation experiment that samples 3-D European Reanalysis-5 (ERA5) T and q following the real-world AIRS time–space sampling from March–November 2019 over much of the continental US. Our time-resolved product is named ERA5-FCST, in correspondence to the AIRS forecast product we are using it to test, named AIRS-FCST. ERA5-FCST errors may arise since processes such as radiative heating and NWP sub-grid convection are ignored. For bulk atmospheric layers, ERA5-FCST captures 59 %–94 % of local hourly variation in T and q. We then consider the relationship between convective available potential energy (CAPE), convective inhibition (CIN), and ERA5 precipitation. The 1∘ latitude–longitude ERA5-FCST grid cells in our highest CAPE and lowest CIN bins are more than 50 times as likely to develop heavy precipitation (> 4 mm hr−1), compared with the baseline probability from randomly selecting a location. This is a substantial improvement compared with using the original CAPE and CIN values at overpass time. The results support the development of similar FCST products for operational atmospheric sounders to provide time-resolved thermodynamics in rapidly changing pre-convective atmospheres.
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Rybka, H., and H. Tost. "Uncertainties in future climate predictions due to convection parameterisations." Atmospheric Chemistry and Physics Discussions 13, no. 10 (October 16, 2013): 26893–931. http://dx.doi.org/10.5194/acpd-13-26893-2013.
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Abstract. In the last decades several convection parameterisations have been developed to consider the impact of small-scale unresolved processes in Earth System Models associated with convective clouds. Global model simulations, which have been performed under current climate conditions with different convection schemes, significantly differ among each other in the simulated transport of trace gases and precipitation patterns due to the parameterisation assumptions and formulations, e.g. the simplified treatment of the cloud microphysics. Here we address sensitivity studies comparing four different convection schemes under alternative climate conditions (doubling of the CO2 concentrations) to identify uncertainties related to convective processes. The increase in surface temperature reveals regional differences up to 4 K dependent on the chosen convection parameterisation. The increase in upper tropospheric temperature affects the amount of water vapour transported to the lower stratosphere. Furthermore, the change in transporting short-lived pollutants within the atmosphere is highly ambiguous for the lower and upper troposphere. Finally, cloud radiative effects have been analysed uncovering a shift in different cloud types in the tropics.
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McTaggart-Cowan, Ron, Paul A. Vaillancourt, Ayrton Zadra, Leo Separovic, Shawn Corvec, and Daniel Kirshbaum. "A Lagrangian Perspective on Parameterizing Deep Convection." Monthly Weather Review 147, no. 11 (October 30, 2019): 4127–49. http://dx.doi.org/10.1175/mwr-d-19-0164.1.
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Abstract The parameterization of deep moist convection as a subgrid-scale process in numerical models of the atmosphere is required at resolutions that extend well into the convective “gray zone,” the range of grid spacings over which such convection is partially resolved. However, as model resolution approaches the gray zone, the assumptions upon which most existing convective parameterizations are based begin to break down. We focus here on one aspect of this problem that emerges as the temporal and spatial scales of the model become similar to those of deep convection itself. The common practice of static tendency application over a prescribed adjustment period leads to logical inconsistencies at resolutions approaching the gray zone, while more frequent refreshment of the convective calculations can lead to undesirable intermittent behavior. A proposed parcel-based treatment of convective initiation introduces memory into the system in a manner that is consistent with the underlying physical principles of convective triggering, thus reducing the prevalence of unrealistic gradients in convective activity in an operational model running with a 10 km grid spacing. The subsequent introduction of a framework that considers convective clouds as persistent objects, each possessing unique attributes that describe physically relevant cloud properties, appears to improve convective precipitation patterns by depicting realistic cloud memory, movement, and decay. Combined, this Lagrangian view of convection addresses one aspect of the convective gray zone problem and lays a foundation for more realistic treatments of the convective life cycle in parameterization schemes.
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Cancelada, Maite, Paola Salio, Daniel Vila, Stephen W. Nesbitt, and Luciano Vidal. "Backward Adaptive Brightness Temperature Threshold Technique (BAB3T): A Methodology to Determine Extreme Convective Initiation Regions Using Satellite Infrared Imagery." Remote Sensing 12, no. 2 (January 20, 2020): 337. http://dx.doi.org/10.3390/rs12020337.
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Thunderstorms in southeastern South America (SESA) stand out in satellite observations as being among the strongest on Earth in terms of satellite-based convective proxies, such as lightning flash rate per storm, the prevalence for extremely tall, wide convective cores and broad stratiform regions. Accurately quantifying when and where strong convection is initiated presents great interest in operational forecasting and convective system process studies due to the relationship between convective storms and severe weather phenomena. This paper generates a novel methodology to determine convective initiation (CI) signatures associated with extreme convective systems, including extreme events. Based on the well-established area-overlapping technique, an adaptive brightness temperature threshold for identification and backward tracking with infrared data is introduced in order to better identify areas of deep convection associated with and embedded within larger cloud clusters. This is particularly important over SESA because ground-based weather radar observations are currently limited to particular areas. Extreme rain precipitation features (ERPFs) from Tropical Rainfall Measurement Mission are examined to quantify the full satellite-observed life cycle of extreme convective events, although this technique allows examination of other intense convection proxies such as the identification of overshooting tops. CI annual and diurnal cycles are analyzed and distinctive behaviors are observed for different regions over SESA. It is found that near principal mountain barriers, a bimodal diurnal CI distribution is observed denoting the existence of multiple CI triggers, while convective initiation over flat terrain has a maximum frequency in the afternoon.
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White, B. A., A. M. Buchanan, C. E. Birch, P. Stier, and K. J. Pearson. "Quantifying the Effects of Horizontal Grid Length and Parameterized Convection on the Degree of Convective Organization Using a Metric of the Potential for Convective Interaction." Journal of the Atmospheric Sciences 75, no. 2 (January 24, 2018): 425–50. http://dx.doi.org/10.1175/jas-d-16-0307.1.
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Abstract The organization of deep convection and its misrepresentation in many global models is the focus of much current interest. A new method is presented for quantifying convective organization based on the identification of convective objects and subsequent derivation of object numbers, areas, and separation distances to describe the degree of convective organization. These parameters are combined into a “convection organization potential” based on the physical principle of an interaction potential between pairs of convective objects. This technique is applied to simulated and observed fields of outgoing longwave radiation (OLR) over the West African monsoon region using data from Met Office Unified Model simulations and satellite observations made by the Geostationary Earth Radiation Budget (GERB) instrument. The method is evaluated by using it to quantify differences between models with different horizontal grid lengths and representations of convection. Distributions of OLR, precipitation and organization parameters, the diurnal cycle of convection, and relationships between the meteorology in different states of organization are compared. Switching from a configuration with parameterized convection to one that allows the model to resolve convective processes at the model grid scale is the leading-order factor improving some aspects of model performance, while increased model resolution is the dominant factor for others. However, no single model configuration performs best compared to observations, indicating underlying deficiencies in both model scaling and process understanding.
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Rivas Soriano, L., JM Sánchez Llorente, A. González Zamora, and F. de Pablo Dávila. "Influence of land cover on lightning and convective precipitation over the European continent." Progress in Physical Geography: Earth and Environment 43, no. 3 (January 24, 2019): 352–64. http://dx.doi.org/10.1177/0309133318825285.
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The impact of land cover on lightning and convective precipitation in the summertime over Europe was analysed using five-year data. Lightning data were obtained with the Optical Transient Detector (OTD) at a 0.5° × 0.5° spatial resolution and the convective precipitation data were calculated by the NCEP/DOE AMIP-II Reanalysis at a ∼1.9° × 1.9° spatial resolution. Data concerning land cover were obtained from the Global Land Cover Facility, although the original 14 categories were grouped into seven categories (water, forest, shrubland, grassland, cropland, bare ground and urban). For all latitude ranges, forested areas tend to increase convective activity during the warm period of the year, and the general effect of shrubland areas is to suppress convective activity. The behaviour of convection in relation to grasslands and croplands depends on latitude. At low latitudes both vegetation types tend to increase convection during the summer. At high latitudes, grassland and cropland areas appear to be associated with the opposite effect in relation to convection: grass suppresses and crops enhance it. Finally, bare soil tends to decrease convective activity. These results seem to be related to the impact of vegetation on soil moisture and roughness. In general, vegetation areas associated with high soil moisture contents and high values in roughness length tend to enhance convective activity.
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Seeley, Jacob T., and Robin D. Wordsworth. "Moist Convection Is Most Vigorous at Intermediate Atmospheric Humidity." Planetary Science Journal 4, no. 2 (February 1, 2023): 34. http://dx.doi.org/10.3847/psj/acb0cb.
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Abstract In Earth’s current climate, moist convective updraft speeds increase with surface warming. This trend suggests that very vigorous convection might be the norm in extremely hot and humid atmospheres, such as those undergoing a runaway greenhouse transition. However, theoretical and numerical evidence suggests that convection is actually gentle in water-vapor-dominated atmospheres, implying that convective vigor may peak at some intermediate humidity level. Here, we perform small-domain convection-resolving simulations of an Earth-like atmosphere over a wide range of surface temperatures and confirm that there is indeed a peak in convective vigor, which we show occurs near T s ≃ 330 K. We show that a similar peak in convective vigor exists when the relative abundance of water vapor is changed by varying the amount of background (noncondensing) gas at fixed T s , which may have implications for Earth’s climate and atmospheric chemistry during the Hadean and Archean eons. We also show that Titan-like thermodynamics (i.e., a thick nitrogen atmosphere with condensing methane and low gravity) produce a peak in convective vigor at T s ≃ 95 K, which is curiously close to the current surface temperature of Titan. Plotted as functions of the saturation-specific humidity at cloud base, metrics of convective vigor from both Earth-like and Titan-like experiments all peak when cloud-base air contains roughly 10% of the condensible gas by mass. Our results point to a potentially common phenomenon in terrestrial atmospheres: that moist convection is most vigorous when the condensible component is between dilute and nondilute abundance.
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Homeyer, Cameron R., and Matthew R. Kumjian. "Microphysical Characteristics of Overshooting Convection from Polarimetric Radar Observations." Journal of the Atmospheric Sciences 72, no. 2 (February 1, 2015): 870–91. http://dx.doi.org/10.1175/jas-d-13-0388.1.
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Abstract The authors present observations of the microphysical characteristics of deep convection that overshoots the altitude of the extratropical tropopause from analysis of the polarimetric radar variables of radar reflectivity factor at horizontal polarization ZH, differential reflectivity ZDR, and specific differential phase KDP. Identified overshooting convective storms are separated by their organization and intensity into three classifications: organized convection, discrete ordinary convection, and discrete supercell convection. Composite analysis of identified storms for each classification reveals microphysical features similar to those found in previous studies of deep convection, with deep columns of highly positive ZDR and KDP representing lofting of liquid hydrometeors within the convective updraft and above the melting level. In addition, organized and discrete supercell classifications show distinct near-zero ZDR minima aligned horizontally with and at altitudes higher than the updraft column features, likely indicative of the frequent presence of large hail in each case. Composites for organized convective systems show a similar ZDR minimum throughout the portion of the convective core that is overshooting the tropopause, corresponding to ZH in the range of 15–30 dBZ and negative KDP observations, in agreement with the scattering properties of small hail and/or lump or conical graupel. Additional analyses of the evolution of overshooting storms reveals that the ZDR minima indicative of hail in the middle and upper troposphere and graupel in the overshooting top are associated with the mature and decaying stages of overshooting, respectively, supporting their inferred contributions to the observed polarimetric fields.
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Howle, L. E., R. P. Behringer, and J. G. Georgiadis. "Convection and flow in porous media. Part 2. Visualization by shadowgraph." Journal of Fluid Mechanics 332 (February 1997): 247–62. http://dx.doi.org/10.1017/s0022112096004004.
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We present results for pattern formation at the onset of convection in fluid-saturated porous media obtained by a novel variation on the shadowgraphic technique (modified shadowgraphic technique). Both ordered and disordered media are used, each exhibiting distinct behaviour. Ordered porous media are constructed from grids of overlapping bars. Convective onset in this type of medium is characterized by a sharp, well-defined bifurcation to straight parallel rolls. The orientation of the convection rolls is determined by the number of bar layers, Nb; odd Nb leads to rolls with axes perpendicular to the direction of the top and bottom bars, and even Nb to rolls at 45° to the bars. Disordered porous layers are produced by stacking randomly drilled disks separated by spacers. In this system, we observe a rounded bifurcation to convection with localized convection near convective onset. More specifically, the flow patterns take on one of several different three-dimensional cellular structures after each cycling through convective onset. These observations may be described by two different mechanisms: random spatial fluctuations in the Rayleigh number (Zimmermann et al. 1993), and/or spatial variation in the thermal conductivity on the length scale of the convection wavelength (Braester & Vadasz 1993).
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Rybka, H., and H. Tost. "Uncertainties in future climate predictions due to convection parameterisations." Atmospheric Chemistry and Physics 14, no. 11 (June 5, 2014): 5561–76. http://dx.doi.org/10.5194/acp-14-5561-2014.
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Abstract. In the last decades several convection parameterisations have been developed to consider the impact of small-scale unresolved processes in Earth System Models associated with convective clouds. Global model simulations, which have been performed under current climate conditions with different convection schemes, significantly differ among each other in the simulated transport of trace gases and precipitation patterns due to the parameterisation assumptions and formulations, e.g. the computation of convective rainfall rates, calculation of entrainment and detrainment rates etc. Here we address sensitivity studies comparing four different convection schemes under alternative climate conditions (with doubling of the CO2 concentrations) to identify uncertainties related to convective processes. The increase in surface temperature reveals regional differences up to 4 K dependent on the chosen convection parameterisation. These differences are statistically significant almost everywhere in the troposphere of the intertropical convergence zone. The increase in upper tropospheric temperature affects the amount of water vapour transported to the lower stratosphere, leading to enhanced water vapour contents between 40% and 60% at the cold point temperature in the Tropics. Furthermore, the change in transporting short-lived pollutants within the atmosphere is highly ambiguous for the lower and upper troposphere. These results reflect that different approaches to compute mass fluxes, detrainment levels or trigger functions determine the transport of short-lived trace gases from the planetary boundary layer to lower, middle or upper tropospheric levels. Finally, cloud radiative effects have been analysed, uncovering a shift in different cloud types in the Tropics, especially for cirrus and deep convective clouds. These cloud types induce a change in net cloud radiative forcing varying from 0.5 W m−2 to 2.0 W m−2.
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Huang, Ying, Ping Long, Guanshi Wang, and Sihai Luo. "Decoupling Method for the Convective-Dominated Leaching Process of Ion-Adsorption-Type Rare-Earth Ores." Minerals 13, no. 1 (January 6, 2023): 89. http://dx.doi.org/10.3390/min13010089.
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Ion-adsorption-type rare-earth ores (IATREOs) that have experienced granite weathering have good permeability, and their leaching process involves the solute transport problem, which is dominated by convection. Because of the oscillation and dispersion errors of existing numerical methods for solving the convective-dominated solute transport equation, the results have low precision in the twin leaching process. In this paper, the convection–dispersion equation is decoupled into the dispersion equation, the convection equation, and the source-sink equation; the Crank–Nicolson and implicit difference methods are used to solve the dispersion equation and the source-sink equation, respectively. The solution of the convection equation is achieved on the basis of its physical interpretation. Therefore, a decoupling method for the convective-dominated solute transport equation is established. In comparison to the two examples with analytical solutions, the calculation errors of the method established in this paper are less than 2.00%, and it can solve the oscillation and dispersion problems. The rationality of the method is further demonstrated through the column leaching experiment of IATREOs. In comparison to the test results, the coefficients of determination of the breakthrough curves of rare-earth ions and ammonium ions calculated by the proposed method are all greater than 0.850, and the peak concentration error of rare-earth ions is less than 7.00%. This indicates that the proposed method can simulate the leaching process well. Furthermore, by combining the multiple/half method and the dichotomy method, an optimization method for determining the leaching agent amount was established to analyze the relationship between the leaching agent and the ratio of dispersion to pillar length. The results can provide a solution that can be used to mine IATREOs from experience to theory.
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Martin, David W., Richard A. Kohrs, Frederick R. Mosher, Carlo Maria Medaglia, and Claudia Adamo. "Over-Ocean Validation of the Global Convective Diagnostic." Journal of Applied Meteorology and Climatology 47, no. 2 (February 1, 2008): 525–43. http://dx.doi.org/10.1175/2007jamc1525.1.
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Abstract The global convective diagnostic (GCD) is a bispectral (infrared and water vapor), day–night scheme for operationally mapping deep convection by means of geostationary satellite images. This article describes a test of GCD performance over tropical and subtropical waters near North America. The test consists of six cases, each involving a convective cloud complex. A seventh case treats convection over land. For each case, a map of deep convection was constructed from image pairs from Geostationary Operational Environmental Satellite-12 (GOES-12). Case by case and for all maritime cases together, the GCD map was compared with a convective parameter derived from the radar on the Tropical Rainfall Measuring Mission (TRMM), a polar-orbiting satellite. In general, each GCD map showed a bloblike feature. In each case, the radar convective pixels typically fell within the GCD blob. However, (except for the land case) the GCD predicted far too many convective pixels. In the maritime cases overprediction was reduced (without correspondingly impairing other measures of performance) by lowering the nominal GCD threshold. With this adjustment in place, for the six maritime cases taken individually, the GCD tended to yield more consistent results than did a monospectral (infrared) convective scheme. With the cases combined, at the lower threshold the GCD performed somewhat better than one of the more stable versions of the infrared scheme. Comparison with lightning events (also observed by TRMM) suggests the possibility of future improvement to the GCD through the incorporation of geostationary satellite observations of lightning.
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Pasquero, Claudia, and Eli Tziperman. "Statistical Parameterization of Heterogeneous Oceanic Convection." Journal of Physical Oceanography 37, no. 2 (February 1, 2007): 214–29. http://dx.doi.org/10.1175/jpo3008.1.
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Abstract A statistical convective adjustment scheme is proposed that attempts to account for the effects of mesoscale and submesoscale variability of temperature and salinity typically observed in the oceanic convective regions. Temperature and salinity in each model grid box are defined in terms of their mean, variance, and mutual correlations. Subgrid-scale instabilities lead to partial mixing between different layers in the water column. This allows for a smooth transition between the only two states (convection on and convection off) allowed in standard convective adjustment schemes. The advantage of the statistical parameterization is that possible instabilities associated with the sharp transition between the two states, which are known to occasionally affect the large-scale model solution, are eliminated. The procedure also predicts the generation of correlations between temperature and salinity and the presence of convectively induced upgradient fluxes that have been obtained in numerical simulations of heterogeneous convection and that cannot be represented by standard convective adjustment schemes.
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Keene, Kelly M., and Russ S. Schumacher. "The Bow and Arrow Mesoscale Convective Structure." Monthly Weather Review 141, no. 5 (May 1, 2013): 1648–72. http://dx.doi.org/10.1175/mwr-d-12-00172.1.
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Abstract The accurate prediction of warm-season convective systems and the heavy rainfall and severe weather associated with them remains a challenge for numerical weather prediction models. This study looks at a circumstance in which quasi-stationary convection forms perpendicular to, and above the cold-pool behind strong bow echoes. The authors refer to this phenomenon as a “bow and arrow” because on radar imagery the two convective lines resemble an archer’s bow and arrow. The “arrow” can produce heavy rainfall and severe weather, extending over hundreds of kilometers. These events are challenging to forecast because they require an accurate forecast of earlier convection and the effects of that convection on the environment. In this study, basic characteristics of 14 events are documented, and observations of 4 events are presented to identify common environmental conditions prior to the development of the back-building convection. Simulations of three cases using the Weather Research and Forecasting Model (WRF) are analyzed in an attempt to understand the mechanisms responsible for initiating and maintaining the convective line. In each case, strong southwesterly flow (inducing warm air advection and gradual isentropic lifting), in addition to directional and speed convergence into the convective arrow appear to contribute to initiation of convection. The linear orientation of the arrow may be associated with a combination of increased wind speeds and horizontal shear in the arrow region. When these ingredients are combined with thermodynamic instability, there appears to be a greater possibility of formation and maintenance of a convective arrow behind a bow echo.
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Janiga, Matthew A., and Chris D. Thorncroft. "The Influence of African Easterly Waves on Convection over Tropical Africa and the East Atlantic." Monthly Weather Review 144, no. 1 (December 29, 2015): 171–92. http://dx.doi.org/10.1175/mwr-d-14-00419.1.
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Abstract Using data from the Tropical Rainfall Measuring Mission (TRMM), the modulation of convection by African easterly waves (AEWs) is investigated over regions of the east Atlantic and tropical Africa. To explain the modulation of convection, the large-scale environment (lift, moisture, conditional instability, and shear) is also examined as a function of AEW phase in each region. Over semiarid portions of tropical Africa, unconditional rain rates are greatest in the northerly phase of AEWs due to the strong adiabatic forcing for ascent. Along the Guinea Coast, the western coast of Africa, and over the east Atlantic—where forcing for ascent is weaker—rainfall is shifted toward the trough where the air is moist. Significant contrasts in the characteristics of convection as a function of AEW phase—comparable in magnitude to regional contrasts—are also observed. In all regions, large and high echo-top convective systems are more sensitive to AEW phase than small and low echo-top systems. In semiarid regions, deep convection and large high echo-top convective systems account for a large fraction of the rainfall in the ridge and northerlies. Stratiform and small low echo-top convective systems dominate in the trough and southerlies. Convective system height and conditional rain rates increase with conditional instability and system sizes may increase with shear. Over the east Atlantic, stratiform fractions and convective system sizes and echo-top heights are greatest in the trough while the ridge is dominated by shallow convection. This is primarily related to the presence of moist air in the trough and dry air in the ridge.
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Cachay Torres, Roberth, and José Roldan López. "Influence of the diffusive term on the modeling of two-dimensional (2D) wave propagation of the law of conservation of mass with constant convective flow velocity." Revista Ciencia y Tecnología 19, no. 1 (March 30, 2023): 11–22. http://dx.doi.org/10.17268/rev.cyt.2023.01.01.
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In this work, the 2D Convection - Diffusion equation was used to model the process of contaminant transport by convection and diffusion. In particular, we assume that we are modeling this pollutant transport process in shallow water and with a unidirectional flow movement in the convective part. The diffusion coefficient is considered constant and depends only on the nature of the substance, a value of 0.004 has been considered. Finite difference numerical schemes are applied to a domain in the XY plane, with side 1. The developed numerical model could be used to predict the distribution of polluting material. The value of the diffusion coefficient strongly influences the step size in time (dt) and the speed values that we give to the convective flow. A faster movement of the contaminant in the direction of the resultant of the convective flow was appreciated, as well as a decrease in the speed of the diffusion process when the local concentration levels decreased and therefore moving only by convection.
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Bruick, Zachary S., Kristen L. Rasmussen, Angela K. Rowe, and Lynn A. McMurdie. "Characteristics of Intense Convection in Subtropical South America as Influenced by El Niño–Southern Oscillation." Monthly Weather Review 147, no. 6 (May 14, 2019): 1947–66. http://dx.doi.org/10.1175/mwr-d-18-0342.1.
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Abstract El Niño–Southern Oscillation (ENSO) is known to have teleconnections to atmospheric circulations and weather patterns around the world. Previous studies have examined connections between ENSO and rainfall in tropical South America, but little work has been done connecting ENSO phases with convection in subtropical South America. The Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) has provided novel observations of convection in this region, including that convection in the lee of the Andes Mountains is among the deepest and most intense in the world with frequent upscale growth into mesoscale convective systems. A 16-yr dataset from the TRMM PR is used to analyze deep and wide convection in combination with ERA-Interim reanalysis storm composites. Results from the study show that deep and wide convection occurs in all phases of ENSO, with only some modest variations in frequency between ENSO phases. However, the most statistically significant differences between ENSO phases occur in the three-dimensional storm structure. Deep and wide convection during El Niño tends to be taller and contain stronger convection, while La Niña storms contain stronger stratiform echoes. The synoptic and thermodynamic conditions supporting the deeper storms during El Niño is related to increased convective available potential energy, a strengthening of the South American low-level jet (SALLJ), and a stronger upper-level jet stream, often with the equatorward-entrance region of the jet stream directly over the convective storm locations. These enhanced synoptic and thermodynamic conditions provide insight into how the structure of some of the most intense convection on Earth varies with phases of ENSO.
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Pereira, L. Gustavo, and Steven A. Rutledge. "Diurnal Cycle of Shallow and Deep Convection for a Tropical Land and an Ocean Environment and Its Relationship to Synoptic Wind Regimes." Monthly Weather Review 134, no. 10 (October 1, 2006): 2688–701. http://dx.doi.org/10.1175/mwr3181.1.
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Abstract The characteristics of shallow and deep convection during the Tropical Rainfall Measuring Mission/Large-Scale Biosphere–Atmosphere Experiment in Amazonia (TRMM/LBA) and the Eastern Pacific Investigation of Climate Processes in the Coupled Ocean–Atmosphere System (EPIC) are evaluated in this study. Using high-quality radar data collected during these two tropical field experiments, the reflectivity profiles, rain rates, fraction of convective area, and fraction of rainfall volume in each region are examined. This study focuses on the diurnal cycle of shallow and deep convection for the identified wind regimes in both regions. The easterly phase in TRMM/LBA and the northerly wind regime in EPIC were associated with the strongest convection, indicated by larger rain rates, higher reflectivities, and deeper convective cores compared to the westerly phase in TRMM/LBA and the southerly regime in EPIC. The diurnal cycle results indicated that convection initiates in the morning and peaks in the afternoon during TRMM/LBA, whereas in the east Pacific the diurnal cycle of convection is very dependent on the wind regime. Deep convection in the northerly regime peaks around midnight, nearly 6 h before its southerly regime counterpart. Moreover, the northerly regime of EPIC was dominated by convective rainfall, whereas the southerly regime was dominated by stratiform rainfall. The diurnal variability was more pronounced during TRMM/LBA than in EPIC. Shallow convection was associated with 10% and 3% of precipitation during TRMM/LBA and EPIC, respectively.
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Lin, Jia-Lin, Myong-In Lee, Daehyun Kim, In-Sik Kang, and Dargan M. W. Frierson. "The Impacts of Convective Parameterization and Moisture Triggering on AGCM-Simulated Convectively Coupled Equatorial Waves." Journal of Climate 21, no. 5 (March 1, 2008): 883–909. http://dx.doi.org/10.1175/2007jcli1790.1.
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Abstract This study examines the impacts of convective parameterization and moisture convective trigger on convectively coupled equatorial waves simulated by the Seoul National University (SNU) atmospheric general circulation model (AGCM). Three different convection schemes are used, including the simplified Arakawa–Schubert (SAS) scheme, the Kuo (1974) scheme, and the moist convective adjustment (MCA) scheme, and a moisture convective trigger with variable strength is added to each scheme. The authors also conduct a “no convection” experiment with deep convection schemes turned off. Space–time spectral analysis is used to obtain the variance and phase speed of dominant convectively coupled equatorial waves, including the Madden–Julian oscillation (MJO), Kelvin, equatorial Rossby (ER), mixed Rossby–gravity (MRG), and eastward inertio-gravity (EIG) and westward inertio-gravity (WIG) waves. The results show that both convective parameterization and the moisture convective trigger have significant impacts on AGCM-simulated, convectively coupled equatorial waves. The MCA scheme generally produces larger variances of convectively coupled equatorial waves including the MJO, more coherent eastward propagation of the MJO, and a more prominent MJO spectral peak than the Kuo and SAS schemes. Increasing the strength of the moisture trigger significantly enhances the variances and slows down the phase speeds of all wave modes except the MJO, and usually improves the eastward propagation of the MJO for the Kuo and SAS schemes, but the effect for the MCA scheme is small. The no convection experiment always produces one of the best signals of convectively coupled equatorial waves and the MJO.
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Manea, Vlad Constantin, Marina Manea, Mihai Pomeran, Lucian Besutiu, and Luminita Zlagnean. "A parallelized particle tracing code for CFD simulations in Earth sciences." Acta Universitaria 22, no. 5 (August 15, 2012): 19–26. http://dx.doi.org/10.15174/au.2012.358.
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The problem of convective flows in a highly viscous fluid represents a common research direction in Earth sciences. For tracing the convective motion of the fluid material, a source passive particles (or tracers) that flow at a local convection velocity and do not affect the pattern of flow it is commonly used. Here we present a parallelized tracer code that uses passive and weightless particles with their position computed from their displacement during a small time interval at the velocity of flow previously calculated for a given point in space and time. The tracer code is integrated in the open source package CitcomS, which is widely used in the solid earth community (www.geodynamics.org). We benchmarked the tracer code on the state-of-the-art CyberDyn parallel machine, a High Performance Computing (HPC) Cluster with 1344 computing cores available at the Institute of Geodynamics of the Romanian Academy.
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Pu, Jingchen, and Xiaolei Zou. "Characteristic Scales of Tropical Convection Based on the Japanese Advanced Himawari-8 Imager Observations." Remote Sensing 14, no. 7 (March 23, 2022): 1553. http://dx.doi.org/10.3390/rs14071553.
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Convective activities play an important role in tropical weather systems. To investigate the characteristic scales of convection, a method combining a principal component (PC) analysis with Fourier decomposition is applied to brightness temperature observations from Advanced Himawari-8 Imager (AHI). Characteristic scales of different modes in tropical convective systems are obtained. The explained variance reduces rapidly from the first to the 60th PC mode by two magnitudes; the horizontal scale decreases from over 2000 km to about 100 km, and the timescale changes from more than 4 days to around 5 h. By a detailed comparison of the first, 20th, 40th, and 60th PC modes, it is found that large-scale (over 2000 km in wavelength) convective activities usually have significant nocturnal enhancement, whereas meso-scale (about 100 km in wavelength) convective activities feature short period and fast change with more intense development in regions of active large-scale convection. This study may be of some importance for the choice of AHI data assimilation cycling interval, the horizontal resolution as well as data thinning for geostationary satellite observations.
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Dhaka, S. K., R. Bhatnagar, Y. Shibagaki, H. Hashiguchi, S. Fukao, T. Kozu, and V. Panwar. "Characteristics of gravity waves generated in a convective and a non-convective environment revealed from hourly radiosonde observation under CPEA-II campaign." Annales Geophysicae 29, no. 12 (December 16, 2011): 2259–76. http://dx.doi.org/10.5194/angeo-29-2259-2011.
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Abstract. Analyses of hourly radiosonde data of temperature, wind, and relative humidity during four days (two with convection and two with no convection) as a part of an intensive observation period in CPEA-2 campaign over Koto Tabang (100.32° E, 0.20° S), Indonesia, are presented. Characteristics of gravity waves in terms of dominant wave frequencies at different heights and their vertical wavelengths are shown in the lower stratosphere during a convective and non-convective period. Gravity waves with periods ~10 h and ~4–5 h were found dominant near tropopause (a region of high stability) on all days of observation. Vertical propagation of gravity waves were seen modified near heights of the three identified strong wind shears (at ~16, 20, and 25 km heights) due to wave-mean flow interaction. Between 17 and 21 km heights, meridional wind fluctuations dominated over zonal wind, whereas from 22 to 30 km heights, wave fluctuations with periods ~3–5 h and ~8–10 h in zonal wind and temperature were highly associated, suggesting zonal orientation of wave propagation. Gravity waves from tropopause region to 30 km heights were analyzed. In general, vertical wavelength of 2–5 km dominated in all the mean-removed (~ weekly mean) wind and temperature hourly profiles. Computed vertical wavelength spectra are similar, in most of the cases, to the source spectra (1–16 km height) except that of zonal wind spectra, which is broad during active convection. Interestingly, during and after convection, gravity waves with short vertical wavelength (~2 km) and short period (~2–3 h) emerged, which were confined in the close vicinity of tropopause, and were not identified on non-convective days, suggesting convection to be the source for them. Some wave features near strong wind shear (at 25 km height) were also observed with short vertical wavelengths in both convective and non-convective days, suggesting wind shear to be the sole cause of generation and seemingly not associated with deep convection below. A drop in the temperature up to ~4–5 K (after removal of diurnal component) was observed at ~16 km height near a strong wind shear (~45–55 m s−1 km−1) during active period of convection.
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Grandpeix, Jean-Yves, Jean-Philippe Lafore, and Frédérique Cheruy. "A Density Current Parameterization Coupled with Emanuel’s Convection Scheme. Part II: 1D Simulations." Journal of the Atmospheric Sciences 67, no. 4 (April 1, 2010): 898–922. http://dx.doi.org/10.1175/2009jas3045.1.
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Abstract The density current parameterization coupled with Emanuel’s convection scheme, described in Part I of this series of papers, is tested in a single-column framework for continental and maritime convective systems. The case definitions and reference simulations are provided by cloud-resolving models (CRMs). For both cases, the wake scheme yields cold pools with temperature and humidity differences relative to the environment in reasonable agreement with observations (with wake depth on the order of 2 km over land and 1 km over ocean). The coupling with the convection scheme yields convective heating, drying, and precipitation similar to those simulated by the CRM. Thus, the representation of the action of the wakes on convection in terms of available lifting energy (ALE) and available lifting power (ALP) appears satisfactory. The sensitivity of the wake–convection system to the basic parameters of the parameterization is widely explored. A range of values for each parameter is recommended to help with implementing the scheme in a full-fledged general circulation model.
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Wagner, Till M., and Hans-F. Graf. "An Ensemble Cumulus Convection Parameterization with Explicit Cloud Treatment." Journal of the Atmospheric Sciences 67, no. 12 (December 1, 2010): 3854–69. http://dx.doi.org/10.1175/2010jas3485.1.
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Abstract The paper describes a convection parameterization employing a new formulation of the quasi-equilibrium closure hypothesis of Arakawa and Schubert. The scheme models a full spectrum of different cumulus clouds and its evolution within one time step of the host global climate model. Each cloud is simulated using a one-dimensional Lagrangian entraining parcel model, which includes mixed phase microphysics and vertical velocity. Hence, the model delivers explicit information on distribution of vertical velocities, precipitation intensities, cloud heights, and cloud coverage. The parameterization is evaluated in the ECHAM single-column model for midlatitude summer and tropical convection. Results show an improved temporal distribution, including the diurnal cycle, of convective heating and moistening in comparison to the Tiedtke–Nordeng scheme, which is the standard convection parameterization within ECHAM. The amount and temporal distribution of precipitation are clearly improved compared with the original parameterization. The convective cloud field model (CCFM) does not produce spurious convection events occurring with the standard parameterization.
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Hughes, T. "Thermal Convection in Ice Sheets: We look but do not see." Journal of Glaciology 31, no. 107 (1985): 39–48. http://dx.doi.org/10.1017/s0022143000004974.
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AbstractThermal convection in the Antarctic and Greenland ice sheets has been dismissed on the grounds that radio-echo stratigraphy is undisturbed for long distances. However, the undisturbed stratigraphy lies, for the most part, above the density inversion in polar ice sheets and therefore does not disprove convection. An echo-free zone is widespread below the density inversion, yet nobody has cited this as a strong indication that convection is indeed present at depth. A generalized Rayleigh criterion for thermal convection in elastic-viscoplastic polycrystalline solids heated from below is developed and applied to ice-sheet convection. An infinite Rayleigh number at the onset of primary creep decreases with time and becomes constant when secondary creep dominates, suggesting that any thermal buoyancy stress can initiate convection but convection cannot be sustained below a buoyancy stress of about 3 kPa. An analysis of the temperature profile down the Byrd Station core hole suggests that about 1000 m of ice below the density inversion will sustain convection. Creep along the Byrd Station strain network, radar sounding in East Antarctica, and seismic sounding in West Antarctica are examined for evidence of convective creep superimposed on advective creep. It is concluded that the evidence for convection is there, if we look for it with the intention of finding it.
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Hopper, Larry J., and Courtney Schumacher. "Modeled and Observed Variations in Storm Divergence and Stratiform Rain Production in Southeastern Texas." Journal of the Atmospheric Sciences 69, no. 4 (March 30, 2012): 1159–81. http://dx.doi.org/10.1175/jas-d-11-092.1.
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Abstract Storm divergence profiles observed by an S-band Doppler radar are compared to ensemble simulations of 10 disparate precipitating systems occurring in warm-season, weakly baroclinic, and strongly baroclinic environments in southeastern Texas. Eight triply nested mesoscale model simulations are conducted for each case using single- and double-moment microphysics with four convective treatments (i.e., two convective parameterizations and explicit versus parameterized convection at 9 km). Observed and simulated radar reflectivities are objectively separated into convective, stratiform, and nonprecipitating anvil columns and comparisons are made between ensemble mean echo coverages and levels of nondivergence (LNDs). In both the model and observations, storms occurring in less baroclinic environments have more convective rain area, less stratiform rain area, and more elevated divergence profiles. The model ensemble and observations agree best for well-organized leading-line trailing-stratiform systems. Excessive convective area fractions are simulated for several less-organized cases, especially those whose ensemble mean LNDs are about 1–2 km more elevated than observed. Simulations parameterizing convection on the intermediate grid produced less-elevated divergence profiles with smaller magnitudes compared to their explicit counterparts. In one warm-season case, utilizing double-moment microphysics when parameterizing convection on both outer grids generated lower LNDs associated with variations in convective intensity and depth, detraining less ice to anvil and stratiform regions at midlevels relative to its single-moment counterpart. Similarly, mesoscale convective vortex simulations employing an ensemble-based versus a single-closure convective parameterization on both outer grids produced the least-elevated heating structures (closer to observed), resulting in the weakest midlevel vortices.
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Stelten, Sean, and William A. Gallus. "Pristine Nocturnal Convective Initiation: A Climatology and Preliminary Examination of Predictability." Weather and Forecasting 32, no. 4 (August 1, 2017): 1613–35. http://dx.doi.org/10.1175/waf-d-16-0222.1.
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Abstract The prediction of convective initiation remains a challenge to forecasters in the Great Plains, especially for elevated events at night. This study examines a subset of 287 likely elevated nocturnal convective initiation events that occurred with little or no direct influence from surface boundaries or preexisting convection over a 4-month period of May–August during the summer of 2015. Events were first classified into one of four types based on apparent formation mechanisms and location relative to any low-level jet. A climatology of each of the four types was performed focusing on general spatial tendencies over a large Great Plains domain and initiation timing trends. Simulations from five convection-allowing models available during the Plains Elevated Convection At Night (PECAN) field campaign, along with four versions of a 4-km Weather Research and Forecasting (WRF) Model, were used to examine the predictability of these types of convective initiation. A dual-peak pattern for initiation timing was revealed, with one peak near 0400 UTC and another around 0700 UTC. The times and prominence of each peak shifted depending on the region analyzed. Positive thermal advection by the geostrophic wind was present in the majority of events for three types but not for the type occurring without a low-level jet. Models were more deficient with location than timing for the five PECAN models, with the four 4-km WRF Models showing similar location errors and problems with initiating convection at a lower altitude than observed.
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Stechmann, Samuel N., and Andrew J. Majda. "Gravity Waves in Shear and Implications for Organized Convection." Journal of the Atmospheric Sciences 66, no. 9 (September 1, 2009): 2579–99. http://dx.doi.org/10.1175/2009jas2976.1.
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Abstract It is known that gravity waves in the troposphere, which are often excited by preexisting convection, can favor or suppress the formation of new convection. Here it is shown that in the presence of wind shear or barotropic wind, the gravity waves can create a more favorable environment on one side of preexisting convection than the other side. Both the nonlinear and linear analytic models developed here show that the greatest difference in favorability between the two sides is created by jet shears, and little or no difference in favorability is created by wind profiles with shear at low levels and no shear in the upper troposphere. A nonzero barotropic wind (or, equivalently, a propagating heat source) is shown to also affect the favorability on each side of the preexisting convection. It is shown that these main features are captured by linear theory, and advection by the background wind is the main physical mechanism at work. These processes should play an important role in the organization of wave trains of convective systems (i.e., convectively coupled waves); if one side of preexisting convection is repeatedly more favorable in a particular background wind shear, then this should determine the preferred propagation direction of convectively coupled waves in this wind shear. In addition, these processes are also relevant to individual convective systems: it is shown that a barotropic wind can lead to near-resonant forcing that amplifies the strength of upstream gravity waves, which are known to trigger new convective cells within a single convective system. The barotropic wind is also important in confining the upstream waves to the vicinity of the source, which can help ensure that any new convective cells triggered by the upstream waves are able to merge with the convective system. All of these effects are captured in a two-dimensional model that is further simplified by including only the first two vertical baroclinic modes. Numerical results are shown with a nonlinear model, and linear theory results are in good agreement with the nonlinear model for most cases.
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Santellanes, Sean R., George S. Young, David J. Stensrud, Matthew R. Kumjian, and Ying Pan. "Environmental Conditions Associated with Horizontal Convective Rolls, Cellular Convection, and No Organized Circulations." Monthly Weather Review 149, no. 5 (May 2021): 1305–16. http://dx.doi.org/10.1175/mwr-d-20-0207.1.
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AbstractTypical environmental conditions associated with horizontal convective rolls (HCRs) and cellular convection have been known for over 50 years. Yet our ability to predict whether HCRs, cellular convection, or no discernable organized (null) circulation will occur within a well-mixed convective boundary layer based upon easily observed environmental variables has been limited. Herein, a large database of 50 cases each of HCR, cellular convection, and null events is created that includes observations of mean boundary layer wind and wind shear, boundary layer depth; surface observations of wind, temperature, and relative humidity; and estimates of surface sensible heat flux. Results from a multiclass linear discriminant analysis applied to these data indicate that environmental conditions can be useful in predicting whether HCRs, cellular convection, or no circulation occurs, with the analysis identifying the correct circulation type on 72% of the case days. This result is slightly better than using a mean convective boundary layer (CBL) wind speed of 6 m s−1 to discriminate between HCRs and cells. However, the mean CBL wind speed has no ability to further separate out cases with no CBL circulation. The key environmental variables suggested by the discriminant analysis are mean sensible heat flux, friction velocity, and the Obukhov length.
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Li, Jianfeng, Zhe Feng, Yun Qian, and L. Ruby Leung. "A high-resolution unified observational data product of mesoscale convective systems and isolated deep convection in the United States for 2004–2017." Earth System Science Data 13, no. 2 (March 3, 2021): 827–56. http://dx.doi.org/10.5194/essd-13-827-2021.
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Abstract. Deep convection possesses markedly distinct properties at different spatiotemporal scales. We present an original high-resolution (4 km, hourly) unified data product of mesoscale convective systems (MCSs) and isolated deep convection (IDC) in the United States east of the Rocky Mountains and examine their climatological characteristics from 2004 to 2017. The data product is produced by applying an updated Flexible Object Tracker algorithm to hourly satellite brightness temperature, radar reflectivity, and precipitation datasets. Analysis of the data product shows that MCSs are much larger and longer-lasting than IDC, but IDC occurs about 100 times more frequently than MCSs, with a mean convective intensity comparable to that of MCSs. Hence both MCS and IDC are essential contributors to precipitation east of the Rocky Mountains, although their precipitation shows significantly different spatiotemporal characteristics. IDC precipitation concentrates in summer in the Southeast with a peak in the late afternoon, while MCS precipitation is significant in all seasons, especially for spring and summer in the Great Plains. The spatial distribution of MCS precipitation amounts varies by season, while diurnally, MCS precipitation generally peaks during nighttime except in the Southeast. Potential uncertainties and limitations of the data product are also discussed. The data product is useful for investigating the atmospheric environments and physical processes associated with different types of convective systems; quantifying the impacts of convection on hydrology, atmospheric chemistry, and severe weather events; and evaluating and improving the representation of convective processes in weather and climate models. The data product is available at https://doi.org/10.25584/1632005 (Li et al., 2020).
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Posselt, Derek J., Susan van den Heever, Graeme Stephens, and Matthew R. Igel. "Changes in the Interaction between Tropical Convection, Radiation, and the Large-Scale Circulation in a Warming Environment." Journal of Climate 25, no. 2 (January 15, 2012): 557–71. http://dx.doi.org/10.1175/2011jcli4167.1.
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Abstract This paper explores the response of the tropical hydrologic cycle to surface warming through the lens of large-domain cloud-system-resolving model experiments run in a radiative–convective equilibrium framework. Simulations are run for 55 days and are driven with fixed insolation and constant sea surface temparatures (SSTs) of 298 K, 300 K, and 302 K. In each experiment, convection organizes into coherent regions of large-scale ascent separated by areas with relatively clear air and troposphere-deep descent. Aspects of the simulations correspond to observed features of the tropical climate system, including the transition to large precipitation rates above a critical value of total column water vapor, and an increase in convective intensity with SST amidst weakening of the large-scale overturning circulation. However, the authors also find notable changes to the interaction between convection and the environment as the surface warms. In particular, organized convection in simulations with SSTs of 298 and 300 K is inhibited by the presence of a strong midtropospheric stable layer and dry upper troposphere. As a result, there is a decrease in the vigor of deep convection and an increase in stratiform precipitation fraction with an increase in SST from 298 to 300 K. With an increase in SST to 302 K, moistening of the middletroposphere and increase in lower-tropospheric buoyancy serve to overcome these limitations, leading to an overall increase in convective intensity and larger increase in upper-tropospheric relative humidity. The authors conclude that, while convective intensity increases with SST, the aggregate nature of deep convection is strongly affected by the details of the thermodynamic environment in which it develops. In particular, the positive feedback between increasing SST and a moistening upper troposphere found in the simulations, operates as a nonmonotonic function of SST and is modulated by a complex interaction between deep convection and the environmental relative humidity and static stability profile. The results suggest that projected changes in convection that assume a monotonic dependence on SST may constitute an oversimplification.
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Vadas, S. L., M. J. Taylor, P. D. Pautet, P. A. Stamus, D. C. Fritts, H. L. Liu, F. T. São Sabbas, V. T. Rampinelli, P. Batista, and H. Takahashi. "Convection: the likely source of the medium-scale gravity waves observed in the OH airglow layer near Brasilia, Brazil, during the SpreadFEx campaign." Annales Geophysicae 27, no. 1 (January 14, 2009): 231–59. http://dx.doi.org/10.5194/angeo-27-231-2009.
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Abstract. Six medium-scale gravity waves (GWs) with horizontal wavelengths of λH=60–160 km were detected on four nights by Taylor et al. (2009) in the OH airglow layer near Brasilia, at 15° S, 47° W, during the Spread F Experiment (SpreadFEx) in Brazil in 2005. We reverse and forward ray trace these GWs to the tropopause and into the thermosphere using a ray trace model which includes thermospheric dissipation. We identify the convective plumes, convective clusters, and convective regions which may have generated these GWs. We find that deep convection is the highly likely source of four of these GWs. We pinpoint the specific deep convective plumes which likely excited two of these GWs on the nights of 30 September and 1 October. On these nights, the source location/time uncertainties were small and deep convection was sporadic near the modeled source locations. We locate the regions containing deep convective plumes and clusters which likely excited the other two GWs. The last 2 GWs were probably also excited from deep convection; however, they must have been ducted ~500–700 km if so. Two of the GWs were likely downwards-propagating initially (after which they reflected upwards from the Earth's surface), while one of the GWs was likely upwards-propagating initially from the convective plume/cluster. We also estimate the amplitudes and vertical scales of these waves at the tropopause, and compare their scales with those from a simple, linear convection model. Finally, we calculate each GW's dissipation altitude, location, and amplitude. We find that the dissipation altitude depends sensitively on the winds at and above the OH layer. We also find that several of these GWs may have penetrated to high enough altitudes to potentially seed equatorial spread F (ESF) if located somewhat farther from the magnetic equator.
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Suselj, Kay, Marcin J. Kurowski, and Joao Teixeira. "A Unified Eddy-Diffusivity/Mass-Flux Approach for Modeling Atmospheric Convection." Journal of the Atmospheric Sciences 76, no. 8 (July 31, 2019): 2505–37. http://dx.doi.org/10.1175/jas-d-18-0239.1.
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Abstract A fully unified parameterization of boundary layer and moist convection (shallow and deep) is presented. The new parameterization is based on the stochastic multiplume eddy-diffusivity/mass-flux (EDMF) approach, which distinguishes between convective plumes and nonconvective mixing. The convective plumes represent both surface-forced updrafts and evaporatively driven downdrafts. The type of convection (i.e., dry, shallow, or deep) represented by the updrafts is not defined a priori, but rather depends on the near-surface updraft properties and the stochastic interactions between the plumes and the environment through lateral entrainment. Consequently, some updrafts may contribute only to the nonlocal transport within the subcloud layer, while others may condense and form shallow or even deep convection. Such a formulation is void of trigger functions and additional closures typical of modular parameterizations. The updrafts are coupled to relatively simple warm-, mixed-, and ice-phase microphysics. Each precipitating updraft forms a downdraft driven by the evaporation of detrained precipitation. The downdrafts control the development of cold pools near the surface that can invigorate convection. The new parameterization is tested in a single-column model against large-eddy simulations (LESs) for cases representing weakly precipitating marine convection and the diurnal cycle of continental deep convection. The results of these EDMF experiments compare well with the LES reference simulations. In particular, the transitions between the different dominant convection regimes are realistically simulated.
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Thielen, J., and A. Gadian. "Influence of different wind directions in relation to topography on the outbreak of convection in Northern England." Annales Geophysicae 14, no. 10 (October 31, 1996): 1078–87. http://dx.doi.org/10.1007/s00585-996-1078-3.
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Abstract. The influence of different wind directions on the outbreak of convection in Northern England, was investigated with a high-resolution numerical model. The Clark model, a 3D finite-difference, non-hydrostatic model was used in this study. It was initialised with the topography of Northern England, a representation of surface characteristics, and used a routinely available meteorological sounding, typical of the unstable conditions. Results showed that convective cells were initially triggered in the lee of the elevated terrain, and that only after the convection had developed, were cells upwind of the elevated terrain produced. The windward slopes themselves seemed sheltered from convection. Under most wind directions, the central Pennines (the Forest of Trawden and the Forest of Rossendale) seemed particularly affected by convective rainfall.
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Mitrovica, J. X., J. Austermann, S. Coulson, J. R. Creveling, M. J. Hoggard, G. T. Jarvis, and F. D. Richards. "Dynamic Topography and Ice Age Paleoclimate." Annual Review of Earth and Planetary Sciences 48, no. 1 (May 30, 2020): 585–621. http://dx.doi.org/10.1146/annurev-earth-082517-010225.
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The connection between the geological record and dynamic topography driven by mantle convective flow has been established over widely varying temporal and spatial scales. As observations of the process have increased and numerical modeling of thermochemical convection has improved, a burgeoning direction of research targeting outstanding issues in ice age paleoclimate has emerged. This review focuses on studies of the Plio-Pleistocene ice age, including investigations of the stability of ice sheets during ice age warm periods and the inception of Northern Hemisphere glaciation. However, studies that have revealed nuanced connections of dynamic topography to biodiversity, ecology, ocean chemistry, and circulation since the start of the current ice-house world are also considered. In some cases, a recognition of the importance of dynamic topography resolves enigmatic events and in others it confounds already complex, unanswered questions. All such studies highlight the role of solid Earth geophysics in paleoclimate research and undermine a common assumption, beyond the field of glacial isostatic adjustment, that the solid Earth remains a rigid, passive substrate during the evolution of the ice age climate system. ▪ Dynamic topography is the large-scale, vertical deflection of Earth's crust driven by mantle convective flow. ▪ This review highlights recent research exploring the implications of the process on key issues in ice age paleoclimate. ▪ This research includes studies of ice sheet stability and inception as well as inferences of peak sea levels during periods of relative ice age warmth. ▪ This review also includes studies on longer timescales, continental-scale ecology and biodiversity, the long-term carbon cycle, and water flux across oceanic gateways.
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Wang, Xinyue, Hironobu Iwabuchi, and Jean-Baptiste Courbot. "Analysis of Diurnal Evolution of Cloud Properties and Convection Tracking over the South China Coastal Area." Remote Sensing 14, no. 19 (October 9, 2022): 5039. http://dx.doi.org/10.3390/rs14195039.
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Different diurnal rainfall cycles occur over the offshore and inland regions of the South China coastal area (SCCA). Inspired by these findings, in this study, we investigated the diurnal evolution features of cloud systems and cloud properties inside such systems for both the SCCA offshore and inland regions, using cloud data retrieved from a recently developed deep neural network model. Rainy day data for June 2017 revealed that the ice cloud optical thickness and top height reached their peak intensities at noon (~12 local standard time (LST)) over the offshore region, approximately 2 h later than the rainfall peak. Over the inland region, cloud and rainfall peaks simultaneously appeared from ~18 to 20 LST. When further examining the cloud-amount variation of different ice-cloud types, we found a clear diurnal oscillation in the medium-thick cloud amount over the offshore region, while for the inland region, this cloud type had no obvious diurnal peak, showing a low cloud amount throughout the day. This phenomenon suggests different inner structures and intensities between offshore and inland convections. To better elucidate the convection features over different regions, a tracking algorithm was applied to obtain various parameters, such as size, number, and duration of mesoscale convective systems. The strongest convections, which lasted over 12 h, tended to be abundant over the offshore region from ~03 to 12 LST, and an inland to offshore migration at ~03 LST was facilitated by the beneficial meteorological conditions observed at 113–116˚E, 20.5–22.5˚N.
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Ghernaout, Badia, Said Bouabdallah, Aissa Atia, and Müslüm Arıcı. "Heat and Fluid Flow in an Open Agricultural Greenhouse in Presence of Plants." Advances in Modelling and Analysis B 64, no. 1-4 (December 31, 2021): 1–8. http://dx.doi.org/10.18280/ama_b.641-401.
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Flow convection in agriculture greenhouse is one of the most important factors on the growth and fruiting of plants. The present work focused on natural convection in an open greenhouse heated by ridge tubes in presence of plants. Analyses are performed for different boundary conditions imposed at the roof such as constant temperature, convective heat flux, and convective and radiative heat flux. The governing equations comprising continuity, momentum and energy equations are solved by Ansys-Fluent software. In each case, the average velocity and temperature of the air are determined. The obtained results are presented in terms velocity and temperature profiles. Isothermal lines and velocity vectors showed that by increasing the convective heat transfer coefficient, the average temperature and average airflow velocity decrease. The outcomes of this study help build greenhouses with dimensions and materials to suitable for the given external conditions.
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Chervov, V. V., N. A. Bushenkova, and G. G. Chernykh. "Tectonic depressions on the East-European and Siberian platforms: numerical modeling of convection beneath the Eurasian continent." Geodynamics & Tectonophysics 12, no. 1 (March 21, 2021): 84–99. http://dx.doi.org/10.5800/gt-2021-12-1-0514.
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In modern concepts, the upper mantle of the Earth is a highly viscous incompressible liquid, and its flow is described using the Navier – Stokes equations in the Oberbeck – Boussinesq and geodynamic approximations. Convective flows in the upper mantle play a decisive role in the kinematics of lithospheric plates and the geological history of continental regions. Mathematical modeling is a basic method for studying convective processes in the mantle. Our paper presents a numerical model of convection, which is based on the implicit artificial compressibility method. This model is tested in detail by comparing our calculation results with the results of a well-known international test. It is demonstrated that the Fedorenko grids sequence method is highly efficient and reduces the computing time almost by a factor of eight. The numerical model is generalized in order to state the problem in a spherical system of coordinates. It is used to analyse the distribution of convective flows in the upper mantle underneath the Eurasian continent. The analysis shows that the thickness and geometrical parameters of the lithospheric blocks are the factors of significant influence on the distribution of convective flows in the upper mantle. The resulting structure of convective flows is manifested in the surface topography of large platform areas wherein the lithosphere thickness is increased. Thus, the locations of extended downward convection flows under the East European and Siberian platforms are clearly comparable to syneclises observed in the study area.
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Inoue, Kuniaki, and Larissa E. Back. "Gross Moist Stability Assessment during TOGA COARE: Various Interpretations of Gross Moist Stability." Journal of the Atmospheric Sciences 72, no. 11 (November 1, 2015): 4148–66. http://dx.doi.org/10.1175/jas-d-15-0092.1.
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Abstract Daily averaged TOGA COARE data are analyzed to investigate the convective amplification/decay mechanisms. The gross moist stability (GMS), which represents moist static energy (MSE) export efficiency by large-scale circulations associated with the convection, is studied together with two quantities, called the critical GMS (a ratio of diabatic forcing to the convective intensity) and the drying efficiency [a version of the effective GMS (GMS minus critical GMS)]. The analyses reveal that convection intensifies (decays) via negative (positive) drying efficiency. The authors illustrate that variability of the drying efficiency during the convective amplifying phase is predominantly explained by the vertical MSE advection (or vertical GMS), which imports MSE via bottom-heavy vertical velocity profiles (associated with negative vertical GMS) and eventually starts exporting MSE via top-heavy profiles (associated with positive vertical GMS). The variability of the drying efficiency during the decaying phase is, in contrast, explained by the horizontal MSE advection. The critical GMS, which is moistening efficiency due to the diabatic forcing, is broadly constant throughout the convective life cycle, indicating that the diabatic forcing always tends to destabilize the convective system in a constant manner. The authors propose various ways of computing quasi-time-independent “characteristic GMS” and demonstrate that all of them are equivalent and can be interpreted as (i) the critical GMS, (ii) the GMS at the maximum precipitation, and (iii) a combination of feedback constants between the radiation, evaporation, and convection. Those interpretations indicate that each convective life cycle is a fluctuation of rapidly changing GMS around slowly changing characteristic GMS.
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Satake, Hidemoto, and Toshio Tagawa. "Influence of Centrifugal Buoyancy in Thermal Convection within a Rotating Spherical Shell." Symmetry 14, no. 10 (September 26, 2022): 2021. http://dx.doi.org/10.3390/sym14102021.
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The dynamo action, which is of importance in the study of the geomagnetism mechanism, is considered to be caused by the convection structure formed inside a rotating spherical shell. This convection structure elongated in the rotation axis is generated by the action of both heat and rotation on the fluid inside a spherical shell. In this study, we analyzed thermal convection in such a rotating spherical shell and attempted to understand the phenomenon of this convective structure. It is known that each value of the Prandtl number, the Ekman number and the Rayleigh number and their balance are important for the generation of such convective structure. We fixed these three parameters and considered the effect of centrifugal buoyancy as the Froude number additionally. To investigate how the effects of centrifugal buoyancy affect the convective structure, we carried out both three-dimensional numerical simulations and linear stability analyses. In particular, we focused on the transition from axisymmetric flow to non-axisymmetric flow having wavenumbers in the toroidal direction and investigated both growth rate and phase velocity of the disturbance. It was found that axisymmetric flow tends to be maintained as the effect of centrifugal buoyancy increases.
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Ni, Xiang, Chuntao Liu, and Edward Zipser. "Ice Microphysical Properties near the Tops of Deep Convective Cores Implied by the GPM Dual-Frequency Radar Observations." Journal of the Atmospheric Sciences 76, no. 9 (September 1, 2019): 2899–917. http://dx.doi.org/10.1175/jas-d-18-0243.1.
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Abstract Using three years of observations from the Dual-Frequency Precipitation Radar (DPR) aboard the Global Precipitation Measurement (GPM) Core Observatory, properties of the cores of deep convection are examined. First, deep convective systems are selected, defined as GPM precipitation features with maximum 20-dBZ echo-top heights above 10 km. The cores of deep convection are described by the profiles of Ku- and Ka-band radar reflectivity at the location of the highest echo top in each deep convective system. Then the dual-frequency ratio (DFR) profile is derived by subtracting Ka-band from Ku-band radar reflectivity. It is found that values of DFR are larger over land than over ocean in general near the top of the convection, which is consistent with larger ice particles in stronger updrafts in continental convection. The magnitude of DFR at 12 km is positively correlated with the convection intensity indicated by 20- and 30-dBZ echo tops. The microphysical properties including volume-weighted mean diameter, ice water content, and total ice particle number concentration are derived using a simple lookup table approach. Under the same particle size distribution assumption, the cores of deep convection over land have larger ice particle size, higher ice water content, and lower particle concentration than those over ocean at levels above 10 km, but with some distinct regional variations.
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Kastman, Joshua, Patrick Market, and Neil Fox. "Dynamic Ensemble Analysis of Frontal Placement Impacts in the Presence of Elevated Thunderstorms during PRECIP Events." Atmosphere 9, no. 9 (August 29, 2018): 339. http://dx.doi.org/10.3390/atmos9090339.
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The Program for Research on Elevated Convection with Intense Precipitation (PRECIP) field campaign sampled 10 cases of elevated convection during 2014 and 2015. These intense observing periods (IOP) mostly featured well-defined stationary or warm frontal zones, over whose inversion elevated convection would form. However, not all frontal zones translated as expected, with some poleward motions being arrested and even returning equatorward. Prior analyses of the observed data highlighted the downdrafts in these events, especially diagnostics for their behavior: the downdraft convective available potential energy (DCAPE) and the downdraft convective inhibition (DCIN). With the current study, the DCAPE and DCIN are examined for four cases: two where frontal motion proceeded poleward, as expected, and two where the frontal motions were slowed significantly or stalled altogether. Using the Weather Research and Forecasting (WRF) model, a multi-model ensemble was created for each of the four cases, and the best performing members were selected for additional deterministic examination. Analyses of frontal motions and surface cold pools are explored in the context of DCAPE and DCIN. These analyses further establish the DCAPE and DCIN, not only as a means to classify elevated convection, but also to aid in explaining frontal motions in the presence of elevated convection.