Minimal models for precipitating turbulent convection

2013 ◽  
Vol 717 ◽  
pp. 576-611 ◽  
Author(s):  
Gerardo Hernandez-Duenas ◽  
Andrew J. Majda ◽  
Leslie M. Smith ◽  
Samuel N. Stechmann
Keyword(s):  
Time Scales ◽  
Environmental Conditions ◽  
Propagation Speed ◽  
Rain Water ◽  
Moist Convection ◽  
Minimal Models ◽  
Cold Pools ◽  
Anelastic Equations ◽  
Water Substance ◽  
Organized Convection

AbstractSimulations of precipitating convection would typically use a non-Boussinesq dynamical core such as the anelastic equations, and would incorporate water substance in all of its phases: vapour, liquid and ice. Furthermore, the liquid water phase would be separated into cloud water (small droplets suspended in air) and rain water (larger droplets that fall). Depending on environmental conditions, the moist convection may organize itself on multiple length and time scales. Here we investigate the question, what is the minimal representation of water substance and dynamics that still reproduces the basic regimes of turbulent convective organization? The simplified models investigated here use a Boussinesq atmosphere with bulk cloud physics involving equations for water vapour and rain water only. As a first test of the minimal models, we investigate organization or lack thereof on relatively small length scales of approximately 100 km and time scales of a few days. It is demonstrated that the minimal models produce either unorganized (‘scattered’) or organized convection in appropriate environmental conditions, depending on the environmental wind shear. For the case of organized convection, the models qualitatively capture features of propagating squall lines that are observed in nature and in more comprehensive cloud resolving models, such as tilted rain water profiles, low-altitude cold pools and propagation speed corresponding to the maximum of the horizontally averaged, horizontal velocity.

Gravity Currents in a Deep Anelastic Atmosphere

2008 ◽  
Vol 65 (2) ◽  
pp. 536-556 ◽  
Author(s):  
George H. Bryan ◽  
Richard Rotunno
Keyword(s):  
Pressure Rise ◽  
Propagation Speed ◽  
Gravity Currents ◽  
Cold Pool ◽  
Exchange Flow ◽  
Channel Depth ◽  
Cold Pools ◽  
Independent Evaluation ◽  
Energy Conserving ◽  
Anelastic Equations

Abstract This study presents analytic results for steady gravity currents in a channel using the deep anelastic equations. Results are cast in terms of a nondimensional parameter H/H0 that relates the channel depth H to a scale depth H0 (the depth at which density goes to zero in an isentropic atmosphere). The classic results based on the incompressible equations correspond to H/H0 = 0. For cold gravity currents (at the bottom of a channel), assuming energy-conserving flow, the nondimensional current depth h/H is much smaller, and nondimensional propagation speed C/(gH)1/2 is slightly smaller as H/H0 increases. For flows with energy dissipation, C/(gH)1/2 decreases as H/H0 increases, even for fixed h/H. The authors conclude that as H/H0 increases the normalized hydrostatic pressure rise in the cold pool increases near the bottom of the channel, whereas drag decreases near the top of the channel; these changes require gravity currents to propagate slower for steady flow to be maintained. From these results, the authors find that steady cold pools have a likely maximum depth of 4 km in the atmosphere (in the absence of shear). For warm gravity currents (at the top of a channel), h/H is slightly larger and C/(gH)1/2 is much larger as H/H0 increases. The authors also conduct two-dimensional numerical simulations of “lock-exchange flow” to provide an independent evaluation of the analytic results. For cold gravity currents the simulations support the analytic results. However, for warm gravity currents the simulations show unsteady behavior that cannot be captured by the analytic theory and which appears to have no analog in incompressible flow.

scholarly journals Coherence of Western Boundary Pressure at the RAPID WAVE Array: Boundary Wave Adjustments or Deep Western Boundary Current Advection?

2013 ◽  
Vol 43 (4) ◽  
pp. 744-765 ◽  
Author(s):  
Shane Elipot ◽  
Chris Hughes ◽  
Sofia Olhede ◽  
John Toole
Keyword(s):  
Continental Slope ◽  
Time Scales ◽  
Propagation Speed ◽  
Large Error ◽  
Western Boundary ◽  
Ocean Bottom ◽  
Boundary Current ◽  
Coherent Signals ◽  
Woods Hole Oceanographic Institution ◽  
Meridional Transport

Abstract This study investigates the coherence between ocean bottom pressure signals at the Rapid Climate Change programme (RAPID) West Atlantic Variability Experiment (WAVE) array on the western North Atlantic continental slope, including the Woods Hole Oceanographic Institution Line W. Highly coherent pressure signals propagate southwestward along the slope, at speeds in excess of 128 m s−1, consistent with expectations of barotropic Kelvin-like waves. Coherent signals are also evidenced in the smaller pressure differences relative to 1000-m depth, which are expected to be associated with depth-dependent basinwide meridional transport variations or an overturning circulation. These signals are coherent and almost in phase for all time scales from 3.6 years down to 3 months. Coherence is still seen at shorter time scales for which group delay estimates are consistent with a propagation speed of about 1 m s−1 over 990 km of continental slope but with large error bounds on the speed. This is roughly consistent with expectations for propagation of coastally trapped waves, though somewhat slower than expected. A comparison with both Eulerian currents and Lagrangian float measurements shows that the coherence is inconsistent with a propagation of signals by advection, except possibly on time scales longer than 6 months.

Role of dynamic and thermodynamic processes for the propagation of organized convection in a large-scale flow

2020 ◽  
Author(s):  
Hyunju Jung ◽  
Ann Kristin Naumann ◽  
Bjorn Stevens
Keyword(s):  
Dynamic Process ◽  
Large Scale ◽  
Surface Wind ◽  
Propagation Speed ◽  
Background Wind ◽  
Near Surface ◽  
Large Scale Flow ◽  
Model Setup ◽  
Self Aggregation ◽  
Organized Convection

<p>Convective self-aggregation in radiative convective equilibrium has been studied due to its similarities to organized convection in the tropics. As tropical atmospheric phenomena are embedded in a large-scale flow, we impose a background wind to the model setup using convection-permitting simulation to analyze the interaction of convective self-aggregation with the background wind. The simulations show that when imposing a background wind, the convective cluster propagates in the direction of the imposed wind but slows down compared to what pure advection would suggest, and eventually becomes stationary. The dynamic process dominates slowing down the propagation speed of the cluster because the surface momentum flux acts as a drag on the near-surface wind, terminating the propagation. The thermodynamic process through the wind-induce surface feedback contributes to only 6% of the propagation speed of the convective cluster and is strongly modified by the dynamic process.</p>

scholarly journals On spatially explicit models of cholera epidemics

2009 ◽  
Vol 7 (43) ◽  
pp. 321-333 ◽  
Author(s):  
E. Bertuzzo ◽  
R. Casagrandi ◽  
M. Gatto ◽  
I. Rodriguez-Iturbe ◽  
A. Rinaldo
Keyword(s):  
Time Scales ◽  
Reactive Transport ◽  
Real Life ◽  
Propagation Speed ◽  
Power Laws ◽  
Compartmental Models ◽  
Spatially Explicit ◽  
Channel Networks ◽  
Spatially Explicit Models ◽  
Regular Lattices

We generalize a recently proposed model for cholera epidemics that accounts for local communities of susceptibles and infectives in a spatially explicit arrangement of nodes linked by networks having different topologies. The vehicle of infection ( Vibrio cholerae ) is transported through the network links that are thought of as hydrological connections among susceptible communities. The mathematical tools used are borrowed from general schemes of reactive transport on river networks acting as the environmental matrix for the circulation and mixing of waterborne pathogens. Using the diffusion approximation, we analytically derive the speed of propagation for travelling fronts of epidemics on regular lattices (either one-dimensional or two-dimensional) endowed with uniform population density. Power laws are found that relate the propagation speed to the diffusion coefficient and the basic reproduction number. We numerically obtain the related, slower speed of epidemic spreading for more complex, yet realistic river structures such as Peano networks and optimal channel networks. The analysis of the limit case of uniformly distributed population sizes proves instrumental in establishing the overall conditions for the relevance of spatially explicit models. To that extent, the ratio between spreading and disease outbreak time scales proves the crucial parameter. The relevance of our results lies in the major differences potentially arising between the predictions of spatially explicit models and traditional compartmental models of the susceptible–infected–recovered (SIR)-like type. Our results suggest that in many cases of real-life epidemiological interest, time scales of disease dynamics may trigger outbreaks that significantly depart from the predictions of compartmental models.

Effects of Moist Convection on Hurricane Predictability

2009 ◽  
Vol 66 (7) ◽  
pp. 1944-1961 ◽  
Author(s):  
Fuqing Zhang ◽  
Jason A. Sippel
Keyword(s):  
Risk Assessment ◽  
Initial Conditions ◽  
Ensemble Prediction ◽  
Tropical Cyclogenesis ◽  
Initial Condition ◽  
Moist Convection ◽  
Cold Pools ◽  
Probabilistic Forecasts ◽  
Prediction Systems ◽  
Analysis System

Abstract This study exemplifies inherent uncertainties in deterministic prediction of hurricane formation and intensity. Such uncertainties could ultimately limit the predictability of hurricanes at all time scales. In particular, this study highlights the predictability limit due to the effects on moist convection of initial-condition errors with amplitudes far smaller than those of any observation or analysis system. Not only can small and arguably unobservable differences in the initial conditions result in different routes to tropical cyclogenesis, but they can also determine whether or not a tropical disturbance will significantly develop. The details of how the initial vortex is built can depend on chaotic interactions of mesoscale features, such as cold pools from moist convection, whose timing and placement may significantly vary with minute initial differences. Inherent uncertainties in hurricane forecasts illustrate the need for developing advanced ensemble prediction systems to provide event-dependent probabilistic forecasts and risk assessment.

scholarly journals Western North Pacific Tropical Cyclones in the Met Office Global Seasonal Forecast System: Performance and ENSO Teleconnections

2020 ◽  
Vol 33 (24) ◽  
pp. 10489-10504
Author(s):  
Xiangbo Feng ◽  
Nicholas P. Klingaman ◽  
Kevin I. Hodges ◽  
Yi-Peng Guo
Keyword(s):  
Environmental Conditions ◽  
North Pacific ◽  
Western North Pacific ◽  
Propagation Speed ◽  
Seasonal Forecast ◽  
Forecast System ◽  
Ensemble Forecasts ◽  
Steering Flow ◽  
Enso Teleconnections ◽  
The Tropical Pacific

AbstractThe performance of the Met Office Global Seasonal Forecast System (GloSea5-GC2) for tropical cyclone (TC) frequency for the western North Pacific (WNP) in July–October is evaluated, using 23 years of ensemble forecasts (1993–2015). Compared to observations, GloSea5 overpredicts the climatological TC frequency in the eastern WNP and underpredicts it in the western and northern WNP. These biases are associated with an El Niño–type bias in TC-related environmental conditions (e.g., low-level convergence and steering flow), which encourages too many TCs to form throughout the tropical Pacific and slows TC propagation speed. For interannual TC frequency variability, GloSea5 overestimates the observed negative TC–ENSO teleconnection in the western and northern WNP, associated with an eastward shift in the ENSO teleconnection to environmental conditions. Consequently, GloSea5 fails to predict interannual TC variability in the northeast WNP (south of Japan); performance is higher in the southwest WNP (e.g., the South China Sea) where the sign of the TC–ENSO teleconnection is correct. This study suggests the need to reduce biases in environmental conditions and associated ENSO teleconnections in GloSea5 to improve the TC prediction performance in the NWP.

Cancellation of Aerosol Indirect Effects in Marine Stratocumulus through Cloud Thinning

2007 ◽  
Vol 64 (7) ◽  
pp. 2657-2669 ◽  
Author(s):  
Robert Wood
Keyword(s):  
Time Scales ◽  
Environmental Conditions ◽  
Indirect Effects ◽  
Marine Boundary Layer ◽  
Cloud Condensation Nuclei ◽  
Cloud Droplet ◽  
Cloud Base ◽  
Aerosol Indirect Effects ◽  
Surface Precipitation ◽  
Cloud Thickness

Abstract Applying perturbation theory within a mixed layer framework, the response of the marine boundary layer (MBL) cloud thickness h to imposed increases of the cloud droplet concentration Nd as a surrogate for increases in cloud condensation nuclei (CCN) concentrations is examined. An analytical formulation is used to quantify the response and demonstrate theoretically that for the range of environmental conditions found over the subtropical eastern oceans, on time scales of less than a day, the cloud thickness feedback response is largely determined by a balance between the moistening/cooling of the MBL resulting from the suppression of surface precipitation, and the drying/warming resulting from enhanced entrainment resulting from increased turbulent kinetic energy. Quantifying the transient cloud response as a ratio of the second to the first indirect effects demonstrates that the nature of the feedback is critically dependent upon the nature of the unperturbed state, with the cloud-base height zcb being the single most important determinant. For zcb < 400 m, increasing Nd leads to cloud thickening in accordance with the Albrecht hypothesis. However, for zcb > 400 m, cloud thinning occurs, which results in a feedback effect that increasingly cancels the Twomey effect as zcb increases. The environmental conditions favoring an elevated cloud base are relatively weak lower-tropospheric stability and a dry free troposphere, although the former is probably more important over the subtropical eastern oceans. On longer time scales an invariable thickening response is found, and thus accurate quantification of the aerosol indirect effects will require a good understanding of the processes that control the time scale over which aerosol perturbations are modified.

Evaluation of Organized Convection Parameterization in Global Climate Models

2020 ◽  
Author(s):  
Chih-Chieh Chen ◽  
Changhai Liu ◽  
Mitch Moncrieff ◽  
Yaga Richter
Keyword(s):  
Climate Models ◽  
Global Climate ◽  
Earth System Model ◽  
Global Climate Models ◽  
System Model ◽  
Moist Convection ◽  
Earth System ◽  
Mesoscale Convective ◽  
Convective Organization ◽  
Organized Convection

<p>The importance of convective organization on the global circulation has been recognized for a long time, but parameterizations of the associated processes are missing in global climate models. Contemporary convective parameterizations commonly use a convective plume model (or a spectrum of plumes). This is perhaps appropriate for unorganized convection but the assumption of a gap between the small cumulus scale and the large-scale motion fails to recognize mesoscale dynamics manifested in mesoscale convective systems (MCSs) and multi-scale cloud systems associated with the MJO. Organized convection is abundant in environments featuring vertical wind shear, and significantly modulates the life cycle of moist convection, the transport of heat and momentum, and accounts for a large percentage of precipitation in the tropics. Mesoscale convective organization is typically associated with counter-gradient momentum transport, and distinct heating profiles between the convective and stratiform regions.</p><p>Moncrieff, Liu and Bogenschutz (2017) recently developed a dynamical based parameterization of organized moisture convection, referred to as multiscale coherent structure parameterization (MCSP), for global climate models. A prototype version of MCSP has been implemented in the NCAR Community Earth System Model (CESM) and the Energy Exascale Earth System Model (E3SM), positively affecting the distribution of tropical precipitation, convectively coupled tropical waves, and the Madden-Julian oscillation. We will show the further development of the MCSP and its impact on the simulation of mean precipitation and variability in the two global climate models.</p>

Using the Translation Speed and Vertical Structure of Gust Fronts to Infer Buoyancy Deficits within Thunderstorm Outflow

2019 ◽  
Vol 147 (10) ◽  
pp. 3575-3594 ◽  
Author(s):  
Abby Hutson ◽  
Christopher Weiss ◽  
George Bryan
Keyword(s):  
Vertical Structure ◽  
Lower Boundary ◽  
Propagation Speed ◽  
Cold Pool ◽  
Slip Model ◽  
Translation Speed ◽  
Acceptable Error ◽  
Cold Pools ◽  
Thunderstorm Outflow ◽  
Gust Front

Abstract This study investigates whether the thermodynamics of supercell rear-flank outflow can be inferred from the propagation speed and vertical structure of the rear-flank gust front. To quantify the relationship between outflow thermodynamic deficit and gust front structure, CM1 is applied as a two-dimensional cold pool model to assess the vertical slope of cold pools of varying strength in different configurations of ambient shear. The model was run with both free-slip and semislip lower boundary conditions and the results were compared to observations of severe thunderstorm outflow captured by the Texas Tech University Ka-band mobile radars. Simulated cold pools in the free-slip model achieve the propagation speeds predicted by cold pool theory, while cold pool speeds in the semislip model propagate slower. Density current theory is applied to the observed cold pools and predicts the cold pool speed to within about 2 m s−1. Both the free-slip and semislip model results reveal that, in the same sheared flow, the edge of a strong cold pool is less inclined than that of a weaker cold pool. Also, a cold pool in weak ambient shear has a steeper slope than the same cold pool in stronger ambient shear. Nonlinear regressions performed on data from both models capture the proper dependence of slope on buoyancy and shear, but the free-slip model does not predict observed slopes within acceptable error, and the semislip model overpredicts the cold pool slope for all observed cases, but with uncertainty due to shear estimation.

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