Journal articles on the topic 'Convection'
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1
Zhang, Nan, Yan Wang, and Xiaomeng Lin. "Mesoscale Observational Analysis of Isolated Convection Associated with the Interaction of the Sea Breeze Front and the Gust Front in the Context of the Urban Heat Humid Island Effect." Atmosphere 13, no. 4 (April 9, 2022): 603. http://dx.doi.org/10.3390/atmos13040603.
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An isolated convection was unexpectedly initiated in the evening of 1 August 2019 around the Tianjin urban region (TUR), which happened at some distance from the shear line at lower level and the preexisting convection to the South, analyzed by using ERA5 reanalysis data and observations from surface weather stations, and a S-band radar. The results show that, 42 min before the initiation of the convection, the atmospheric thermodynamic conditions around TUR were favorable for the initiation of the isolated convection, although the southerly and vertical shear of the horizontal wind at the lower level was weak. A sea-breeze front approached the TUR and continued to move West, leading to the triggering of the isolated convection in the context of the urban humid heat island (UHHI) effect. Subsequently, the gust front, which was formed between the cold pool away from the TUR and the warm and humid air of the UHHI, moved northward, approached the convection, and collided with sea breeze front, resulting in five reflectivity centers of isolated convection being merged and the convection’s development. Finally, the isolated convection split into two convections that moved away from the TUR and disappeared at 20:36 Beijing Time. The isolated convection was initiated and developed by the interaction of the sea breeze front and gust front in the context of the UHHI effect. The sea breeze front triggered the isolated convection around TUR in the context of the UHHI effect, and the gust front produced by the early convective storms to the south played a vital role in the development of the isolated convection.
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Folkins, Ian, S. Fueglistaler, G. Lesins, and T. Mitovski. "A Low-Level Circulation in the Tropics." Journal of the Atmospheric Sciences 65, no. 3 (March 1, 2008): 1019–34. http://dx.doi.org/10.1175/2007jas2463.1.
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Abstract Deep convective tropical systems are strongly convergent in the midtroposphere. Horizontal wind measurements from a variety of rawinsonde arrays in the equatorial Pacific and Caribbean are used to calculate the mean dynamical divergence profiles of large-scale arrays (≥1000 km in diameter) in actively convecting regions. Somewhat surprisingly, the magnitude of the midtropospheric divergence calculated from these arrays is usually small. In principle, the midlevel convergence of deep convective systems could be balanced on larger scales either by a vertical variation in the radiative mass flux of the background clear sky atmosphere, or by a divergence from shallow cumuli. The vertical variation of the clear sky mass flux in the midtroposphere is small, however, so that the offsetting divergence must be supplied by shallow cumuli. On spatial scales of ∼1000 km, the midlevel convergent inflow toward deep convection appears to be internally compensated, or “screened,” by a divergent outflow from surrounding precipitating shallow convection. Deep convective systems do not induce a large-scale inflow of midlevel air toward actively convecting regions from the rest of the tropics, but instead help generate a secondary low-level circulation, in which the net downward mass flux from mesoscale and convective-scale downdrafts is balanced by a net upward mass flux from precipitating shallow cumuli. The existence of this circulation is consistent with observational evidence showing that deep and shallow convection are spatiotemporally coupled on a wide range of both spatial and temporal scales. One of the mechanisms proposed for coupling shallow convection to deep convection is the tendency for deep convection to cool the lower troposphere. The authors use radiosonde temperature profiles and the Tropical Rainfall Measuring Mission (TRMM) 3B42 gridded rainfall product to argue that the distance over which deep convection cools the lower troposphere is approximately 1000 km.
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Romps, David M., and Zhiming Kuang. "A Transilient Matrix for Moist Convection." Journal of the Atmospheric Sciences 68, no. 9 (September 1, 2011): 2009–25. http://dx.doi.org/10.1175/2011jas3712.1.
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Abstract A method is introduced for diagnosing a transilient matrix for moist convection. This transilient matrix quantifies the nonlocal transport of air by convective eddies: for every height z, it gives the distribution of starting heights z′ for the eddies that arrive at z. In a cloud-resolving simulation of deep convection, the transilient matrix shows that two-thirds of the subcloud air convecting into the free troposphere originates from within 100 m of the surface. This finding clarifies which initial height to use when calculating convective available potential energy from soundings of the tropical troposphere.
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Wang, Yin, Pik-Yin Lai, Hao Song, and Penger Tong. "Mechanism of large-scale flow reversals in turbulent thermal convection." Science Advances 4, no. 11 (November 2018): eaat7480. http://dx.doi.org/10.1126/sciadv.aat7480.
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It is commonly believed that heat flux passing through a closed thermal convection system is balanced so that the convection system can remain at a steady state. Here, we report a new kind of convective instability for turbulent thermal convection, in which the convective flow stays over a long steady “quiet period” having a minute amount of heat accumulation in the convection cell, followed by a short and intermittent “active period” with a massive eruption of thermal plumes to release the accumulated heat. The rare massive eruption of thermal plumes disrupts the existing large-scale circulation across the cell and resets its rotational direction. A careful analysis reveals that the distribution of the plume eruption amplitude follows the generalized extreme value statistics with an upper bound, which changes with the fluid properties of the convecting medium. The experimental findings have important implications to many closed convection systems of geophysical scale, in which massive eruptions and sudden changes in large-scale flow pattern are often observed.
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Pertiwi, Nada Aulia, Hendi Rohendi, and Setiawan Setiawan. "Penyusunan Model Laporan Keuangan Entitas Mikro Kecil Menengah Berdasarkan SAK EMKM pada EMKM Konveksi." Jurnal Accounting Information System (AIMS) 3, no. 1 (April 20, 2020): 36–50. http://dx.doi.org/10.32627/aims.v3i1.334.
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This study aims to know the knowledge of EMKM Convections jeans in Soreang District regarding SAK EMKM, to find out the financial records carried out by EMKM, and to compile a model of EMKM Convection’s financial statements that are under SAK EMKM. In data collection, used snowball sampling and data analysis techniques are carried out through the stages of data reduction, evaluating EMKM’s knowledge of SAK EMKM, evaluating the suitability of the records carried out by EMKM Convection with applicable standards, namely SAK EMKM, seeing the similarity of activities contained in EMKM convection to create a model suitable for all EMKM convection and compile a model of financial statements for EMKM Convections following SAK EMKM. The results showed that the EMKM Convections in Soreang District still did not know about the existence of SAK EMKM, EMKM actors also still did a simple recording, there was no further record up to the preparation of financial statements. Besides, this study also produced a financial statement model for EMKM convections that were by the SAK EMKM.
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Neggers, Roel A. J., J. David Neelin, and Bjorn Stevens. "Impact Mechanisms of Shallow Cumulus Convection on Tropical Climate Dynamics*." Journal of Climate 20, no. 11 (June 1, 2007): 2623–42. http://dx.doi.org/10.1175/jcli4079.1.
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Abstract Subtropical shallow cumulus convection is shown to play an important role in tropical climate dynamics, in which convective mixing between the atmospheric boundary layer and the free troposphere initiates a chain of large-scale feedbacks. It is found that the presence of shallow convection in the subtropics helps set the width and intensity of oceanic ITCZs, a mechanism here termed the shallow cumulus humidity throttle because of the control exerted on the moisture supply to the deep convection zones. These conclusions are reached after investigations based on a tropical climate model of intermediate complexity, with sufficient vertical degrees of freedom to capture (i) the effects of shallow convection on the boundary layer moisture budget and (ii) the dependency of deep convection on the free-tropospheric humidity. An explicit shallow cumulus mixing time scale in this simple parameterization is varied to assess sensitivity, with moist static energy budget analysis aiding to identify how the local effect of shallow convection is balanced globally. A reduction in the mixing efficiency of shallow convection leads to a more humid atmospheric mixed layer, and less surface evaporation, with a drier free troposphere outside of the convecting zones. Advection of drier free-tropospheric air from the subtropics by transients such as dry intrusions, as well as by mean inflow, causes a substantial narrowing of the convection zones by inhibition of deep convection at their margins. In the tropical mean, the reduction of convection by this narrowing more than compensates for the reduction in surface evaporation. Balance is established via a substantial decrease in tropospheric temperatures throughout the Tropics, associated with the reduction in convective heating. The temperature response—and associated radiative contribution to the net flux into the column—have broad spatial scales, while the reduction of surface evaporation is concentrated outside of the convecting zones. This results in differential net flux across the convecting zone, in a sense that acts to destabilize those areas that do convect. This results in stronger large-scale convergence and more intense convection within a narrower area. Finally, mixed layer ocean experiments show that in a coupled ocean–atmosphere system this indirect feedback mechanism can lead to SST differences up to +2 K between cases with different shallow cumulus mixing time, tending to counteract the direct radiative impact of low subtropical clouds on SST.
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Bouffard, Damien, and Alfred Wüest. "Convection in Lakes." Annual Review of Fluid Mechanics 51, no. 1 (January 5, 2019): 189–215. http://dx.doi.org/10.1146/annurev-fluid-010518-040506.
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Lakes and other confined water bodies are not exposed to tides, and their wind forcing is usually much weaker compared to ocean basins and estuaries. Hence, convective processes are often the dominant drivers for shaping mixing and stratification structures in inland waters. Due to the diverse environments of lakes—defined by local morphological, geochemical, and meteorological conditions, among others—a fascinating variety of convective processes can develop with remarkably unique signatures. Whereas the classical cooling-induced and shear-induced convections are well-known phenomena due to their dominant roles in ocean basins, other convective processes are specific to lakes and often overlooked, for example, sidearm, under-ice, and double-diffusive convection or thermobaric instability and bioconvection. Additionally, the peculiar properties of the density function at low salinities/temperatures leave distinctive traces. In this review, we present these various processes and connect observations with theories and model results.
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Zhao, N., X. Y. Shen, Y. H. Ding, and M. Takahashi. "A possible theory for the interaction between convective activities and vortical flows." Nonlinear Processes in Geophysics 18, no. 5 (October 31, 2011): 779–89. http://dx.doi.org/10.5194/npg-18-779-2011.
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Abstract. Theoretical studies usually attribute convections to the developments of instabilities such as the static or symmetric instabilities of the basic flows. However, the following three facts make the validities of these basic theories unconvincing. First, it seems that in most cases the basic flow with balance property cannot exist as the exact solution, so one cannot formulate appropriate problems of stability. Second, neither linear nor nonlinear theories of dynamical instability are able to describe a two-way interaction between convection and its background, because the basic state which must be an exact solution of the nonlinear equations of motion is prescribed in these issues. And third, the dynamical instability needs some extra initial disturbance to trigger it, which is usually another point of uncertainty. The present study suggests that convective activities can be recognized in the perspective of the interaction of convection with vortical flow. It is demonstrated that convective activities can be regarded as the superposition of free modes of convection and the response to the forcing induced by the imbalance of the unstably stratified vortical flow. An imbalanced vortical flow provides not only an initial condition from which unstable free modes of convection can develop but also a forcing on the convection. So, convection is more appropriately to be regarded as a spontaneous phenomenon rather than a disturbance-triggered phenomenon which is indicated by any theory of dynamical instability. Meanwhile, convection, particularly the forced part, has also a reaction on the basic flow by preventing the imbalance of the vortical flow from further increase and maintaining an approximately balanced flow.
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Fuentes, J. R., Andrew Cumming, Matias Castro-Tapia, and Evan H. Anders. "Heat Transport and Convective Velocities in Compositionally Driven Convection in Neutron Star and White Dwarf Interiors." Astrophysical Journal 950, no. 1 (June 1, 2023): 73. http://dx.doi.org/10.3847/1538-4357/accb56.
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Abstract We investigate heat transport associated with compositionally driven convection driven by crystallization at the ocean–crust interface in accreting neutron stars, or growth of the solid core in cooling white dwarfs. We study the effect of thermal diffusion and rapid rotation on the convective heat transport, using both mixing length theory and numerical simulations of Boussinesq convection. We determine the heat flux, composition gradient, and Péclet number, Pe (the ratio of thermal diffusion time to convective turnover time) as a function of the composition flux. We find two regimes of convection with a rapid transition between them as the composition flux increases. At small Pe, the ratio between the heat flux and composition flux is independent of Pe, because the loss of heat from convecting fluid elements due to thermal diffusion is offset by the smaller composition gradient needed to overcome the reduced thermal buoyancy. At large Pe, the temperature gradient approaches the adiabatic gradient, saturating the heat flux. We discuss the implications for cooling of neutron stars and white dwarfs. Convection in neutron stars spans both regimes. We find rapid mixing of neutron star oceans, with a convective turnover time of the order of weeks to minutes depending on rotation. Except during the early stages of core crystallization, white dwarf convection is in the thermal-diffusion-dominated fingering regime. We find convective velocities much smaller than recent estimates for crystallization-driven dynamos. The small fraction of energy carried as kinetic energy calls into question the effectiveness of crystallization-driven dynamos as an explanation for observed magnetic fields in white dwarfs.
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Ma, Nancy, John Walker, David Bliss, and George Bryant. "Forced Convection During Liquid Encapsulated Crystal Growth With an Axial Magnetic Field." Journal of Fluids Engineering 120, no. 4 (December 1, 1998): 844–50. http://dx.doi.org/10.1115/1.2820749.
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This paper treats the forced convection, which is produced by the rotation of the crystal about its vertical centerline during the liquid-encapsulated Czochralski or Kyropoulos growth of compound semiconductor crystals, with a uniform vertical magnetic field. The model assumes that the magnetic field strength is sufficiently large that convective heat transfer and all inertial effects except the centripetal acceleration are negligible. With the liquid encapsulant in the radial gap between the outside surface of the crystal and the vertical wall of the crucible, the forced convection is fundamentally different from that with a free surface between the crystal and crucible for the Czochralski growth of silicon crystals. Again unlike the case for silicon growth, the forced convection for the actual nonzero electrical conductivity of an indium-phosphide crystal is virtually identical to that for an electrically insulating crystal. The electromagnetic damping of the forced convection is stronger than that of the buoyant convection. In order to maintain a given balance between the forced and buoyant convections, the angular velocity of the crystal must be increased as the magnetic field strength is increased.
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Spyksma, K., and P. Bartello. "Predictability in Wet and Dry Convective Turbulence." Journal of the Atmospheric Sciences 65, no. 1 (January 1, 2008): 220–34. http://dx.doi.org/10.1175/2007jas2307.1.
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Abstract There is a growing interest in understanding the role that moisture plays in atmospheric dynamics, particularly in its effect on predictability. Current research indicates that when moisture effects are added to an atmospheric model, the error growth produced by the new moist dynamics reduces the predictability times, especially at the scales of moist convection. The issue of moist convection’s effect on predictability is addressed herein. By performing high-resolution large-ensemble runs, it is shown that although nonprecipitating moist convection is less predictable than dry convection resulting from the same forcing, this effect can be explained by the energy injected into the system through the latent heating and cooling arising from the convective motion. This extra energy is spread evenly over most scales of the convective dynamics. When the predictability times are scaled to account for the extra kinetic energy, and the resulting earlier growth of error energy, wet and dry convection have very similar error growth characteristics. Sensitivity tests are performed to ensure that the results from the large ensembles have converged and that they are consistent with either changing resolution, diffusion levels, initial error energy length scales, or forcing amplitude.
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Tulich, Stefan N., and Brian E. Mapes. "Multiscale Convective Wave Disturbances in the Tropics: Insights from a Two-Dimensional Cloud-Resolving Model." Journal of the Atmospheric Sciences 65, no. 1 (January 1, 2008): 140–55. http://dx.doi.org/10.1175/2007jas2353.1.
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Abstract Multiscale convective wave disturbances with structures broadly resembling observed tropical waves are found to emerge spontaneously in a nonrotating, two-dimensional cloud model forced by uniform cooling. To articulate the dynamics of these waves, model outputs are objectively analyzed in a discrete truncated space consisting of three cloud types (shallow convective, deep convective, and stratiform) and three dynamical vertical wavelength bands. Model experiments confirm that diabatic processes in deep convective and stratiform regions are essential to the formation of multiscale convective wave patterns. Specifically, upper-level heating (together with low-level cooling) serves to preferentially excite discrete horizontally propagating wave packets with roughly a full-wavelength structure in troposphere and “dry” phase speeds cn in the range 16–18 m s−1. These wave packets enhance the triggering of new deep convective cloud systems, via low-level destabilization. The new convection in turn causes additional heating over cooling, through delayed development of high-based deep convective cells with persistent stratiform anvils. This delayed forcing leads to an intensification and then widening of the low-level cold phases of wave packets as they move through convecting regions. Additional widening occurs when slower-moving (∼8 m s−1) “gust front” wave packets excited by cooling just above the boundary layer trigger additional deep convection in the vicinity of earlier convection. Shallow convection, meanwhile, provides positive forcing that reduces convective wave speeds and destroys relatively small-amplitude-sized waves. Experiments with prescribed modal wind damping establish the critical role of short vertical wavelengths in setting the equivalent depth of the waves. However, damping of deep vertical wavelengths prevents the clustering of mesoscale convective wave disturbances into larger-scale envelopes, so these circulations are important as well.
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LaPointe, Stephen J., Nancy Ma, and D. W. Mueller. "Growth of Binary Alloyed Semiconductor Crystals by the Vertical Bridgman-Stockbarger Process with a Strong Magnetic Field." Journal of Fluids Engineering 127, no. 3 (January 4, 2005): 523–28. http://dx.doi.org/10.1115/1.1899169.
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This paper presents a model for the unsteady species transport for the growth of alloyed semiconductor crystals during the vertical Bridgman-Stockbarger process with a steady axial magnetic field. During growth of alloyed semiconductors such as germanium-silicon (GeSi) and mercury-cadmium-telluride (HgCdTe), the solute’s concentration is not small, so that density differences in the melt are very large. These compositional variations drive compositionally driven buoyant convection, or solutal convection, in addition to thermally driven buoyant convection. These buoyant convections drive convective transport, which produces nonuniformities in the concentration in both the melt and the crystal. This transient model predicts the distribution of species in the entire crystal grown in a steady axial magnetic field. The present study presents results of concentration in the crystal and in the melt at several different stages during crystal growth.
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Wang, Zhuo. "What is the Key Feature of Convection Leading up to Tropical Cyclone Formation?" Journal of the Atmospheric Sciences 75, no. 5 (May 2018): 1609–29. http://dx.doi.org/10.1175/jas-d-17-0131.1.
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Infrared brightness temperature data are used to investigate convective evolution during tropical cyclone (TC) formation in a quasi-Lagrangian framework. More than 150 named Atlantic storms during 1989–2010 were examined. It is found that both convective intensity and convective frequency increase with time in the inner pouch region but change little, or even weaken slightly, in the outer pouch region. Convection thus appears to concentrate toward the circulation center as genesis is approached. However, large variability is found from storm to storm in convective intensity, area, and duration, and the convective evolution of individual storms does not resemble the composite mean. Further analysis suggests that the composite mean or the median represents the probability of occurrence of convection instead of a recurrent pattern. Three distinct spatial patterns of convection are identified using cluster analysis. Substantial differences in convection intensity and area are found among the clusters and can be attributed to the impacts of environmental conditions. These differences suggest that convection intensity or area is not a key feature of convection for tropical cyclogenesis. In particular, a small and weak convective system is not necessarily associated with a weak vortex. A simple proxy of the radial gradient of convection is found to be similar among the clusters. Furthermore, convection is most effective in strengthening the TC protovortex when its maximum occurs near the pouch center. These findings suggest that organized convection near the pouch center is a key feature of convection for tropical cyclogenesis and that emphasizing convective intensity or frequency without considering the spatial pattern may be misleading.
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Vreugdenhil, Catherine A., and Bishakhdatta Gayen. "Ocean Convection." Fluids 6, no. 10 (October 12, 2021): 360. http://dx.doi.org/10.3390/fluids6100360.
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Ocean convection is a key mechanism that regulates heat uptake, water-mass transformation, CO2 exchange, and nutrient transport with crucial implications for ocean dynamics and climate change. Both cooling to the atmosphere and salinification, from evaporation or sea-ice formation, cause surface waters to become dense and down-well as turbulent convective plumes. The upper mixed layer in the ocean is significantly deepened and sustained by convection. In the tropics and subtropics, night-time cooling is a main driver of mixed layer convection, while in the mid- and high-latitude regions, winter cooling is key to mixed layer convection. Additionally, at higher latitudes, and particularly in the sub-polar North Atlantic Ocean, the extensive surface heat loss during winter drives open-ocean convection that can reach thousands of meters in depth. On the Antarctic continental shelf, polynya convection regulates the formation of dense bottom slope currents. These strong convection events help to drive the immense water-mass transport of the globally-spanning meridional overturning circulation (MOC). However, convection is often highly localised in time and space, making it extremely difficult to accurately measure in field observations. Ocean models such as global circulation models (GCMs) are unable to resolve convection and turbulence and, instead, rely on simple convective parameterizations that result in a poor representation of convective processes and their impact on ocean circulation, air–sea exchange, and ocean biology. In the past few decades there has been markedly more observations, advancements in high-resolution numerical simulations, continued innovation in laboratory experiments and improvement of theory for ocean convection. The impacts of anthropogenic climate change on ocean convection are beginning to be observed, but key questions remain regarding future climate scenarios. Here, we review the current knowledge and future direction of ocean convection arising from sea–surface interactions, with a focus on mixed layer, open-ocean, and polynya convection.
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Ecke, Robert E., Hans Haucke, and John Wheatley. "Convectively driven superfluid turbulence in dilute solutions of 3He in superfluid 4He." Canadian Journal of Physics 65, no. 11 (November 1, 1987): 1322–27. http://dx.doi.org/10.1139/p87-208.
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A dilute solution of 3He in superfluid 4He usually behaves as a single-component classical fluid in the context of thermal convection. However, certain convective states can be excited that do not seem to exist in classical convection. These states are characterized by noisy temperature fluctuations and a pronounced decrease in heat transport relative to the classical convecting states. Critical convective-flow fields are observed analogous to critical velocities for superfluid turbulence in pipes. The magnitudes of the average critical velocities for these two types of superfluid turbulence are in good agreement. Also, a quantitative estimate of energy dissipation due to the interaction of normal fluid and quantized vortex lines is consistent with the large decrease in heat transport for the turbulent states. These states are identified as states of convectively driven superfluid turbulence.
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Qin, Lang, Ang Zhang, Jinglian Du, Zhihua Dong, Feng Liu, and Bin Jiang. "Effect of Forced Convection on Magnesium Dendrite: Comparison between Constant and Altering Flow Fields." Materials 16, no. 24 (December 18, 2023): 7695. http://dx.doi.org/10.3390/ma16247695.
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Convection has a nonnegligible effect on the growth of the magnesium dendrite with six-primary-branch pattern. Most work, however, investigates the effect of the convection by simplifying the melt flow as a constant horizontal flow. In this work, four convection behaviors, including equally distributed convection, linearly distributed convection, sinusoidal-wave convection, and square-wave convection, are imposed and simulated through the phase-field lattice-Boltzmann schemes. The effects of constant (the former two) and altering (the latter two) flow fields are quantified by the length ratio of the upstream primary arm to the downstream one. The results show that the dendrite asymmetry increases under the constant forced convections but presents nonmonotonic change under the altering convections. A simple mathematical relation is fitted to summarize the dependence of the dendrite asymmetry on the input velocity, the undercooling, and the flow frequency. Deep understanding of the convection effects can guide the prediction and control of the magnesium dendrite under more complex situations.
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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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Hohenegger, C., and C. S. Bretherton. "Simulating deep convection with a shallow convection scheme." Atmospheric Chemistry and Physics Discussions 11, no. 3 (March 11, 2011): 8385–430. http://dx.doi.org/10.5194/acpd-11-8385-2011.
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Abstract. Convective processes profoundly affect the global water and energy balance of our planet but remain a challenge for global climate modeling. Here we develop and investigate the suitability of a unified convection scheme, capable of handling both shallow and deep convection, to simulate cases of tropical oceanic convection, mid-latitude continental convection, and maritime shallow convection. To that aim, we employ large-eddy simulations (LES) as a benchmark to test and refine a unified convection scheme implemented in the Single-Column Community Atmosphere Model (SCAM). Our approach is motivated by previous cloud-resolving modeling studies, which have documented the gradual transition between shallow and deep convection and its possible importance for the simulated precipitation diurnal cycle. Analysis of the LES reveals that differences between shallow and deep convection, regarding cloud-base properties as well as entrainment/detrainment rates, can be related to the evaporation of precipitation. Parameterizing such effects and accordingly modifying the University of Washington shallow convection scheme, it is found that the new unified scheme can represent both shallow and deep convection as well as tropical and continental convection. Compared to the default SCAM version, the new scheme especially improves relative humidity, cloud cover and mass flux profiles. The new unified scheme also removes the well-known too early onset and peak of convective precipitation over mid-latitude continental areas.
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Hohenegger, C., and C. S. Bretherton. "Simulating deep convection with a shallow convection scheme." Atmospheric Chemistry and Physics 11, no. 20 (October 19, 2011): 10389–406. http://dx.doi.org/10.5194/acp-11-10389-2011.
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Abstract. Convective processes profoundly affect the global water and energy balance of our planet but remain a challenge for global climate modeling. Here we develop and investigate the suitability of a unified convection scheme, capable of handling both shallow and deep convection, to simulate cases of tropical oceanic convection, mid-latitude continental convection, and maritime shallow convection. To that aim, we employ large-eddy simulations (LES) as a benchmark to test and refine a unified convection scheme implemented in the Single-column Community Atmosphere Model (SCAM). Our approach is motivated by previous cloud-resolving modeling studies, which have documented the gradual transition between shallow and deep convection and its possible importance for the simulated precipitation diurnal cycle. Analysis of the LES reveals that differences between shallow and deep convection, regarding cloud-base properties as well as entrainment/detrainment rates, can be related to the evaporation of precipitation. Parameterizing such effects and accordingly modifying the University of Washington shallow convection scheme, it is found that the new unified scheme can represent both shallow and deep convection as well as tropical and mid-latitude continental convection. Compared to the default SCAM version, the new scheme especially improves relative humidity, cloud cover and mass flux profiles. The new unified scheme also removes the well-known too early onset and peak of convective precipitation over mid-latitude continental areas.
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Callies, Jörn, and Raffaele Ferrari. "Baroclinic Instability in the Presence of Convection." Journal of Physical Oceanography 48, no. 1 (January 2018): 45–60. http://dx.doi.org/10.1175/jpo-d-17-0028.1.
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AbstractBaroclinic mixed-layer instabilities have recently been recognized as an important source of submesoscale energy in deep winter mixed layers. While the focus has so far been on the balanced dynamics of these instabilities, they occur in and depend on an environment shaped by atmospherically forced small-scale turbulence. In this study, idealized numerical simulations are presented that allow the development of both baroclinic instability and convective small-scale turbulence, with simple control over the relative strength. If the convection is only weakly forced, baroclinic instability restratifies the layer and shuts off convection, as expected. With increased forcing, however, it is found that baroclinic instabilities are remarkably resilient to the presence of convection. Even if the instability is too weak to restratify the layer and shut off convection, the instability still grows in the convecting environment and generates baroclinic eddies and fronts. This suggests that despite the vigorous atmospherically forced small-scale turbulence in winter mixed layers, baroclinic instabilities can persistently grow, generate balanced submesoscale turbulence, and modify the bulk properties of the upper ocean.
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Stellingwerf, R. F. "The Effects of Convection in RR Lyrae Stars." International Astronomical Union Colloquium 82 (1985): 280–83. http://dx.doi.org/10.1017/s0252921100109522.
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AbstractThe effect of convection in RR Lyrae stars has been investigated using nonlinear models that include the effects of time dependence, turbulent pressure, convective overshooting, and convection-pulsation interaction. The convective structure reduces to that of mixing-length theory in the limit of no time dependence and thick convective zones. Initial tests suggested that convection has an important effect on the stability and pulsation mode of these stars. We now have investigated the nonlinear nature of convection at limiting amplitude. Convection serves as a limiting amplitude mechanism for these stars by causing turbulence near the phase of minimum radius that damps the driving. A comparison with Geneva observations of RR Lyrae shows good agreement in the phase dependence and amplitude of the turbulent motions.
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Groot, Edward, Patrick Kuntze, Annette Miltenberger, and Holger Tost. "Divergent convective outflow in ICON deep-convection-permitting and parameterised deep convection simulations." Weather and Climate Dynamics 5, no. 2 (May 31, 2024): 779–803. http://dx.doi.org/10.5194/wcd-5-779-2024.
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Abstract. Upper-tropospheric deep convective outflows during an event on 10–11 June 2019 over central Europe are analysed in ensembles of the operational Icosahedral Nonhydrostatic (ICON) numerical weather prediction model. Both a parameterised and an explicit representation of deep convective systems is studied. Near-linear response of deep convective outflow strength to net latent heating is found for parameterised convection, while different but physically coherent patterns of outflow variability are found in convection-permitting simulations at 1 km horizontal grid spacing. We investigate if the conceptual model for outflow strength proposed in our previous idealised large-eddy simulation (LES) study is able to explain the variation in outflow strength in a real-case scenario. Convective organisation and aggregation induce a non-linear increase in the magnitude of deep convective outflows with increasing net latent heating in convection-permitting simulations, consistent with the conceptual model. However, in contrast to expectations from the conceptual model, a dependence of the outflow strength on the dimensionality of convective overturning (two-dimensional versus three-dimensional) cannot be fully corroborated from the real-case simulations. Our results strongly suggest that the interactions between gravity waves emitted by heating in individual deep convective elements within larger organised convective systems are of prime importance for the representation of divergent outflow strength from organised convection in numerical models.
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Deng, Liping, and Xiaoqing Wu. "Effects of Convective Processes on GCM Simulations of the Madden–Julian Oscillation." Journal of Climate 23, no. 2 (January 15, 2010): 352–77. http://dx.doi.org/10.1175/2009jcli3114.1.
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Abstract Weak temporal variability in tropical climates such as the Madden–Julian oscillation (MJO) is one of major deficiencies in general circulation models (GCMs). The uncertainties in the representation of convection and cloud processes are responsible for these deficiencies. With the improvement made to the convection scheme, the Iowa State University (ISU) GCM, which is based on a version of the NCAR Community Climate Model, is able to simulate many features of MJO as revealed by observations. In this study, four 10-yr (1979–88) ISU GCM simulations with observed sea surface temperatures are analyzed and compared to examine the effects of the revised convection closure, convection trigger condition, and convective momentum transport (CMT) on the MJO simulations. The modifications made in the convection scheme improve the simulations of amplitude, spatial distribution, eastward propagation, and horizontal and vertical structures, especially for the coherent feature of eastward-propagating convection and the precursor sign of convective center. The revised convection closure plays a key role in the improvement of the eastward propagation of MJO. The convection trigger helps produce less frequent but more vigorous moist convection and enhance the amplitude of the MJO signal. The inclusion of CMT results in a more coherent structure for the MJO deep convective center and its corresponding atmospheric variances.
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Castro-Tapia, Matias, Andrew Cumming, and J. R. Fuentes. "Fast and Slow Crystallization-driven Convection in White Dwarfs." Astrophysical Journal 969, no. 1 (June 21, 2024): 10. http://dx.doi.org/10.3847/1538-4357/ad4152.
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Abstract We investigate crystallization-driven convection in carbon–oxygen white dwarfs. We present a version of the mixing length theory that self-consistently includes the effects of thermal diffusion and composition gradients, and provides solutions for the convective parameters based on the local heat and composition fluxes. Our formulation smoothly transitions between the regimes of fast adiabatic convection at large Peclet number and slow thermohaline convection at low Peclet number. It also allows for both thermally driven and compositionally driven convection, including correctly accounting for the direction of heat transport for compositionally driven convection in a thermally stable background. We use the MESA stellar evolution code to calculate the composition and heat fluxes during crystallization in different models of cooling white dwarfs, and determine the regime of convection and the convective velocity. We find that convection occurs in the regime of slow thermohaline convection during most of the cooling history of the star. However, at the onset of crystallization, the composition flux is large enough to drive fast overturning convection for a short time (∼10 Myr). We estimate the convective velocities in both of these phases and discuss the implications for explaining observed white dwarf magnetic fields with crystallization-driven dynamos.
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Han, Jongil, Weiguo Wang, Young C. Kwon, Song-You Hong, Vijay Tallapragada, and Fanglin Yang. "Updates in the NCEP GFS Cumulus Convection Schemes with Scale and Aerosol Awareness." Weather and Forecasting 32, no. 5 (October 1, 2017): 2005–17. http://dx.doi.org/10.1175/waf-d-17-0046.1.
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Abstract The current operational NCEP Global Forecast System (GFS) cumulus convection schemes are updated with a scale-aware parameterization where the cloud mass flux decreases with increasing grid resolution. The ratio of advective time to convective turnover time is also taken into account for the scale-aware parameterization. In addition, the present deep cumulus convection closure using the quasi-equilibrium assumption is no longer used for grid sizes smaller than a threshold value. For the shallow cumulus convection scheme, the cloud-base mass flux is modified to be given by a function of mean updraft velocity. A simple aerosol-aware parameterization where rain conversion in the convective updraft is modified by aerosol number concentration is also included in the update. Along with the scale- and aerosol-aware parameterizations, more changes are made to the schemes. The cloud-base mass-flux computation in the deep convection scheme is modified to use convective turnover time as the convective adjustment time scale. The rain conversion rate is modified to decrease with decreasing air temperature above the freezing level. Convective inhibition in the subcloud layer is used as an additional trigger condition. Convective cloudiness is enhanced by considering suspended cloud condensate in the updraft. The lateral entrainment in the deep convection scheme is also enhanced to more strongly suppress convection in a drier environment. The updated NCEP GFS cumulus convection schemes display significant improvements especially in the summertime continental U.S. precipitation forecasts.
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Grandpeix, Jean-Yves, and Jean-Philippe Lafore. "A Density Current Parameterization Coupled with Emanuel’s Convection Scheme. Part I: The Models." Journal of the Atmospheric Sciences 67, no. 4 (April 1, 2010): 881–97. http://dx.doi.org/10.1175/2009jas3044.1.
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Abstract The aim of the present series of papers is to develop a density current parameterization for global circulation models. This first paper is devoted to the presentation of this new wake parameterization coupled with Emanuel’s convective scheme. The model represents a population of identical circular cold pools (the wakes) with vertical frontiers. The wakes are cooled by the precipitating downdrafts while the outside area is warmed by the subsidence induced by the saturated drafts. The budget equations for mass, energy, and water yield evolution equations for the prognostic variables (the vertical profiles of the temperature and humidity differences between the wakes and their exterior). They also provide additional terms for the equations of the mean variables. The driving terms of the wake equations are the differential heating and drying due to convective drafts. The action of the convection on the wakes is implemented by splitting the convective tendency and attributing the effect of the precipitating downdrafts to the wake region and the effect of the saturated drafts to their exterior. Conversely, the action of the wakes on convection is implemented by introducing two new variables representing the convergence at the leading edge of the wakes. The available lifting energy (ALE) determines the triggers of deep convection: convection occurs when ALE exceeds the convective inhibition. The available lifting power (ALP) determines the intensity of convection; it is equal to the power input into the system by the collapse of the wakes. The ALE/ALP closure, together with the splitting of the convective heating and drying, implements the full coupling between wake and convection. The coupled wake–convection scheme thus created makes it possible to represent the moist convective processes more realistically, to prepare the coupling of convection with boundary layer and orographic processes, and to consider simulating the propagation of convective systems.
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Brisson, Erwan, Christoph Brendel, Stephan Herzog, and Bodo Ahrens. "Lagrangian evaluation of convective shower characteristics in a convection-permitting model." Meteorologische Zeitschrift 27, no. 1 (January 29, 2018): 59–66. http://dx.doi.org/10.1127/metz/2017/0817.
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Bellenger, H., Y. N. Takayabu, T. Ushiyama, and K. Yoneyama. "Role of Diurnal Warm Layers in the Diurnal Cycle of Convection over the Tropical Indian Ocean during MISMO." Monthly Weather Review 138, no. 6 (June 1, 2010): 2426–33. http://dx.doi.org/10.1175/2010mwr3249.1.
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Abstract The role of air–sea interaction in the diurnal variations of convective activity during the suppressed and developing stages of an intraseasonal convective event is analyzed using in situ observations from the Mirai Indian Ocean cruise for the Study of the Madden–Julian oscillation (MJO)-convection Onset (MISMO) experiment. For the whole period, convection shows a clear average diurnal cycle with a primary maximum in the early morning and a secondary one in the afternoon. Episodes of large diurnal sea surface temperature (SST) variations are observed because of diurnal warm layer (DWL) formation. When no DWL is observed, convection exhibits a diurnal cycle characterized by a maximum in the early morning, whereas when DWL forms, convection increases around noon and peaks in the afternoon. Boundary layer processes are found to control the diurnal evolution of convection. In particular, when DWL forms, the change in surface heat fluxes can explain the decrease of convective inhibition and the intensification of the convection during the early afternoon.
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Heavens, Nicholas G., David M. Kass, James H. Shirley, Sylvain Piqueux, and Bruce A. Cantor. "An Observational Overview of Dusty Deep Convection in Martian Dust Storms." Journal of the Atmospheric Sciences 76, no. 11 (October 16, 2019): 3299–326. http://dx.doi.org/10.1175/jas-d-19-0042.1.
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Abstract Deep convection, as used in meteorology, refers to the rapid ascent of air parcels in Earth’s troposphere driven by the buoyancy generated by phase change in water. Deep convection undergirds some of Earth’s most important and violent weather phenomena and is responsible for many aspects of the observed distribution of energy, momentum, and constituents (particularly water) in Earth’s atmosphere. Deep convection driven by buoyancy generated by the radiative heating of atmospheric dust may be similarly important in the atmosphere of Mars but lacks a systematic description. Here we propose a comprehensive framework for this phenomenon of dusty deep convection (DDC) that is supported by energetic calculations and observations of the vertical dust distribution and exemplary dusty deep convective structures within local, regional, and global dust storm activity. In this framework, DDC is distinct from a spectrum of weaker dusty convective activity because DDC originates from preexisting or concurrently forming mesoscale circulations that generate high surface dust fluxes, oppose large-scale horizontal advective–diffusive processes, and are thus able to maintain higher dust concentrations than typically simulated. DDC takes two distinctive forms. Mesoscale circulations that form near Mars’s highest volcanoes in dust storms of all scales can transport dust to the base of the upper atmosphere in as little as 2 h. In the second distinctive form, mesoscale circulations at low elevations within regional and global dust storm activity generate freely convecting streamers of dust that are sheared into the middle atmosphere over the diurnal cycle.
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Parodi, Antonio, and Kerry Emanuel. "A Theory for Buoyancy and Velocity Scales in Deep Moist Convection." Journal of the Atmospheric Sciences 66, no. 11 (November 1, 2009): 3449–63. http://dx.doi.org/10.1175/2009jas3103.1.
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Abstract Buoyancy and velocity scales for dry convection in statistical equilibrium were derived in the early twentieth century by Prandtl, but the scaling of convective velocity and buoyancy, as well as the fractional area coverage of convective clouds, is still unresolved for moist convection. In this paper, high-resolution simulations of an atmosphere in radiative–convective equilibrium are performed using the Weather Research and Forecasting (WRF) model, a three-dimensional, nonhydrostatic, convection-resolving, limited-area model. The velocity and buoyancy scales for moist convection in statistical equilibrium are characterized by prescribing different constant cooling rates to the system. It is shown that the spatiotemporal properties of deep moist convection and buoyancy and velocity scales at equilibrium depend on the terminal velocity of raindrops and a hypothesis is developed to explain this behavior. This hypothesis is evaluated and discussed in the context of the numerical results provided by the WRF model. The influence of domain size on radiative–convective equilibrium statistics is also assessed. The dependence of finescale spatiotemporal properties of convective structures on numerical and physical details is investigated.
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Schulz, Hauke, and Bjorn Stevens. "Observing the Tropical Atmosphere in Moisture Space." Journal of the Atmospheric Sciences 75, no. 10 (October 2018): 3313–30. http://dx.doi.org/10.1175/jas-d-17-0375.1.
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Measurements from the Barbados Cloud Observatory are analyzed to identify the processes influencing the distribution of moist static energy and the large-scale organization of tropical convection. Five years of water vapor and cloud profiles from a Raman lidar and cloud radar are composed to construct the structure of the observed atmosphere in moisture space. The large-scale structure of the atmosphere is similar to that now familiar from idealized studies of convective self-aggregation, with shallow clouds prevailing over a moist marine layer in regions of low-rank humidity, and deep convection in a nearly saturated atmosphere in regions of high-rank humidity. With supplementary reanalysis datasets the overall circulation pattern is reconstructed in moisture space, and shows evidence of a substantial lower-tropospheric component to the circulation. This shallow component of the circulation helps support the differentiation between the moist and dry columns, similar to what is found in simulations of convective self-aggregation. Radiative calculations show that clear-sky radiative differences can explain a substantial part of this circulation, with further contributions expected from cloud radiative effects. The shallow component appears to be important for maintaining the low gross moist stability of the convecting column. A positive feedback between a shallow circulation driven by differential radiative cooling and the low-level moisture gradients that help support it is hypothesized to play an important role in conditioning the atmosphere for deep convection. The analysis suggests that the radiatively driven shallow circulations identified by modeling studies as contributing to the self-aggregation of convection in radiative–convective equilibrium similarly play a role in shaping the intertropical convergence zone and, hence, the large-scale structure of the tropical atmosphere.
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Hart, Neil C. G., Richard Washington, and Ross I. Maidment. "Deep Convection over Africa: Annual Cycle, ENSO, and Trends in the Hotspots." Journal of Climate 32, no. 24 (December 4, 2019): 8791–811. http://dx.doi.org/10.1175/jcli-d-19-0274.1.
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Abstract Africa is one of the three key regions of deep convection in the global tropics. There is a wealth of information on the intensity, variability, and change of convection and associated rainfall in regions across the continent but almost all of this literature is regionally focused and confined to specific seasons. This fragmented approach precludes a continent-wide view of deep convection leaving the following key issues unanswered: When is deep convection the most widespread across Africa? Where on the continent is deep convection most active? Where does widespread convection have the most interannual variability? This paper confronts these questions using a satellite-derived integral of deep convection. At the continental scale, March exhibits the most extensive deep convection whereas the West African monsoon during June–July exhibits the least. El Niño generally suppresses pan-African convective activity while La Niña enhances this activity. These pan-African signals are largely determined by regional hotspots: the eastern Congo hosts the most persistent widespread deep convection, southeastern southern Africa displays the highest interannual variability, and regional highlands maintain local convective activity hotspots. Furthermore, pan-African annual mean convective activity has increased ~10% between 1983 and 2015 with increases of >20% recorded in local hotspots. Results in this study provide a climatological baseline for both observational and model-based studies of African climates and offer insights into when African convection has the greatest potential impact on the general circulation.
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WITHAM, FRED, and JEREMY C. PHILLIPS. "The dynamics and mixing of turbulent plumes in a turbulently convecting environment." Journal of Fluid Mechanics 602 (April 25, 2008): 39–61. http://dx.doi.org/10.1017/s0022112008000682.
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The turbulent motion of buoyant plumes released into turbulently convecting environments is studied. By assuming that the turbulent environment removes fluid from the plume at a rate proportional to a characteristic environmental velocity scale, we derive a model describing the fluid behaviour. For the example of pure buoyancy plumes, entrainment dominates near the source and the plume radius increases with distance, while further from the source removal, or extrainment, of plume material dominates, and the plume radius decreases to zero. Theoretical predictions are consistent with laboratory experiments, a major feature of which is the natural variability of the convection. We extend the study to include the evolution of a finite confined environment, the end-member regimes of which are a well-mixed environment at all times (high convective velocities), and a ‘filling-box’ model similar to that of Baines & Turner (1969) (low convective velocities). These regimes, and the motion of the interface in a ‘filling-box’ experiment, match experimental observations. We find that the convecting filling box is not stable indefinitely, but that the density stratification will eventually be overcome by thermal convection. The model presented here has important applications in volcanology, ventilation studies and environmental science.
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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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Anders, Evan H., Adam S. Jermyn, Daniel Lecoanet, Adrian E. Fraser, Imogen G. Cresswell, Meridith Joyce, and J. R. Fuentes. "Schwarzschild and Ledoux are Equivalent on Evolutionary Timescales." Astrophysical Journal Letters 928, no. 1 (March 1, 2022): L10. http://dx.doi.org/10.3847/2041-8213/ac5cb5.
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Abstract Stellar evolution models calculate convective boundaries using either the Schwarzschild or Ledoux criterion, but confusion remains regarding which criterion to use. Here we present a 3D hydrodynamical simulation of a convection zone and adjacent radiative zone, including both thermal and compositional buoyancy forces. As expected, regions that are unstable according to the Ledoux criterion are convective. Initially, the radiative zone adjacent to the convection zone is Schwarzschild unstable but Ledoux stable due to a composition gradient. Over many convective overturn timescales, the convection zone grows via entrainment. The convection zone saturates at the size originally predicted by the Schwarzschild criterion, although in this final state the Schwarzschild and Ledoux criteria agree. Therefore, the Schwarzschild criterion should be used to determine the size of stellar convection zones, except possibly during short-lived evolutionary stages in which entrainment persists.
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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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Pasetto, Stefano, Cesare Chiosi, Mark Cropper, and Eva K. Grebel. "Scale-free convection theory." Proceedings of the International Astronomical Union 11, A29B (August 2015): 747. http://dx.doi.org/10.1017/s1743921316006700.
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AbstractConvection is one of the fundamental mechanisms to transport energy, e.g., in planetology, oceanography, as well as in astrophysics where stellar structure is customarily described by the mixing-length theory, which makes use of the mixing-length scale parameter to express the convective flux, velocity, and temperature gradients of the convective elements and stellar medium. The mixing-length scale is taken to be proportional to the local pressure scale height of the star, and the proportionality factor (the mixing-length parameter) must be determined by comparing the stellar models to some calibrator, usually the Sun. No strong arguments exist to claim that the mixing-length parameter is the same in all stars and all evolutionary phases. Because of this, all stellar models in the literature are hampered by this basic uncertainty. In a recent paper (Pasetto et al. 2014) we presented the first fully analytical scale-free theory of convection that does not require the mixing-length parameter. Our self-consistent analytical formulation of convection determines all the properties of convection as a function of the physical behaviour of the convective elements themselves and the surrounding medium (be it a star, an ocean, or a primordial planet). The new theory of convection is formulated starting from a conventional solution of the Navier-Stokes/Euler equations, i.e. the Bernoulli equation for a perfect fluid, but expressed in a non-inertial reference frame co-moving with the convective elements. In our formalism, the motion of convective cells inside convective-unstable layers is fully determined by a new system of equations for convection in a non-local and time dependent formalism. We obtained an analytical, non-local, time-dependent solution for the convective energy transport that does not depend on any free parameter. The predictions of the new theory in astrophysical environment are compared with those from the standard mixing-length paradigm in stars with exceptional results for atmosphere models of the Sun and all the stars in the Hertzsprung-Russell diagram.
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Masunaga, Hirohiko. "A Satellite Study of the Atmospheric Forcing and Response to Moist Convection over Tropical and Subtropical Oceans." Journal of the Atmospheric Sciences 69, no. 1 (January 1, 2012): 150–67. http://dx.doi.org/10.1175/jas-d-11-016.1.
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Abstract Satellite data are analyzed to explore the thermodynamic evolution of tropical and subtropical atmospheres prior and subsequent to moist convection in order to offer an observational test bed for convective adjustment, which is central to the quasi-equilibrium hypothesis. Tropical Rainfall Measuring Mission (TRMM) and Aqua satellite measurements are projected onto a composite temporal sequence over an hourly to daily time scale by exploiting the temporal gap between the local satellite overpasses, which changes from one day to another. The atmospheric forcing and response to convection are investigated separately for deep convective and congestus clouds. In the deep tropics, systematic moisture transport from the atmospheric boundary layer (ABL) to the free troposphere is evident in association with deep convection. The quick ABL ventilation suggests a swift convective adjustment but is preceded by a steady buildup of ABL moisture, which does not imply continuous adjustment to equilibrium. The evolution of convective available potential energy (CAPE) is controlled not only by the ABL moisture but also largely by a coincident ABL cooling linked with a bipolar anomaly of tropospheric temperature. The ABL moisture and temperature effects together lead to a rapid drop of CAPE for 12 h preceding convection, followed by a restoring phase that emerges as the cool anomaly recovers for a day or two. When moist convection is brought by congestus clouds with no deep convection nearby, CAPE gently increases over a period of 1–2 days until congestus occurs and then declines as slowly, suggestive of no efficient convective adjustment. The subtropical atmosphere shows no sign of convective adjustment whether or not vigorous convection is present.
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Tomassini, Lorenzo. "The Interaction between Moist Convection and the Atmospheric Circulation in the Tropics." Bulletin of the American Meteorological Society 101, no. 8 (August 1, 2020): E1378—E1396. http://dx.doi.org/10.1175/bams-d-19-0180.1.
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Abstract Theories of the interaction between moist convection and the atmospheric circulation in the tropics are reviewed. Two main schools of thought are highlighted: (i) one that emphasizes the lower-level control of convection through moisture convergence and variations in convective inhibition, and (ii) one that sees convection as an adjustment process in reaction to larger-scale instabilities, referred to as convective quasi-equilibrium theory. Conceptually the two views consider moist convection to have fundamentally different roles in the tropical circulation. In one case the presence of low-level inhibition and the conditional nature of the atmospheric instability allows for convective vertical motion and latent heating to drive and reinforce synoptic-scale disturbances and overturning circulations; in the other case, because low-level inhibition is not acknowledged to be a widespread controlling barrier, convection is believed to balance and dampen vertical instabilities at the rate they are created by larger-scale processes over the vertical extent of the atmosphere. More recently, investigations of the moisture dynamics surrounding organized convective structures have led to an emerging consensus on the theory of convection–circulation coupling in the tropics that acknowledges the important role of lower- to midtropospheric moisture variations, and the significance of moist convection and convective clouds for initiating and establishing circulations. However, the implementation of these new insights in numerical models lags behind. This is exemplified by the apparent inadequacy of climate models to correctly represent decadal variability in the tropical Pacific, a fact that potentially has implications for the confidence in climate change projections based on such models.
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Birch, Cathryn E., Malcolm J. Roberts, Luis Garcia-Carreras, Duncan Ackerley, Michael J. Reeder, Adrian P. Lock, and Reinhard Schiemann. "Sea-Breeze Dynamics and Convection Initiation: The Influence of Convective Parameterization in Weather and Climate Model Biases." Journal of Climate 28, no. 20 (October 13, 2015): 8093–108. http://dx.doi.org/10.1175/jcli-d-14-00850.1.
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Abstract There are some long-established biases in atmospheric models that originate from the representation of tropical convection. Previously, it has been difficult to separate cause and effect because errors are often the result of a number of interacting biases. Recently, researchers have gained the ability to run multiyear global climate model simulations with grid spacings small enough to switch the convective parameterization off, which permits the convection to develop explicitly. There are clear improvements to the initiation of convective storms and the diurnal cycle of rainfall in the convection-permitting simulations, which enables a new process-study approach to model bias identification. In this study, multiyear global atmosphere-only climate simulations with and without convective parameterization are undertaken with the Met Office Unified Model and are analyzed over the Maritime Continent region, where convergence from sea-breeze circulations is key for convection initiation. The analysis shows that, although the simulation with parameterized convection is able to reproduce the key rain-forming sea-breeze circulation, the parameterization is not able to respond realistically to the circulation. A feedback of errors also occurs: the convective parameterization causes rain to fall in the early morning, which cools and wets the boundary layer, reducing the land–sea temperature contrast and weakening the sea breeze. This is, however, an effect of the convective bias, rather than a cause of it. Improvements to how and when convection schemes trigger convection will improve both the timing and location of tropical rainfall and representation of sea-breeze circulations.
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Zehnder, Joseph A., Liyan Zhang, Dianne Hansford, Anshuman Radzan, Nancy Selover, and Constance M. Brown. "Using Digital Cloud Photogrammetry to Characterize the Onset and Transition from Shallow to Deep Convection over Orography." Monthly Weather Review 134, no. 9 (September 1, 2006): 2527–46. http://dx.doi.org/10.1175/mwr3194.1.
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Abstract An automated method for segmenting digital images of orographic cumulus and a simple metric for characterizing the transition from shallow to deep convection are presented. The analysis is motivated by the hypothesis that shallow convection conditions the atmosphere for further deep convection by moistening it and preventing the evaporation of convective turrets through the entrainment of dry air. Time series of convective development are compared with sounding and surface data for 6 days during the summer of 2003. The observations suggest the existence of a threshold for the initiation of shallow convection based on the surface equivalent potential temperature and the saturated equivalent potential temperature above the cloud base. This criterion is similar to that controlling deep convection over the tropical oceans. The subsequent evolution of the convection depends on details of the environment. Surface fluxes of sensible and latent heat, along with the transport of boundary layer air by upslope flow, increase the surface equivalent potential temperature and once the threshold value is exceeded, shallow convection begins. The duration of the shallow convection period and growth rate of the deep convection are determined by the kinematic and thermodynamic structure of the mid- and upper troposphere.
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Zhang, Tong, Shanshan Geng, Xin Mu, Jiamin Chen, Junyi Wang, and Zan Wu. "Thermal Characteristics of a Stratospheric Airship with Natural Convection and External Forced Convection." International Journal of Aerospace Engineering 2019 (September 8, 2019): 1–11. http://dx.doi.org/10.1155/2019/4368046.
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Though convective heat transfer is one of the main factors that dominate the thermal characteristics of stratospheric airships, there is no specific correlation equations for the calculation of convective heat transfer of airships. The equations based on flat plate and sphere models are all in use. To ameliorate the confusing situation of diverse convective heat transfer equations and to end the misuse of them in the thermal characteristic analysis of stratospheric airships, a multinode steady-state model for ellipsoid airships is built. The accuracy of the five widely accepted equations for natural convective heat transfer is compared and analysed on the proposed large-scale airship model by numerical simulation, so does that of the five equations for external forced convective heat transfer. The simulation method is verified by the available experimental data. Simulation results show that the difference of the five natural convection equations is negligible, while that of the five external forced convection equations must be considered in engineering. Forced convection equations with high precision and wide application should be further investigated.
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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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Tulich, Stefan N., and Brian E. Mapes. "Transient Environmental Sensitivities of Explicitly Simulated Tropical Convection." Journal of the Atmospheric Sciences 67, no. 4 (April 1, 2010): 923–40. http://dx.doi.org/10.1175/2009jas3277.1.
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Abstract A three-dimensional cloud-resolving model, maintained in a statistically steady convecting state by tropics-like forcing, is subjected to sudden (10 min) stimuli consisting of horizontally homogeneous temperature and/or moisture sources with various profiles. Ensembles of simulations are used to increase the statistical robustness of the results and to assess the deterministic nature of the model response for domain sizes near contemporary global model resolution. The response to middle- and upper-tropospheric perturbations is predominantly local in the vertical: convection damps the imposed stimulus over a few hours. Low-level perturbations are similarly damped, but also produce a vertically nonlocal response: enhancement or suppression of new deep convective clouds extending above the perturbed level. Experiments show that the “effective inhibition layer” for deep convection is about 4 km deep, far deeper than traditional convective inhibition defined for undilute lifted parcels. Both the local and nonlocal responses are remarkably linear but can be highly stochastic, especially if deep convection is only intermittently present (small domains, weak forcing). Quantitatively, temperature-versus-moisture perturbations in a ratio corresponding to adiabatic vertical displacements produce responses of roughly equal magnitude. However, moisture perturbations seem to provoke the nonlocal (upward spreading) type of response more effectively. This nonlocal part of the response is also more effective when background forcing intensity is weak. Only at very high intensity does the response approach the limits of purely local damping and pure determinism that would be most convenient for theory and parameterization.
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Torn, Ryan D., Glen S. Romine, and Thomas J. Galarneau. "Sensitivity of Dryline Convection Forecasts to Upstream Forecast Errors for Two Weakly Forced MPEX Cases." Monthly Weather Review 145, no. 5 (April 19, 2017): 1831–52. http://dx.doi.org/10.1175/mwr-d-16-0457.1.
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Abstract The sensitivity of convective forecasts along the Texas dryline to upstream forecast fields at earlier lead times is evaluated for two consecutive days (27–28 May) characterized by no clear synoptic forcing for convection initiation (CI) during the 2013 Mesoscale Predictability Experiment (MPEX) by applying the ensemble-based sensitivity technique to convection-allowing WRF ensemble forecasts. For both cases, the members with stronger convection are characterized by higher water vapor just above the top of the boundary layer, which is associated with lower convective inhibition (CIN) at the time of CI. Forecast convection is sensitive to the lower-tropospheric water vapor and zonal wind at earlier lead times farther south along the dryline, such that increasing the water vapor and/or making the wind more easterly is associated with more convection. For 28 May, the water vapor along the dryline is also sensitive to the convection that occurs around 0600 UTC, which leads to cold pool–induced surface divergence that subsequently shifts the dryline north or south. Ensemble members that correctly have decreased convection in the Texas Panhandle on 28 May have more accurate forecasts of water vapor and meridional wind with respect to dropwindsondes in the sensitive region 9 h prior to CI compared to members with more extensive convection. Reducing the 0-h water vapor within the sensitive region can suppress convection in members with extensive convection; however, increasing the 0-h water vapor does not lead to more convection in members without convection.
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Čorný, Ivan. "Aspects of Determination of Convective Coefficients." Advances in Thermal Processes and Energy Transformation 1, no. 2 (2018): 31–33. http://dx.doi.org/10.54570/atpet2018/01/02/0031.
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The paper deals with selected aspects in the determination of convective coefficients and the formation of air convection models on solid surfaces. The goal is to assess the suitability of individual approaches for application in specific situations, considering that convective heat transfer coefficients have a significant impact on the resulting accuracy of the facilities/process design. Convection is a dynamic phenomenon, its exact physical description is complex, convection coefficients cannot be measured directly. Correct determination of convective coefficients is therefore very important in various areas where thermal energy is transmitted in this way. The paper gives an overview of selected aspects of various methods for determining convective heat transfer parameters.
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Frierson, Dargan M. W. "The Dynamics of Idealized Convection Schemes and Their Effect on the Zonally Averaged Tropical Circulation." Journal of the Atmospheric Sciences 64, no. 6 (June 2007): 1959–76. http://dx.doi.org/10.1175/jas3935.1.
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In this paper, the effect of a simple convection scheme on the zonally averaged tropical general circulation is examined within an idealized moist GCM to obtain broad classifications of the influence of convection on the Tropics. This is accomplished with a simplified convection scheme in the style of Betts and Miller. The scheme is utilized in a moist GCM with simplified physical parameterizations (gray radiation, with zonally symmetric, slab mixed layer ocean boundary conditions). Comparisons are made with simulations without a convection scheme [i.e., with large-scale condensation (LSC) only], with the moist convective adjustment (MCA) parameterization, and with various formulations and parameter sets with a simplified Betts–Miller (SBM) scheme. With the control run using the SBM scheme, the Tropics become quieter and less dependent on horizontal resolution as compared with the LSC or MCA simulations. The Hadley circulation mass transport is significantly reduced with the SBM scheme, as is the ITCZ precipitation. An important factor determining this behavior is the parameterization of shallow convection: without shallow convection, the convection scheme is largely ineffective at preventing convection from occurring at the grid scale. The sensitivities to convection scheme parameters are also examined. The simulations are remarkably insensitive to the convective relaxation time, and only mildly sensitive to the relative humidity of the reference profile, provided significant large-scale condensation is not allowed to occur. The changes in the zonally averaged tropical circulation that occur in all the simulations are understood based on the convective criteria of the schemes and the gross moist stability of the atmosphere.
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Barber, Katelyn A., and Gretchen L. Mullendore. "The Importance of Convective Stage on Out-of-Cloud Convectively Induced Turbulence from High-Resolution Simulations." Monthly Weather Review 148, no. 11 (November 2020): 4587–605. http://dx.doi.org/10.1175/mwr-d-20-0065.1.
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AbstractTurbulence (clear-air, mountain wave, convectively induced) is an aviation hazard that is a challenge to forecast due to the coarse resolution ultilized in operational weather models. Turbulence indices are commonly used to aid pilots in avoiding turbulence, but these indices have been designed and calibrated for midlatitude clear-air turbulence prediction (e.g., the Ellrod index). A significant limitation with current convectively induced turbulence (CIT) prediction is the lack of storm stage dependency. In this study, six high-resolution simulations of tropical oceanic and midlatitude continental convection are performed to characterize the turbulent environment near various convective types during the developing and mature stages. Second-order structure functions, a diagnostic commonly used to identify turbulence in turbulence prediction systems, are used to characterize the probability of turbulence for various convective types. Turbulence likelihood was found to be independent of region (i.e., tropical vs midlatitude) but dependent on convective stage. The probability of turbulence increased near developing convection for the majority of cases. Additional analysis of static stability and vertical wind shear, indicators of turbulence potential, showed that the convective environment near developing convection was more favorable for turbulence production than mature convection. Near developing convection, static stability decreased and vertical wind shear increased. Vertical wind shear near mature and developing convection was found to be weakly correlated to turbulence intensity in both the tropics and the midlatitudes. This study emphasizes the need for turbulence avoidance guidelines for the aviation community that are dependent on convective stage.
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Guzik, Joyce A., T. H. Morgan, N. J. Nelson, C. Lovekin, K. Kosak, I. N. Kitiashvili, N. N. Mansour, and A. Kosovichev. "2-D and 3-D models of convective turbulence and oscillations in intermediate-mass main-sequence stars." Proceedings of the International Astronomical Union 11, A29B (August 2015): 540–43. http://dx.doi.org/10.1017/s1743921316006086.
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AbstractWe present multidimensional modeling of convection and oscillations in main-sequence stars somewhat more massive than the Sun, using three separate approaches: 1) Using the 3-D planar StellarBox radiation hydrodynamics code to model the envelope convection zone and part of the radiative zone. Our goals are to examine the interaction of stellar pulsations with turbulent convection in the envelope, excitation of acoustic modes, and the role of convective overshooting; 2) Applying the spherical 3-D MHD ASH (Anelastic Spherical Harmonics) code to simulate the core convection and radiative zone. Our goal is to determine whether core convection can excite low-frequency gravity modes, and thereby explain the presence of low frequencies for some hybrid γ Dor/δ Sct variables for which the envelope convection zone is too shallow for the convective blocking mechanism to drive gravity modes; 3) Applying the ROTORC 2-D stellar evolution and dynamics code to calculate evolution with a variety of initial rotation rates and extents of core convective overshooting. The nonradial adiabatic pulsation frequencies of these nonspherical models are calculated using the 2-D pulsation code NRO. We present new insights into pulsations of 1-2 M⊙ stars gained by multidimensional modeling.