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1
Franzke, Christian, Daan Crommelin, Alexander Fischer, and Andrew J. Majda. "A Hidden Markov Model Perspective on Regimes and Metastability in Atmospheric Flows." Journal of Climate 21, no. 8 (April 15, 2008): 1740–57. http://dx.doi.org/10.1175/2007jcli1751.1.
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Abstract In this study, data from three atmospheric models are analyzed to investigate the existence of atmospheric flow regimes despite nearly Gaussian statistics of the planetary waves in these models. A hierarchy of models is used, which describes the atmospheric circulation with increasing complexity. To systematically identify atmospheric regimes, the presence of metastable states in the data is searched for by fitting so-called hidden Markov models (HMMs) to the time series. A hidden Markov model is designed to describe the situation in which part of the information of the system is unknown or hidden and another part is observed. Within the context of this study, some representative variable of planetary-scale flow (e.g., mean zonal flow or leading principal component) is known (“observed”), but its dynamics may depend crucially on the overall flow configuration, which is unknown. The behavior of this latter, “hidden” variable is described by a Markov chain. If the Markov chain possesses metastable (or quasi persistent) states, they are identified as regimes. In this perspective, regimes can be present even though the observed data have a nearly Gaussian probability distribution. The parameters of the HMMs are fit to the time series using a maximum-likelihood approach; well-established and robust numerical methods are available to do this. Possible metastability of the Markov chain is assessed by inspecting the eigenspectrum of the associated transition probability matrix. The HMM procedure is first applied to data from a simplified model of barotropic flow over topography with a large-scale mean flow. This model exhibits regime behavior of its large-scale mean flow for sufficiently high topography. In the case of high topography, the authors find three regimes, two of which correspond to zonal flow and the third to blocking. Next, a three-layer quasigeostrophic model is used as a prototype atmospheric general circulation model (GCM). Its first empirical orthogonal function (EOF) is similar to the Arctic Oscillation (AO) and exhibits metastability. For this model, two regime states are found: one corresponding to the positive phase of the AO with large amplitude and decreased variability of the streamfunction field, and another corresponding to the negative AO phase with small amplitude and increased variability. Finally, the authors investigate a comprehensive GCM. The leading four EOFs of this model show no signs of metastability. The results of the barotropic flow over topography and of the quasigeostrophic model suggest that the observed small skewness of planetary wave probability density functions (PDFs) is an imprint of blocked circulation states.
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Teng, Haiyan, and Grant Branstator. "A Zonal Wavenumber 3 Pattern of Northern Hemisphere Wintertime Planetary Wave Variability at High Latitudes." Journal of Climate 25, no. 19 (April 16, 2012): 6756–69. http://dx.doi.org/10.1175/jcli-d-11-00664.1.
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Abstract A prominent pattern of variability of the Northern Hemisphere wintertime tropospheric planetary waves, referred to here as the Wave3 pattern, is identified from the NCEP–NCAR reanalysis. It is worthy of attention because its structure is similar to the linear trend pattern as well as the leading pattern of multidecadal variability of the planetary waves during the past half century. The Wave3 pattern is defined as the second empirical orthogonal function (EOF) of detrended December–February mean 300-hPa meridional wind V300 and denotes a zonal shift of the ridges and troughs of the climatological flow. Although its interannual variance is roughly comparable to that of EOF1 of V300, which represents the Pacific–North America (PNA) pattern, its multidecadal variance is nearly twice as large as that of the PNA. Wave3 is not completely structurally or temporally distinct from the northern annular mode (NAM) but, for some attributes, the linkage of the observed trend to Wave3 is clearer than to NAM. The prominence of the Wave3 pattern is further supported by attributes of many climate models that participated in phase 3 of the Coupled Model Intercomparison Project (CMIP3). In particular, in the Community Climate System Model, version 3 (CCSM3), the Wave3 pattern is present as EOF3 of V300 in both a fully coupled integration and a stand-alone atmospheric integration forced by climatological sea surface temperatures. Its existence in the latter experiment indicates that the pattern can be produced by atmospheric processes alone.
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Suwa, Yudai. "On the importance of the equation of state for the neutrino-driven supernova explosion mechanism." Proceedings of the International Astronomical Union 7, S279 (April 2011): 397–98. http://dx.doi.org/10.1017/s174392131201352x.
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AbstractWe present two-dimensional numerical simulations of core-collapse supernova including multi-energy neutrino radiative transfer. We aim to examine the influence of the equation of state (EOS) for the dense nuclear matter. We employ four sets of EOSs, namely, those by Lattimer and Swesty (LS) and Shen et al., which became standard EOSs in the core-collapse supernova community. We reconfirm that not every EOS produces an explosion in spherical symmetry, which is consistent with previous works. In two-dimensional simulations, we find that the structure of the accretion flow is significantly different between LS EOS and Shen EOS, inducing an even qualitatively different evolution of the shock wave, namely, the LS EOS leads to shock propagation beyond 2000 km from the center, while the Shen EOS shows only oscillations within 500 km. The possible origins of the difference are discussed.
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Dubos, Thomas, Philippe Drobinski, and Pierre Carlotti. "Turbulence Anisotropy Carried by Streaks in the Neutral Atmospheric Surface Layer." Journal of the Atmospheric Sciences 65, no. 8 (August 1, 2008): 2631–45. http://dx.doi.org/10.1175/2007jas2333.1.
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Abstract The authors investigate the relationships between coherent structures and turbulence anisotropy in the neutral planetary boundary layer by means of empirical orthogonal function (EOF) analysis of large-eddy simulation (LES) data. The simulated flow contains near-surface transient streaks. The EOF analysis extracts the most energetic patterns from the velocity fluctuations based on their second-order spatial correlations. The scale and direction of streaks obtained from a level-by-level analysis of the LES flow field do correspond to that of the EOFs. It is found that two characteristics of the turbulence anisotropy depend on whether or not the velocity fluctuations with a given horizontal wave vector present distinct patterns: (i) the vertical extent up to which the turbulent kinetic energy (TKE) is concentrated and (ii) the ratio of the vertical TKE EV to the horizontal TKE EH. Although still present in the complete signal, this anisotropy is strongly emphasized when the signal is projected onto the EOF structures. Hence the coherent structures do indeed carry more anisotropy than the remaining turbulent fluctuations. Furthermore, at horizontal wave vectors where energetic patterns are dominant, the ratio EV/EH takes values close to 0.2, representative of the ratio EV/EH based on the total LES flow and on in situ measurements.
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Hu, Feng, Leying Zhang, Qiao Liu, and Dorina Chyi. "Environmental Factors Controlling the Precipitation in California." Atmosphere 12, no. 8 (August 2, 2021): 997. http://dx.doi.org/10.3390/atmos12080997.
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Using observational data covering 1948–2020, the environmental factors controlling the winter precipitation in California were investigated. Empirical orthogonal function (EOF) analysis was applied to identify the dominant climate regimes contributing to the precipitation. The first EOF mode described a consistent change, with 70.1% variance contribution, and the second mode exhibited a south–east dipole change, with 11.7% contribution. For EOF1, the relationship was positive between PC1(principal component) and SST (sea surface temperature) in the central Pacific Ocean, while it was negative with SST in the southeast Indian Ocean. The Pacific–North America mode, induced by the positive SST and precipitation in the central Pacific Ocean, leads to California being occupied by southwesterlies, which would transport warm and wet flow from the ocean, beneficial for precipitation. As for the negative relationship, California is controlled by biotrophically high pressure, representing part of the Rossby wave train induced by the positive SST in the Indian ocean, which is unfavorable for the precipitation. For EOF2, California is controlled by positive vorticity at the upper level, whereas at the lower level, there is positive vorticity to the south and negative vorticity to the north, the combination of which leads to the dipole mode change in the precipitation.
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Rennert, Kevin J., and John M. Wallace. "Cross-Frequency Coupling, Skewness, and Blocking in the Northern Hemisphere Winter Circulation." Journal of Climate 22, no. 21 (November 1, 2009): 5650–66. http://dx.doi.org/10.1175/2009jcli2669.1.
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Abstract Variability in daily wintertime [December–February (DJF)] 500-hPa heights on low [L: <(30 day)−1], intermediate [M: (6–30 day)−1], and high [H: >(6 day)−1] frequencies is examined using 40-yr ECMWF Re-Analysis (ERA-40) data. Leading EOFs of L correspond to planetary-scale teleconnection patterns; those of M to retrograding, eastward-dispersing long waves oriented along great circle routes; and those of H to baroclinic waves in the climatological-mean storm tracks. In the Atlantic sector, EOF 1 of M appears to be embedded in EOF 1 of L. Cross-frequency coupling between L and M exhibits distinctive patterns. In the Atlantic sector the negative polarity of the North Atlantic Oscillation (NAO) with above-normal heights over Greenland is associated with enhanced M variability over Greenland. An analogous relationship is observed in the Pacific sector between an NAO-like pattern and the variance of M over Alaska. Cross-frequency coupling between L and H in both sectors is indicative of a reinforcement of the background flow by the baroclinic waves. Cross-frequency coupling between L and M is responsible for most of the skewness of the anomalies in the 500-hPa height field. Linear wave dynamics evidently play an important role in M. Composites of high amplitude anomalies of contrasting signs over Baffin Bay exhibit similar spatial structures (apart from the sign reversal) and they exhibit a similar evolution, with westward phase propagation and downstream development characteristic of the behavior of Rossby waves. It is argued that teleconnection patterns exhibit memories much longer than the 7–10-day decorrelation time of daily indices formed by projecting unfiltered daily fields onto their spatial patterns.
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Dias, Juliana, and George N. Kiladis. "The Relationship between Equatorial Mixed Rossby–Gravity and Eastward Inertio-Gravity Waves. Part II." Journal of the Atmospheric Sciences 73, no. 5 (May 1, 2016): 2147–63. http://dx.doi.org/10.1175/jas-d-15-0231.1.
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Abstract Space–time spectral analysis of tropical cloudiness data shows strong evidence that convectively coupled n = 0 mixed Rossby–gravity waves (MRGs) and eastward inertio-gravity waves (EIGs) occur primarily within the western/central Pacific Ocean. Spectral filtering also shows that MRG and EIG cloudiness patterns are antisymmetric with respect to the equator, and they propagate coherently toward the west and east, respectively, with periods between 3 and 5 days, in agreement with Matsuno’s linear shallow-water theory. In contrast to the spectral approach, in a companion paper it has been shown that empirical orthogonal functions (EOFs) of 2–6-day-filtered cloudiness data within the tropical Pacific Ocean also suggest an antisymmetric pattern, but with the leading EOFs implying a zonally standing but poleward-propagating oscillation, along with the associated tropospheric flow moving to the west. In the present paper, these two views are reconciled by applying an independent approach based on a tracking method to assess tropical convection organization. It is shown that, on average, two-thirds of MRG and EIG events develop independently of one another, and one-third of the events overlap in space and time. This analysis also verifies that MRG and EIG cloudiness fields tend to propagate meridionally away from the equator. It is demonstrated that the lack of zonal propagation implied from the EOF analysis is likely due to the interference between eastward- and westward-propagating disturbances. In addition, it is shown that the westward-propagating circulation associated with the leading EOF is consistent with the expected theoretical behavior of an interference between MRGs and EIGs.
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Evans, K. F., J. R. Wang, D. O'C Starr, G. Heymsfield, L. Li, L. Tian, R. P. Lawson, A. J. Heymsfield, and A. Bansemer. "Ice hydrometeor profile retrieval algorithm for high-frequency microwave radiometers: application to the CoSSIR instrument during TC4." Atmospheric Measurement Techniques 5, no. 9 (September 25, 2012): 2277–306. http://dx.doi.org/10.5194/amt-5-2277-2012.
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Abstract. A Bayesian algorithm to retrieve profiles of cloud ice water content (IWC), ice particle size (Dme), and relative humidity from millimeter-wave/submillimeter-wave radiometers is presented. The first part of the algorithm prepares an a priori file with cumulative distribution functions (CDFs) and empirical orthogonal functions (EOFs) of profiles of temperature, relative humidity, three ice particle parameters (IWC, Dme, distribution width), and two liquid cloud parameters. The a priori CDFs and EOFs are derived from CloudSat radar reflectivity profiles and associated ECMWF temperature and relative humidity profiles combined with three cloud microphysical probability distributions obtained from in situ cloud probes. The second part of the algorithm uses the CDF/EOF file to perform a Bayesian retrieval with a hybrid technique that uses Monte Carlo integration (MCI) or, when too few MCI cases match the observations, uses optimization to maximize the posterior probability function. The very computationally intensive Markov chain Monte Carlo (MCMC) method also may be chosen as a solution method. The radiative transfer model assumes mixtures of several shapes of randomly oriented ice particles, and here random aggregates of spheres, dendrites, and hexagonal plates are used for tropical convection. A new physical model of stochastic dendritic snowflake aggregation is developed. The retrieval algorithm is applied to data from the Compact Scanning Submillimeter-wave Imaging Radiometer (CoSSIR) flown on the ER-2 aircraft during the Tropical Composition, Cloud and Climate Coupling (TC4) experiment in 2007. Example retrievals with error bars are shown for nadir profiles of IWC, Dme, and relative humidity, and nadir and conical scan swath retrievals of ice water path and average Dme. The ice cloud retrievals are evaluated by retrieving integrated 94 GHz backscattering from CoSSIR for comparison with the Cloud Radar System (CRS) flown on the same aircraft. The rms difference in integrated backscattering is around 3 dB over a 30 dB range. A comparison of CoSSIR retrieved and CRS measured reflectivity shows that CoSSIR has the ability to retrieve low-resolution ice cloud profiles in the upper troposphere.
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Evans, K. F., J. R. Wang, D. O'C Starr, G. Heymsfield, L. Li, L. Tian, R. P. Lawson, A. J. Heymsfield, and A. Bansemer. "Ice hydrometeor profile retrieval algorithm for high frequency microwave radiometers: application to the CoSSIR instrument during TC4." Atmospheric Measurement Techniques Discussions 5, no. 2 (April 27, 2012): 3117–98. http://dx.doi.org/10.5194/amtd-5-3117-2012.
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Abstract. A Bayesian algorithm to retrieve profiles of cloud ice water content (IWC), ice particle size (Dme), and relative humidity from millimeter-wave/submillimeter-wave radiometers is presented. The first part of the algorithm prepares an a priori file with cumulative distribution functions (CDFs) and empirical orthogonal functions (EOFs) of profiles of temperature, relative humidity, three ice particle parameters (IWC, Dme, distribution width), and two liquid cloud parameters. The a priori CDFs and EOFs are derived from CloudSat radar reflectivity profiles and associated ECMWF temperature and relative humidity profiles combined with three cloud microphysical probability distributions obtained from in situ cloud probes. The second part of the algorithm uses the CDF/EOF file to perform a Bayesian retrieval with a hybrid technique that uses Monte Carlo integration (MCI) or, when too few MCI cases match the observations, uses optimization to maximize the posterior probability function. The very computationally intensive Markov chain Monte Carlo (MCMC) method also may be chosen as a solution method. The radiative transfer model assumes mixtures of several shapes of randomly oriented ice particles, and here random aggregates of hexagonal plates, spheres, and dendrites are used for tropical convection. A new physical model of stochastic dendritic snowflake aggregation is developed. The retrieval algorithm is applied to data from the Compact Scanning Submillimeter-wave Imaging Radiometer (CoSSIR) flown on the ER-2 aircraft during the Tropical Composition, Cloud and Climate Coupling (TC4) experiment in 2007. Example retrievals with error bars are shown for nadir profiles of IWC, Dme, and relative humidity, and nadir and conical scan swath retrievals of ice water path and average Dme. The ice cloud retrievals are evaluated by retrieving integrated 94 GHz backscattering from CoSSIR for comparison with the Cloud Radar System (CRS) flown on the same aircraft. The rms difference in integrated backscattering is around 3 dB over a 30 dB range. A comparison of CoSSIR retrieved and CRS measured reflectivity shows that CoSSIR has the ability to retrieve low-resolution ice cloud profiles in the upper troposphere.
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Lorenz, David J. "Understanding Midlatitude Jet Variability and Change Using Rossby Wave Chromatography: Wave–Mean Flow Interaction." Journal of the Atmospheric Sciences 71, no. 10 (September 22, 2014): 3684–705. http://dx.doi.org/10.1175/jas-d-13-0201.1.
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Abstract Rossby wave chromatography (RWC) is implemented in a linearized barotropic model as a tool to understand the interaction between the midlatitude jet and the eddy momentum fluxes (uυ) in an idealized GCM. Given the background zonal-mean flow and the space–time structure of the baroclinic wave activity source, RWC calculates the space–time structure of the upper-tropospheric uυ. RWC allows a clean separation of the effects of phase speed changes and index of refraction changes (i.e., changes in background flow) on uυ. It is found that uυ reinforces imposed zonal-mean zonal wind (u) anomalies that are collocated with the centers of action of the first empirical orthogonal function (EOF1) of the GCM. Critical-level dynamics are essential for the positive feedback when u is equatorward of the mean jet, and “reflecting level” dynamics are essential for the positive feedback when u is poleward of the jet. The eddy momentum flux caused by changes in the phase speeds of the wave sources, on the other hand, are associated with a negative feedback. When the imposed u is out of phase with EOF1, the eddies tend to shift the imposed u poleward (equatorward) for anomalies that are equatorward (poleward) of the poleward center of action of EOF1. Critical (reflecting)-level dynamics is most important for the poleward shift in the subtropics (midlatitudes). Because there are no baroclinic feedbacks in these experiments, these results suggest that barotropic feedbacks alone can account for the structure of the u variability in the midlatitudes.
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Zeiden, Kristin L., Jennifer A. MacKinnon, Matthew H. Alford, Daniel L. Rudnick, Gunnar Voet, and Hemantha Wijesekera. "Broadband Submesoscale Vorticity Generated by Flow around an Island." Journal of Physical Oceanography 51, no. 4 (April 2021): 1301–17. http://dx.doi.org/10.1175/jpo-d-20-0161.1.
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AbstractAn array of moorings deployed off the coast of Palau is used to characterize submesoscale vorticity generated by broadband upper-ocean flows around the island. Palau is a steep-sided archipelago lying in the path of strong zonal geostrophic currents, but tides and inertial oscillations are energetic as well. Vorticity is correspondingly broadband, with both mean and variance O(f) in a surface and subsurface layer (where f is the local Coriolis frequency). However, while subinertial vorticity is linearly related to the incident subinertial current, the relationship between superinertial velocity and superinertial vorticity is weak. Instead, there is a strong nonlinear relationship between subinertial velocity and superinertial vorticity. A key observation of this study is that during periods of strong westward flow, vorticity in the tidal bands increases by an order of magnitude. Empirical orthogonal functions (EOFs) of velocity show this nonstationary, superinertial vorticity variance is due to eddy motion at the scale of the array. Comparison of kinetic energy and vorticity time series suggest that lateral shear against the island varies with the subinertial flow, while tidal currents lead to flow reversals inshore of the recirculating wake and possibly eddy shedding. This is a departure from the idealized analog typically drawn on in island wake studies: a cylinder in a steady flow. In that case, eddy formation occurs at a frequency dependent on the scale of the obstacle and strength of the flow alone. The observed tidal formation frequency likely modulates the strength of submesoscale wake eddies and thus their dynamic relationship to the mesoscale wake downstream of Palau.
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Sandler, Dor, and Nili Harnik. "Future wintertime meridional wind trends through the lens of subseasonal teleconnections." Weather and Climate Dynamics 1, no. 2 (August 24, 2020): 427–43. http://dx.doi.org/10.5194/wcd-1-427-2020.
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Abstract. Large-scale atmospheric circulation is expected to change considerably in the upcoming decades, and with it the interaction between Rossby waves and the jet stream. A common feature of midlatitude wintertime variability is upper-tropospheric quasi-stationary number 5 wave packets, which often propagate zonally along the jet. These are collectively referred to as the circumglobal teleconnection pattern (CTP). Their likeness seemingly emerges as a robust signal in future meridional wind trend projections in the Northern Hemisphere, which take the form of a zonal wave encompassing the midlatitudes. We attempt to elucidate this link across timescales (daily, monthly, and climatological), focusing on wave propagation in the jet waveguide in reanalysis and a 36-member ensemble of CMIP5 models. Using empirical orthogonal function (EOF) analysis on 300 hPa subseasonal V anomalies, we first establish the ensemble's skill in capturing the pattern. Then, by investigating EOF phase space, we characterize the CTP's behavior in present-day climatology and how it is projected to change. Under RCP8.5 forcing, most models develop a gradual preference for monthly-mean waves with certain longitudinal phases. The ensemble is thus divided into subgroups based on region of increased wave activity. For each model, this region corresponds to a more pronounced local trend, which helps explain the ensemble projection spread. Additionally, in two test-case models, this coincides with an increasing number of preferably phased wave packets at the synoptic scale. Some signs suggest that differences in CTP dynamics might stem from mean flow interaction, while no evidence was found for the role of tropical diabatic forcing. Thus, we conclude that this climate change response, seemingly a single large-scale wave, is actually comprised of several regional effects which are related to shifts in CTP phase distributions. The strong dynamical disagreement in the ensemble then manifests as significantly different circulation trends, which in turn might affect projected local temperature and precipitation patterns.
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Nakamura, Mototaka, and Shozo Yamane. "Dominant Anomaly Patterns in the Near-Surface Baroclinicity and Accompanying Anomalies in the Atmosphere and Oceans. Part I: North Atlantic Basin." Journal of Climate 22, no. 4 (February 15, 2009): 880–904. http://dx.doi.org/10.1175/2008jcli2297.1.
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Abstract Variability in the monthly mean flow and storm track in the North Atlantic basin is examined with a focus on the near-surface baroclinicity, B = Bxi + Byj. Dominant patterns of anomalous B found from empirical orthogonal function (EOF) analyses generally show patterns of shift and changes in the strength of B. Composited anomalies in the monthly mean wind at various pressure levels based on the signals in the EOFs display robust accompanying anomalies in the mean flow up to 50 hPa in the winter and up to 100 hPa in other seasons. Anomalous eddy fields accompanying the anomalous Bx patterns exhibit, broadly speaking, structures anticipated from linear theories of baroclinic instabilities and suggest a tendency for anomalous wave fluxes to accelerate/decelerate the surface westerly accordingly. Atmospheric anomalies accompanying By anomalies have patterns different from those that accompany Bx anomalies but are as large as those found for Bx. Anomalies in the sea surface temperature (SST) found for the anomalous patterns of Bx often show large values of small spatial scales along the Gulf Stream (GS), indicating that a meridional shift in the position of the GS and/or changes in the heat transport by the GS may be responsible for the anomalous Bx and concomitant tropospheric and lower-stratospheric anomalies. Anomalies in the net surface heat flux, SST in preceding months, and meridional eddy heat flux in the lower troposphere support this interpretation.
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Jiang, Boyang, and James Kaihatu. "MULTI-DIMENSIONAL ERROR ANALYSIS OF NEARSHORE WAVE MODELING TOOLS, WITH APPLICATION TOWARD DATA-DRIVEN BOUNDARY CORRECTION." Coastal Engineering Proceedings 1, no. 32 (January 30, 2011): 67. http://dx.doi.org/10.9753/icce.v32.waves.67.
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As the forecasting models become more sophisticated in their physics and possible depictions of the nearshore hydrodynamics, they also become increasingly sensitive to errors in the inputs, such as errors in the specification of boundary information (lateral boundary conditions, initial boundary conditions, etc). Evaluation of the errors on the boundary is less straightforward, and is the subject of this study. The model under investigation herein is the Delft3D modeling suite, developed at Deltares (formerly Delft Hydraulics) in Delft, the Netherlands. Coupling of the wave (SWAN) and hydrodynamic (FLOW) model requires care at the lateral boundaries in order to balance run time and error growth. To this extent, we will use perturbation method and spatio-temporal analysis method such as Empirical Orthogonal Function (EOF) analysis to determine the various scales of motion in the flow field and the extent of their response to imposed boundary errors.
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Nakamura, Mototaka, and Shozo Yamane. "Dominant Anomaly Patterns in the Near-Surface Baroclinicity and Accompanying Anomalies in the Atmosphere and Oceans. Part II: North Pacific Basin." Journal of Climate 23, no. 24 (December 15, 2010): 6445–67. http://dx.doi.org/10.1175/2010jcli3017.1.
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Abstract Variability in the monthly-mean flow and storm track in the North Pacific basin is examined with a focus on the near-surface baroclinicity. Dominant patterns of anomalous near-surface baroclinicity found from empirical orthogonal function (EOF) analyses generally show mixed patterns of shift and changes in the strength of near-surface baroclinicity. Composited anomalies in the monthly-mean wind at various pressure levels based on the signals in the EOFs show accompanying anomalies in the mean flow up to 50 hPa in the winter and up to 100 hPa in other seasons. Anomalous eddy fields accompanying the anomalous near-surface baroclinicity patterns exhibit, broadly speaking, structures anticipated from simple linear theories of baroclinic instability, and suggest a tendency for anomalous wave fluxes to accelerate–decelerate the surface westerly accordingly. However, the relationship between anomalous eddy fields and anomalous near-surface baroclinicity in the midwinter is not consistent with the simple linear baroclinic instability theories. Composited anomalous sea surface temperature (SST) accompanying anomalous near-surface baroclinicity often exhibits moderate values and large spatial scales in the basin, rather than large values concentrated near the oceanic fronts. In the midsummer and in some cases in cold months, however, large SST anomalies are found around the Kuroshio–Oyashio Extensions. Accompanying anomalies in the net surface heat flux, SST in the preceding and following months, and meridional eddy heat flux in the lower troposphere suggest active roles played by the ocean in generating the concomitant anomalous large-scale atmospheric state in some of these cases.
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Mounier, Flore, George N. Kiladis, and Serge Janicot. "Analysis of the Dominant Mode of Convectively Coupled Kelvin Waves in the West African Monsoon." Journal of Climate 20, no. 8 (April 15, 2007): 1487–503. http://dx.doi.org/10.1175/jcli4059.1.
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Abstract The dominant mode of convectively coupled Kelvin waves has been detected over the Atlantic and Africa during northern summer by performing composite analyses on observational fields based on an EOF reconstructed convection index over West Africa. Propagating eastward, many waves originate from the Pacific sector, interact with deep convection of the marine ITCZ over the Atlantic and the continental ITCZ over West and central Africa, and then weaken over East Africa and the Indian Ocean. It has been shown that they are able to modulate the life cycle and track of individual westward-propagating convective systems. Their mean kinematic characteristics comprise a wavelength of 8000 km, and a phase speed of 15 m s−1, leading to a period centered on 6 to 7 days. The African Kelvin wave activity displays large seasonal variability, being highest outside of northern summer when the ITCZ is close to the equator, facilitating the interactions between convection and these equatorially trapped waves. The convective and dynamical patterns identified over the Atlantic and Africa show some resemblance to the theoretical equatorially trapped Kelvin wave solution on an equatorial β plane. Most of the flow is in the zonal direction as predicted by theory, and there is a tendency for the dynamical fields to be symmetric about the equator, even though the ITCZ is concentrated well north of the equator at the full development of the African monsoon. In the upper troposphere and the stratosphere, the temperature contours slope sharply eastward with height, as expected from an eastward-moving heat source that forces a dry Kelvin wave response. It is finally shown that the mean impact of African Kelvin waves on rainfall and convection is of the same level as African easterly waves.
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Wai, OWH, and KW Bedford. "Empirical orthogonal functional analysis of sediment concentration profiles subjected to waves and currents." Marine and Freshwater Research 46, no. 1 (1995): 373. http://dx.doi.org/10.1071/mf9950373.
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Spatial and temporal eigenfunctions for profiles of suspended-sediment concentrations collected during three distinct flow conditions (current-dominated, wave-dominated, and wave- current-dominated) were used to study the non-linear sediment dynamics in the water column. The eigenfunctions were obtained by the method of Empirical Orthogonal Function (EOF) analysis. The variance distribution of the first spatial eigenfunction associated with the largest eigenvalue reflects the characteristic structure of the original profiles, and the second largest spatial eigenfunction indicates the location of possible structural or boundary layer changes in the profiles. The first temporal eigenfunctions for the current- and wave-driven profiles correlate with the turbulence-wave kinetic energy. Because of the complexity of the wave-current flow field, the first two temporal eigenfunctions for the wave-current-driven profiles have a weak relation with the major driving forces. Orthogonal functions can be used to reconstruct sediment concentration profiles efficiently and accurately. To reconstruct 97% of the variation of 10-min averaged profiles in a 2-h data record, only two eigenvalues, and their corresponding orthogonal functions are required, even in the complex wave-current flow field.
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BABA, KENJI, and JAMES RENWICK. "Aspects of intraseasonal variability of Antarctic sea ice in austral winter related to ENSO and SAM events." Journal of Glaciology 63, no. 241 (September 11, 2017): 838–46. http://dx.doi.org/10.1017/jog.2017.49.
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ABSTRACTWe performed an Empirical Orthogonal Function (EOF) analysis to assess the intraseasonal variability of 5–60 day band-pass filtered Antarctic sea-ice concentration in austral winter using a 20-year daily dataset from 1995 to 2014. Zonal wave number 3 dominated in the Antarctic, especially so across the west Antarctic. Results showed the coexistence of stationary and propagating wave components. A spectral analysis of the first two principal components (PCs) showed a similar structure for periods up to 15 days but generally more power in PC1 at longer periods. Regression analysis upon atmospheric fields using the first two PCs of sea-ice concentration showed a coherent wave number 3 pattern. The spatial phase delay between the sea-ice and mean sea-level pressure patterns suggests that meridional flow and associated temperature advection are important for modulating the sea-ice field. EOF analyses carried out separately for El Niño, La Niña and neutral years, and for Southern Annular Mode positive, negative and neutral periods, suggest that the spatial patterns of wave number 3 shift between subsets. The results also indicate that El Niño-Southern Oscillation and Southern Annular Mode affect stationary wave interactions between sea-ice and atmospheric fields on intraseasonal timescales.
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Zheng, J. G., B. C. Khoo, and Z. M. Hu. "Simulation of Wave-Flow-Cavitation Interaction Using a Compressible Homogenous Flow Method." Communications in Computational Physics 14, no. 2 (August 2013): 328–54. http://dx.doi.org/10.4208/cicp.050412.140812a.
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AbstractA numerical method based on a homogeneous single-phase flow model is presented to simulate the interaction between pressure wave and flow cavitation. To account for compressibility effects of liquid water, cavitating flow is assumed to be compressible and governed by time-dependent Euler equations with proper equation of state (EOS). The isentropic one-fluid formulation is employed to model the cavitation inception and evolution, while pure liquid phase is modeled by Tait equation of state. Because of large stiffness of Tait EOS and great variation of sound speed in flow field, some of conventional compressible gasdynamics solvers are unstable and even not applicable when extended to calculation of flow cavitation. To overcome the difficulties, a Godunov-type, cell-centered finite volume method is generalized to numerically integrate the governing equations on triangular mesh. The boundary is treated specially to ensure stability of the approach. The method proves to be stable, robust, accurate, time-efficient and oscillation-free.Novel numerical experiments are designed to investigate unsteady dynamics of the cavitating flow impacted by pressure wave, which is of great interest in engineering applications but has not been studied systematically so far. Numerical simulation indicates that cavity over cylinder can be induced to collapse if the object is accelerated suddenly and extremely high pressure pulse results almost instantaneously. This, however, may be avoided by changing the traveling speed smoothly. The accompanying huge pressure increase may damage underwater devices. However, cavity formed at relatively high upstream speed may be less distorted or affected by shock wave and can recover fully from the initial deformation. It is observed that the cavitating flow starting from a higher freestream velocity is more stable and more resilient with respect to perturbation than the flow with lower background speed. These findings may shed some light on how to control cavitation development to avoid possible damage to operating devices.
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Hunt, F. K., R. Tailleux, and J. J. M. Hirschi. "The vertical structure of oceanic Rossby waves: a comparison of high-resolution model data to theoretical vertical structures." Ocean Science 8, no. 1 (January 13, 2012): 19–35. http://dx.doi.org/10.5194/os-8-19-2012.
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Abstract. Tests of the new Rossby wave theories that have been developed over the past decade to account for discrepancies between theoretical wave speeds and those observed by satellite altimeters have focused primarily on the surface signature of such waves. It appears, however, that the surface signature of the waves acts only as a rather weak constraint, and that information on the vertical structure of the waves is required to better discriminate between competing theories. Due to the lack of 3-D observations, this paper uses high-resolution model data to construct realistic vertical structures of Rossby waves and compares these to structures predicted by theory. The meridional velocity of a section at 24° S in the Atlantic Ocean is pre-processed using the Radon transform to select the dominant westward signal. Normalized profiles are then constructed using three complementary methods based respectively on: (1) averaging vertical profiles of velocity, (2) diagnosing the amplitude of the Radon transform of the westward propagating signal at different depths, and (3) EOF analysis. These profiles are compared to profiles calculated using four different Rossby wave theories: standard linear theory (SLT), SLT plus mean flow, SLT plus topographic effects, and theory including mean flow and topographic effects. Our results support the classical theoretical assumption that westward propagating signals have a well-defined vertical modal structure associated with a phase speed independent of depth, in contrast with the conclusions of a recent study using the same model but for different locations in the North Atlantic. The model structures are in general surface intensified, with a sign reversal at depth in some regions, notably occurring at shallower depths in the East Atlantic. SLT provides a good fit to the model structures in the top 300 m, but grossly overestimates the sign reversal at depth. The addition of mean flow slightly improves the latter issue, but is too surface intensified. SLT plus topography rectifies the overestimation of the sign reversal, but overestimates the amplitude of the structure for much of the layer above the sign reversal. Combining the effects of mean flow and topography provided the best fit for the mean model profiles, although small errors at the surface and mid-depths are carried over from the individual effects of mean flow and topography respectively. Across the section the best fitting theory varies between SLT plus topography and topography with mean flow, with, in general, SLT plus topography performing better in the east where the sign reversal is less pronounced. None of the theories could accurately reproduce the deeper sign reversals in the west. All theories performed badly at the boundaries. The generalization of this method to other latitudes, oceans, models and baroclinic modes would provide greater insight into the variability in the ocean, while better observational data would allow verification of the model findings.
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Hunt, F. K., R. Tailleux, and J. J. M. Hirschi. "The vertical structure of oceanic Rossby waves: a comparison of high-resolution model data to theoretical vertical structures." Ocean Science Discussions 8, no. 3 (May 13, 2011): 1089–129. http://dx.doi.org/10.5194/osd-8-1089-2011.
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Abstract. Tests of the new Rossby wave theories that have been developed over the past decade to account for discrepancies between theoretical wave speeds and those observed by satellite altimeters have focused primarily on the surface signature of such waves. It appears, however, that the surface signature of the waves acts only as a rather weak constraint, and that information on the vertical structure of the waves is required to better discriminate between competing theories. Due to the lack of 3-D observations, this paper uses high-resolution model data to construct realistic vertical structures of Rossby waves and compares these to structures predicted by theory. The meridional velocity of a section at 24° S in the Atlantic Ocean is pre-processed using the Radon transform to select the dominant westward signal. Normalized profiles are then constructed using three complementary methods based respectively on: (1) averaging vertical profiles of velocity, (2) diagnosing the amplitude of the Radon transform of the westward propagating signal at different depths, and (3) EOF analysis. These profiles are compared to profiles calculated using four different Rossby wave theories: standard linear theory (SLT), SLT plus mean flow, SLT plus topographic effects, and theory including mean flow and topographic effects. The model data supports the classical theoretical assumption that westward propagating signals have a well-defined vertical modal structure associated with a phase speed independent of depth, in contrast with the conclusions of a recent study using the same model. The model structures were surface intensified, with a sign reversal at depth in some regions, notably occurring at shallower depths in the East Atlantic. SLT provides a good fit to the model structures in the top 300 m, but grossly overestimates the sign reversal at depth. The addition of mean flow slightly improves the latter issue, but is too surface intensified. SLT plus topography rectifies the overestimation of the sign reversal, but overestimates the amplitude of the structure for much of the layer above the sign reversal. Combining the effects of mean flow and topography provided the best fit for the mean model profiles, although small errors at the surface and mid-depths are carried over from the individual effects of mean flow and topography, respectively. Across the section the best fitting theory varies between SLT plus topography and topography with mean flow, with, in general, SLT plus topography performing better in the east where the sign reversal is less pronounced. None of the theories could accurately reproduce the deeper sign reversals in the west. All theories performed badly at the boundaries. The generalization of this method to other latitudes, oceans, models and baroclinic modes would provide greater insight into the variability in the ocean, while better observational data would allow verification of the model findings.
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Zhao, Siyu, Yi Deng, and Robert X. Black. "An Intraseasonal Mode of Atmospheric Variability Relevant to the U.S. Hydroclimate in Boreal Summer: Dynamic Origin and East Asia Connection." Journal of Climate 31, no. 24 (December 2018): 9855–68. http://dx.doi.org/10.1175/jcli-d-18-0206.1.
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Intraseasonal modes of atmospheric variability over the Northern Hemisphere (NH) midlatitudes in boreal summer are identified via an empirical orthogonal function (EOF) analysis of the daily 10–90-day bandpass-filtered 250-hPa streamfunction for the period of 1950–2016. The first two EOF modes are characterized, respectively, by (i) a single-signed streamfunction anomaly that extends across the NH and (ii) a regional dipole structure with centers over the Aleutian Islands and northeastern Pacific. The third EOF mode (EOF-3) is a quasi-stationary wave train over the Pacific–North American sector with an equivalent barotropic structure in the vertical. EOF-3 is associated with a northwest–southeast oriented anomalous precipitation dipole over the United States. A nonmodal instability analysis of the boreal summer climatological flow in terms of the 250-hPa streamfunction reveals that one of the top “optimal mode” disturbances mimicking the EOF-3 structure grows from an initial precursor disturbance over East Asia through extracting kinetic energy from background flow and attains its maximum amplitude in around nine days. An additional lag regression analysis illustrates that anomalous latent heating associated with cloud and precipitation formation over East Asia is responsible for generating the precursor disturbance for the EOF-3-like optimal mode. This result suggests the existence of an important connection between the hydrological cycles of East Asia and North America, which is dynamically intrinsic to the boreal summer upper-tropospheric flow. Knowledge of such a connection will help us better understand and model hydroclimate variability over these two continents.
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Zhang, Gan, and Zhuo Wang. "Interannual Variability of the Atlantic Hadley Circulation in Boreal Summer and Its Impacts on Tropical Cyclone Activity." Journal of Climate 26, no. 21 (October 16, 2013): 8529–44. http://dx.doi.org/10.1175/jcli-d-12-00802.1.
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Abstract A novel method was developed to define the regional Hadley circulation (HC) in terms of the meridional streamfunction. The interannual variability of the Atlantic HC in boreal summer was examined using EOF analysis. The leading mode (M1), explaining more than 45% of the variances, is associated with the intensity change of the ITCZ. M1 is significantly correlated to multiple climate factors and has strong impacts on Atlantic tropical cyclone (TC) activity. In the positive (negative) phase of M1, the ITCZ is stronger (weaker) than normal, and more (fewer) TCs form over the main development region (MDR) with a larger (smaller) fraction of storms intensifying into major hurricanes. Analyses showed that the large-scale dynamic and thermodynamic conditions associated with a stronger ITCZ are more favorable for TC activity. The roles of tropical easterly waves in modulating the Atlantic TC activity are highlighted. In the positive phase of M1, the wave activity is significantly enhanced over the MDR and the Caribbean Sea, which can be attributed to stronger coastal convection and mean flow structure changes. In the context of the recently proposed marsupial paradigm, the frequency and structure of wave pouches were examined. In the positive phase of M1, the pouch frequency increases, and the number of pouches with a vertically coherent structure also rises significantly. A deep and vertically aligned wave pouch has been shown to be highly favorable for TC formation. The HC perspective thus unifies both dynamic and thermodynamic conditions impacting Atlantic TC activity and helps explain the statistical linkages between Atlantic TC activity and different climate factors.
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Levy, Gad. "Southern hemisphere low level wind circulation statisticsfrom the Seasat scatterometer." Annales Geophysicae 12, no. 1 (January 31, 1994): 65–79. http://dx.doi.org/10.1007/s00585-994-0065-9.
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Abstract. Analyses of remotely sensed low-level wind vector data over the Southern Ocean are performed. Five-day averages and monthly means are created and the month-to-month variability during the winter (July-September) of 1978 is investigated. The remotely sensed winds are compared to the Australian Bureau of Meteorology (ABM) and the National Meteorological Center (NMC) surface analyses. In southern latitudes the remotely sensed winds are stronger than what the weather services' analyses suggest, indicating underestimation by ABM and NMC in these regions. The evolution of the low-level jet and the major stormtracks during the season are studied and different flow regimes are identified. The large-scale variability of the meridional flow is studied with the aid of empirical orthogonal function (EOF) analysis. The dominance of quasi-stationary wave numbers 3, 4, and 5 in the winter flow is evident in both the EOF analysis and the mean flow. The signature of an exceptionally strong blocking situation is evident in July and the special conditions leading to it are discussed. A very large intraseasonal variability with different flow regimes at different months is documented.
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Chen, Xiao-Peng, and Ming Liu. "Simulation of Acoustic Behavior of Bubbly Liquids with Hybrid Lattice Boltzmann and Homogeneous Equilibrium Models." Communications in Computational Physics 17, no. 4 (April 2015): 925–36. http://dx.doi.org/10.4208/cicp.2014.m283.
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AbstractHomogeneous equilibrium model (HEM) has been widely used in cavitating flow simulations. The major feature of this model is that a single equation of state (EOS) is proposed to describe the thermal behavior of bubbly liquid, where both kinematic and thermal equilibrium is assumed between two phases. In this paper, the HEM was coupled with multi-relaxation-time lattice Boltzmann model (MRT-LBM) and the acoustic behavior was simulated. Two approaches were applied alternatively: adjusting speed of sound (Buick, J. Phys. A, 2006, 39:13807-13815) and setting real gas EOS. Both approaches result in high accuracy in acoustic speed predictions for different void (gas) volume of fractions. It is demonstrated that LBM could be successfully applied as a Navier-Stokes equation solver for industrial applications. However, further dissipation and dispersion analysis shows that Shan-Chen type approaches of LBM are deficient, especially in large wave-number region.
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LaCasce, J. H., and Sjoerd Groeskamp. "Baroclinic Modes over Rough Bathymetry and the Surface Deformation Radius." Journal of Physical Oceanography 50, no. 10 (October 1, 2020): 2835–47. http://dx.doi.org/10.1175/jpo-d-20-0055.1.
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AbstractThe deformation radius is widely used as an indication of the eddy length scale at different latitudes. The radius is usually calculated assuming a flat ocean bottom. However, bathymetry alters the baroclinic modes and hence their deformation radii. In a linear quasigeostrophic two-layer model with realistic parameters, the deep flow for a 100-km wave approaches zero with a bottom ridge roughly 10 m high, leaving a baroclinic mode that is mostly surface trapped. This is in line with published current meter studies showing a primary EOF that is surface intensified and has nearly zero flow at the bottom. The deformation radius associated with this “surface mode” is significantly larger than that of the flat bottom baroclinic mode. Using World Ocean Atlas data, the surface radius is found to be 20%–50% larger over much of the globe, and 100% larger in some regions. This in turn alters the long Rossby wave speed, which is shown to be 1.5–2 times faster than over a flat bottom. In addition, the larger deformation radius is easier to resolve in ocean models.
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Kikuchi, Kazuyoshi, and Bin Wang. "Global Perspective of the Quasi-Biweekly Oscillation*." Journal of Climate 22, no. 6 (March 15, 2009): 1340–59. http://dx.doi.org/10.1175/2008jcli2368.1.
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Abstract The quasi-biweekly oscillation (QBW: here defined as a 12–20-day oscillation) is one of the major systems that affect tropical and subtropical weather and seasonal mean climate. However, knowledge is limited concerning its temporal and spatial structures and dynamics, particularly in a global perspective. To advance understanding of the QBW, its life cycle is documented using a tracking method and extended EOF analysis. Both methods yield consistent results. The analyses reveal a wide variety of QBW activity in terms of initiation, movement, development, and dissipation. The convective anomalies associated with the QBW are predominant in the latitude bands between 10° and 30° in both hemispheres. The QBW modes tend to occur regionally and be associated with monsoons. Three boreal summer modes are identified in the Asia–Pacific, Central America, and subtropical South Pacific regions. Five austral summer modes are identified in the Australia–southwest Pacific, South Africa–Indian Ocean, South America–Atlantic, subtropical North Pacific, and North Atlantic–North Africa regions. The QBW modes are classified into two categories: westward- and eastward-propagating modes. The westward mode is found in the Asia–Pacific and Central America regions during boreal summer; it originates in the tropics and dissipates in the subtropics. The behavior of the westward-propagating mode can be understood in terms of equatorial Rossby waves in the presence of monsoon mean flow and convective coupling. The eastward-propagating mode, on the other hand, connects with upstream extratropical Rossby wave trains and propagates primarily eastward and equatorward. Barotropic Rossby wave trains play an essential role in controlling initiation, development, and propagation of the eastward QBW mode in the subtropics. The results therefore suggest that not only tropical but also extratropical dynamics are required for fully understanding the behavior of the QBW systems worldwide. The new conceptual picture of QBW obtained here based on long-term observation provides valuable information on the behavior of QBW systems in a global perspective, which is important for a thorough understanding of tropical variability on a time scale between day-to-day weather and the Madden–Julian oscillation.
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Keller, Julia H., Sarah C. Jones, and Patrick A. Harr. "An Eddy Kinetic Energy View of Physical and Dynamical Processes in Distinct Forecast Scenarios for the Extratropical Transition of Two Tropical Cyclones*." Monthly Weather Review 142, no. 8 (August 1, 2014): 2751–71. http://dx.doi.org/10.1175/mwr-d-13-00219.1.
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Abstract The extratropical transition (ET) of Hurricane Hanna (2008) and Typhoon Choi-Wan (2009) caused a variety of forecast scenarios in the European Centre for Medium-Range Weather Forecasts (ECMWF) Ensemble Prediction System (EPS). The dominant development scenarios are extracted for two ensemble forecasts initialized prior to the ET of those tropical storms, using an EOF and fuzzy clustering analysis. The role of the transitioning tropical cyclone and its impact on the midlatitude flow in the distinct forecast scenarios is examined by conducting an analysis of the eddy kinetic energy budget in the framework of downstream baroclinic development. This budget highlights sources and sinks of eddy kinetic energy emanating from the transitioning tropical cyclone or adjacent upstream midlatitude flow features. By comparing the budget for several forecast scenarios for the ET of each of the two tropical cyclones, the role of the transitioning storms on the development in downstream regions is investigated. Distinct features during the interaction between the tropical cyclone and the midlatitude flow turned out to be important. In the case of Hurricane Hanna, the duration of baroclinic conversion from eddy available potential into eddy kinetic energy was important for the amplification of the midlatitude wave pattern and the subsequent reintensification of Hanna as an extratropical cyclone. In the case of Typhoon Choi-Wan, the phasing between the storm and the midlatitude flow was one of the most critical factors for the future development.
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Giddings, Sarah N., Stephen G. Monismith, Derek A. Fong, and Mark T. Stacey. "Using Depth-Normalized Coordinates to Examine Mass Transport Residual Circulation in Estuaries with Large Tidal Amplitude Relative to the Mean Depth." Journal of Physical Oceanography 44, no. 1 (January 1, 2014): 128–48. http://dx.doi.org/10.1175/jpo-d-12-0201.1.
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Abstract Residual (subtidal) circulation profiles in estuaries with a large tidal amplitude-to-depth ratio often are quite complex and do not resemble the traditional estuarine gravitational circulation profile. This paper describes how a depth-normalized σ-coordinate system allows for a more physical interpretation of residual circulation profiles than does a fixed vertical coordinate system in an estuary with a tidal amplitude comparable to the mean depth. Depth-normalized coordinates permit the approximation of Lagrangian residuals, performance of empirical orthogonal function (EOF) analysis, estimation of terms in the along-stream momentum equations throughout depth, and computation of a tidally averaged momentum balance. The residual mass transport velocity has an enhanced two-layer exchange flow relative to an Eulerian mean because of the Stokes wave transport velocity directed upstream at all depths. While the observed σ-coordinate profiles resemble gravitational circulation, and pressure and friction are the dominant terms in the tidally varying and tidally averaged momentum equations, the two-layer shear velocity from an EOF analysis does not correlate with the along-stream density gradient. To directly compare to theoretical profiles, an extension of a pressure–friction balance in σ coordinates is solved. While the barotropic riverine residual matches theory, the mean longitudinal density gradient and mean vertical mixing cannot explain the magnitude of the observed two-layer shear residual. In addition, residual shear circulation in this system is strongly driven by asymmetries during the tidal cycle, particularly straining and advection of the salinity field, creating intratidal variation in stratification, vertical mixing, and shear.
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Zhu, Jinshuang, Yudi Liu, Ruiqing Xie, and Haijie Chang. "A Comparative Analysis of the Impacts of Two Types of El Niño on the Central and Eastern Pacific ITCZ." Atmosphere 9, no. 7 (July 15, 2018): 266. http://dx.doi.org/10.3390/atmos9070266.
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The precipitation data from the Global Precipitation Climatology Project (GPCP) and CPC Merged Analysis of Precipitation (CMAP) were used to investigate the discrepancy of Centre and Eastern Pacific ITCZ (CEP-ITCZ) during two types of El Niño years. Two models of the heat source distribution during two types of El Niño events were constructed, and the causes of different CEP-ITCZ anomalies for two types of El Niño events were analyzed through the Gill model. The results show that the CEP-ITCZ precipitation is approximately 4.0° southward, and the intensity is enhanced by 3.6 mm/day during the mature period of Eastern Pacific El Niño (EP-El Niño), while during the mature period of Central Pacific El Niño (CP-El Niño), it is only 0.8° southward, and the intensity is enhanced by 3.2 mm/day. The meridional mode of the SST anomaly by means of EOF (Empirical Orthogonal Function) can indirectly affect the CEP-ITCZ by influencing the atmospheric Rossby wave response. In CP-El Niño years, the meridional mode of the SST anomaly is weak, and the atmospheric Rossby wave response enhances the northern and southern trade-wind zones at the same time. The anomaly of cross-equatorial flow is weak and the CEP-ITCZ moves southward a little. At the same time, the wind convergence zone is enhanced, and it is more conducive to the vertical transport of water vapor. In EP-El Niño years, the meridional mode of the SST anomaly is strong, and the atmospheric Rossby wave response strengthens the meridional wind on the northern side of the equator, leading to the southward shift of the CEP-ITCZ. At the same time, the wind convergence zone is weakened and widened, and to a certain extent, it suppresses the vertical transport increase of water vapor caused by the sea surface evaporation.
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Blanes-Vidal, Victoria, Tomas Majtner, Luis David Avendaño-Valencia, Knud B. Yderstraede, and Esmaeil S. Nadimi. "Invisible Color Variations of Facial Erythema: A Novel Early Marker for Diabetic Complications?" Journal of Diabetes Research 2019 (September 2, 2019): 1–7. http://dx.doi.org/10.1155/2019/4583895.
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Aim. (1) To quantify the invisible variations of facial erythema that occur as the blood flows in and out of the face of diabetic patients, during the blood pulse wave using an innovative image processing method, on videos recorded with a conventional digital camera and (2) to determine whether this “unveiled” facial red coloration and its periodic variations present specific characteristics in diabetic patients different from those in control subjects. Methods. We video recorded the faces of 20 diabetic patients with peripheral neuropathy, retinopathy, and/or nephropathy and 10 nondiabetic control subjects, using a Canon EOS camera, for 240 s. Only one participant presented visible facial erythema. We applied novel image processing methods to make the facial redness and its variations visible and automatically detected and extracted the redness intensity of eight facial patches, from each frame. We compared average and standard deviations of redness in the two groups using t-tests. Results. Facial redness varies, imperceptibly and periodically, between redder and paler, following the heart pulsation. This variation is consistently and significantly larger in diabetic patients compared to controls (p value < 0.001). Conclusions. Our study and its results (i.e., larger variations of facial redness with the heartbeats in diabetic patients) are unprecedented. One limitation is the sample size. Confirmation in a larger study would ground the development of a noninvasive cost-effective automatic tool for early detection of diabetic complications, based on measuring invisible redness variations, by image processing of facial videos captured at home with the patient’s smartphone.
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Kolovos, Konstantinos, Phoevos Koukouvinis, Robert M. McDavid, and Manolis Gavaises. "Transient Cavitation and Friction-Induced Heating Effects of Diesel Fuel during the Needle Valve Early Opening Stages for Discharge Pressures up to 450 MPa." Energies 14, no. 10 (May 18, 2021): 2923. http://dx.doi.org/10.3390/en14102923.
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An investigation of the fuel heating, vapor formation, and cavitation erosion location patterns inside a five-hole common rail diesel fuel injector, occurring during the early opening period of the needle valve (from 2 μm to 80 μm), discharging at pressures of up to 450 MPa, is presented. Numerical simulations were performed using the explicit density-based solver of the compressible Navier–Stokes (NS) and energy conservation equations. The flow solver was combined with tabulated property data for a four-component diesel fuel surrogate, derived from the perturbed chain statistical associating fluid theory (PC-SAFT) equation of state (EoS), which allowed for a significant amount of the fuel’s physical and transport properties to be quantified. The Wall Adapting Local Eddy viscosity (WALE) Large Eddy Simulation (LES) model was used to resolve sub-grid scale turbulence, while a cell-based mesh deformation arbitrary Lagrangian–Eulerian (ALE) formulation was used for modelling the injector’s needle valve movement. Friction-induced heating was found to increase significantly when decreasing the pressure. At the same time, the Joule–Thomson cooling effect was calculated for up to 25 degrees K for the local fuel temperature drop relative to the fuel’s feed temperature. The extreme injection pressures induced fuel jet velocities in the order of 1100 m/s, affecting the formation of coherent vortical flow structures into the nozzle’s sac volume.
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Wettstein, Justin J., and John M. Wallace. "Observed Patterns of Month-to-Month Storm-Track Variability and Their Relationship to the Background Flow*." Journal of the Atmospheric Sciences 67, no. 5 (May 1, 2010): 1420–37. http://dx.doi.org/10.1175/2009jas3194.1.
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Abstract Month-to-month storm-track variability is investigated via EOF analyses performed on ERA-40 monthly-averaged high-pass filtered daily 850-hPa meridional heat flux and the variances of 300-hPa meridional wind and 500-hPa height. The analysis is performed both in hemispheric and sectoral domains of the Northern and Southern Hemispheres. Patterns characterized as “pulsing” and “latitudinal shifting” of the climatological-mean storm tracks emerge as the leading sectoral patterns of variability. Based on the analysis presented, storm-track variability on the spatial scale of the two Northern Hemisphere sectors appears to be largely, but perhaps not completely, independent. Pulsing and latitudinally shifting storm tracks are accompanied by zonal wind anomalies consistent with eddy-forced accelerations and geopotential height anomalies that project strongly on the dominant patterns of geopotential height variability. The North Atlantic Oscillation (NAO)–Northern Hemisphere annular mode (NAM) is associated with a pulsing of the Atlantic storm track and a meridional displacement of the upper-tropospheric jet exit region, whereas the eastern Atlantic (EA) pattern is associated with a latitudinally shifting storm track and an extension or retraction of the upper-tropospheric jet. Analogous patterns of storm-track and upper-tropospheric jet variability are associated with the western Pacific (WP) and Pacific–North America (PNA) patterns. Wave–mean flow relationships shown here are more clearly defined than in previous studies and are shown to extend through the depth of the troposphere. The Southern Hemisphere annular mode (SAM) is associated with a latitudinally shifting storm track over the South Atlantic and Indian Oceans and a pulsing South Pacific storm track. The patterns of storm-track variability are shown to be related to simple distortions of the climatological-mean upper-tropospheric jet.
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Weijer, Wilbert, Sarah T. Gille, and Frédéric Vivier. "Modal Decay in the Australia–Antarctic Basin." Journal of Physical Oceanography 39, no. 11 (November 1, 2009): 2893–909. http://dx.doi.org/10.1175/2009jpo4209.1.
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Abstract The barotropic intraseasonal variability in the Australia–Antarctic Basin (AAB) is studied in terms of the excitation and decay of topographically trapped barotropic modes. The main objective is to reconcile two widely differing estimates of the decay rate of sea surface height (SSH) anomalies in the AAB that are assumed to be related to barotropic modes. First, an empirical orthogonal function (EOF) analysis is applied to almost 15 years of altimeter data. The analysis suggests that several modes are involved in the variability of the AAB, each related to distinct areas with (almost) closed contours of potential vorticity. Second, the dominant normal modes of the AAB are determined in a barotropic shallow-water (SW) model. These stationary modes are confined by the closed contours of potential vorticity that surround the eastern AAB, and the crest of the Southeast Indian Ridge. For reasonable values of horizontal eddy viscosity and bottom friction, their decay time scale is on the order of several weeks. Third, the SW model is forced with realistic winds and integrated for several years. Projection of the modal velocity patterns onto the output fields shows that the barotropic modes are indeed excited in the model, and that they decay slowly on the frictional 𝒪(3 weeks) time scale. However, the SSH anomalies in the modal areas display rapid 𝒪(4 days) decay. Additional analysis shows that this rapid decay reflects the adjustment of unbalanced flow components through the emission of Rossby waves. Resonant excitation of the dominant free modes accounts for about 20% of the SSH variability in the forced-model run. Other mechanisms are suggested to explain the region of high SSH variability in the AAB.
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Annamalai, H., and K. R. Sperber. "Regional Heat Sources and the Active and Break Phases of Boreal Summer Intraseasonal (30–50 Day) Variability*." Journal of the Atmospheric Sciences 62, no. 8 (August 1, 2005): 2726–48. http://dx.doi.org/10.1175/jas3504.1.
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Abstract The boreal summer intraseasonal variability (BSISV) associated with the 30–50-day mode is represented by the coexistence of three components: poleward propagation of convection over the Indian and tropical west Pacific longitudes and eastward propagation along the equator. The hypothesis that the three components influence each other has been investigated using observed outgoing longwave radiation (OLR), NCEP–NCAR reanalysis, and solutions from an idealized linear model. The null hypothesis is that the three components are mutually independent. Cyclostationary EOF (CsEOF) analysis is applied on filtered OLR to extract the life cycle of the BSISV. The dominant CsEOF mode is significantly tied to the observed spatial rainfall pattern associated with the active/break phases over the Indian subcontinent. The components of the heating patterns from CsEOF analysis serve as prescribed forcings for the dry version of the linear model. This allows one to investigate the possible roles that the regional heat sources and sinks play in driving the large-scale monsoon circulation at various stages of the BSISV life cycle. To understand the interactive nature between convection and circulation, the moist version of the model is forced with intraseasonal SST anomalies. The linear models reproduce the major features of the BSISV seen in the reanalysis. The linear model suggests three new findings: (i) The circulation anomalies that develop as a Rossby wave response to suppressed convection over the equatorial Indian Ocean associated with the previous break phase of the BSISV results in low-level convergence and tropospheric moisture enhancement over the equatorial western Indian Ocean and helps trigger the next active phase of the BSISV. (ii) The development of convection over the tropical west Pacific forces descent anomalies to the west. This, in conjunction with the weakened cross-equatorial flow due to suppressed convective anomalies over the equatorial Indian Ocean, reduces the tropospheric moisture over the Arabian Sea and promotes westerly wind anomalies that do not recurve over India. As a result the low-level cyclonic vorticity shifts from India to Southeast Asia and break conditions are initiated over India. (iii) The circulation anomalies forced by equatorial Indian Ocean convective anomalies significantly influence the active/break phases over the tropical west Pacific. The model solutions support the hypothesis that the three components of the BSISV influence each other but do not imply that such an influence is responsible for the space–time evolution of the BSISV. Further, the applicability of the model results to the observed system is constrained by the assumption that linear interactions are sufficient to address the BSISV and that air–sea interaction and transient forcing are excluded.
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Ng, Chiu-On, and Bo Chen. "Dispersion in Electro-Osmotic Flow Through a Slit Channel With Axial Step Changes of Zeta Potential." Journal of Fluids Engineering 135, no. 10 (August 6, 2013). http://dx.doi.org/10.1115/1.4024958.
Full textAbstract:
An analytical study is presented in this paper on hydrodynamic dispersion due to steady electro-osmotic flow (EOF) in a slit microchannel with longitudinal step changes of ζ potential. The channel wall is periodically patterned with alternating stripes of distinct ζ potentials. Existing studies in the literature have considered dispersion in EOF with axial nonuniformity of ζ potential only in the limiting case where the length scale for longitudinal variation is much longer than the cross-sectional dimension of the channel. Hence, the existing theories on EOF dispersion subject to nonuniform charge distributions are all based on the lubrication approximation, by which cross-sectional mixing is ignored. In the present study, the general case where the length of one periodic unit of wall pattern (which involves a step change of ζ potential) is comparable with the channel height, as well as the long-wave limiting case, are investigated. The problem for the hydrodynamic dispersion coefficient is solved numerically in the general case, and analytically in the long-wave lubrication limit. The dispersion coefficient and the plate height are found to have strong, or even nonmonotonic, dependence on the controlling parameters, including the period length of the wall pattern, the area fraction of the EOF-suppressing region, the Debye parameter, the Péclet number, and the ratio of the two ζ potentials.
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"Seismic modelling of the Earth’s large-scale three-dimensional structure." Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences 328, no. 1599 (July 4, 1989): 291–308. http://dx.doi.org/10.1098/rsta.1989.0037.
Full textAbstract:
Several different kinds of seismological data, spanning more than three orders of magnitude in frequency, have been employed in the study of the Earth’s large-scale three-dimensional structure. These yield different but overlapping information, which is leading to a coherent picture of the Earth’s internal heterogeneity. In this article we describe several methods of seismic inversion and intercom pare the resulting models. Models of upper-mantle shear velocity based upon mantle waveforms (Woodhouse & Dziewonski ( J. geophys. Res . 89 , 5953-5986 (1984))) ( f ≲ 7 mHz) and long-period body waveforms ( f ≲ 20 mHz; Woodhouse & Dziewonski ( Eos, Wash . 67 , 307 (1986))) show the mid-oceanic ridges to be the major low-velocity anomalies in the uppermost mantle, together with regions in the western Pacific, characterized by back-arc volcanism. High velocities are associated with the continents, and in particular with the continental shields, extending to depths in excess of 300 km. By assuming a given ratio between density and wave velocity variations, and a given mantle viscosity structure, such models have been successful in explaining some aspects of observed plate motion in terms of thermal convection in the mantle (Forte & Peltier ( J. geophys. Res . 92 , 3645-3679 (1987))). An im portant qualitative conclusion from such analysis is that the magnitude of the observed seismic anomalies is of the order expected in a convecting system having the viscosity, tem perature derivatives and flow rates which characterize the mantle. Models of the lower mantle based upon P-wave arrival times ( f ≈ 1 Hz; Dziewonski ( J. geophys. Res . 89 , 5929-5952 (1984)); Morelli & Dziewonski ( Eos, Wash . 67 , 311 (1986))) SH waveforms ( f ≈ mHz; Woodhouse & Dziewonski (1986)) and free oscillations (Giardini et al . ( Nature, Lond . 325 , 405-411 (1987); J. geophys. Res. 93 , 13716—13742 (1988))) ( f ≈ 0.5-5 mHz) show a very long wavelength pattern, largely contained in spherical harmonics of degree 2, which is present over a large range of depths (1000-2700 km). This anomaly has been detected in both compressional and shear wave velocities, and yields a ratio of relative perturbations in v s and v P in the lower mantle in the range 2-2.5. Such values, which are much larger than has sometimes been assumed, roughly correspond to the case that perturbations in shear modulus dominate those in bulk modulus. It is this anomaly that is mainly responsible for the observed low-degree geoid undulations (Hager et al. Nature, Lond . 313 , 541-545 (1985))). In the upper part of the lower mantle this pattern consists of a high-velocity feature skirting the subduction zones of the Pacific and extending from Indonesia to the Mediterranean, with low velocities elsewhere; thus it appears to be associated with plate convergence and subduction. The pattern of wave speeds in the lowermost mantle is such that approximately 80% of hot spots are in regions of lower than average velocities in the D" region. The topography of the core-mantle boundary, determined from the arrival times of reflected and transmitted waves (Morelli & Dziewonski ( Nature, Lond . 325 , 678-683 (1987))), exhibits a pattern of depressions encircling the Pacific, having an amplitude of approximately ± 5 km, which has been shown to be consistent with the stresses induced by density anomalies inferred from tom ographic models of the lower mantle (Forte & Peltier ( Tectonphysics (In the press.) (1989))). By using both free oscillations (Woodhouse et al . ( Geophys. Res. Lett . 13 , 1549-1552 (1986))) and travel-time data (Morelli et al . ( Geophys. Res. Lett . 13 , 1545—1548 (1986))), the inner core has been found to be anisotropic, exhibiting high velocities for waves propagating parallel to the Earth ’s rotation axis and low velocities in the equatorial plane. Tomographic models represent an instantaneous, low-resolution image of a convecting system. They require for their detailed interpretation knowledge of mineral and rock properties that are, as yet, poorly known but that laboratory experiments can potentially determ ine. The fact that the present distribution of seismic anomalies must represent the current configuration of therm al and compositional heterogeneity advected by m antle flow, imposes a complex set of constraints on the possible modes of convection in the m antle of which the implications have not yet been worked out; this will require num erical modelling of convection in three dimensions, which only recently has become feasible. Thus the interpretation of the ‘geographical’ information from seismology in terms of geodynamical processes is a matter of considerable complexity, and we may expect that a number of the conclusions to be drawn from the seismological results lie in the future.