scholarly journals Tropical Continental Downdraft Characteristics: Mesoscale Systems versus Unorganized Convection

2017 ◽  
Author(s):  
Kathleen A. Schiro ◽  
J. David Neelin
Keyword(s):  
High Probability ◽  
Climate Models ◽  
Large Fraction ◽  
Potential Temperature ◽  
Strong Relationship ◽  
Diagnostic Tools ◽  
Cold Pool ◽  
Equivalent Potential Temperature ◽  
Isolated Cells ◽  
Equivalent Potential

Abstract. Downdrafts and cold pool characteristics for mesoscale convective systems (MCSs) and isolated, unorganized deep precipitating convection are analyzed using multi-instrument data from the GOAmazon campaign. For both MCSs and isolated cells, there are increases in column water vapor (CWV) observed in the two hours leading the convection and an increase in wind speed, decrease in surface moisture and temperature, and increase in relative humidity coincident with system passage. Composites of vertical velocity data and radar reflectivity from a radar wind profiler show that the downdrafts associated with the sharpest decreases in surface equivalent potential temperature (θe) have a probability that increases towards lower levels below the freezing level. Both MCSs and unorganized convection show similar mean downdraft magnitudes and probabilities with height. This is consistent with thermodynamic arguments: if θe were approximately conserved following descent, it would imply that a large fraction of the air reaching the surface originates at altitudes in the lowest 2 km, with probability of lower θe dropping exponentially. Mixing computations suggest that, on average, air originating at heights greater than 3 km must undergo substantial mixing, particularly in the case of isolated cells, to match the observed cold pool θe, likewise implying a low typical origin level. Precipitation conditionally averaged on decreases in surface equivalent potential temperature (Δθe) exhibits a strong relationship because the largest Δθe values are associated with high probability of precipitation. The more physically motivated conditional average of Δθe on precipitation levels off with increasing precipitation rate, bounded by the maximum difference between surface θe and its minimum in the profile aloft. Precipitation values greater than about 10 mm h−1 are associated with high probability of Δθe decreases. Robustness of these statistics observed across scales and regions suggests their potential use as model diagnostic tools for the improvement of downdraft parameterizations in climate models.

scholarly journals Tropical continental downdraft characteristics: mesoscale systems versus unorganized convection

2018 ◽  
Vol 18 (3) ◽  
pp. 1997-2010 ◽  
Author(s):  
Kathleen A. Schiro ◽  
J. David Neelin
Keyword(s):  
Climate Models ◽  
Potential Temperature ◽  
Strong Relationship ◽  
Diagnostic Tools ◽  
Cold Pool ◽  
Equivalent Potential Temperature ◽  
Isolated Cells ◽  
Convective Systems ◽  
Freezing Level ◽  
Equivalent Potential

Abstract. Downdrafts and cold pool characteristics for strong mesoscale convective systems (MCSs) and isolated, unorganized deep precipitating convection are analyzed using multi-instrument data from the DOE Atmospheric Radiation Measurement (ARM) GoAmazon2014/5 campaign. Increases in column water vapor (CWV) are observed leading convection, with higher CWV preceding MCSs than for isolated cells. For both MCSs and isolated cells, increases in wind speed, decreases in surface moisture and temperature, and increases in relative humidity occur coincidentally with system passages. Composites of vertical velocity data and radar reflectivity from a radar wind profiler show that the downdrafts associated with the sharpest decreases in surface equivalent potential temperature (θe) have a probability of occurrence that increases with decreasing height below the freezing level. Both MCSs and unorganized convection show similar mean downdraft magnitudes and probabilities with height. Mixing computations suggest that, on average, air originating at heights greater than 3 km must undergo substantial mixing, particularly in the case of isolated cells, to match the observed cold pool θe, implying a low typical origin level. Precipitation conditionally averaged on decreases in surface equivalent potential temperature (Δθe) exhibits a strong relationship because the most negative Δθe values are associated with a high probability of precipitation. The more physically motivated conditional average of Δθe on precipitation shows that decreases in θe level off with increasing precipitation rate, bounded by the maximum difference between surface θe and its minimum in the profile aloft. Robustness of these statistics observed across scales and regions suggests their potential use as model diagnostic tools for the improvement of downdraft parameterizations in climate models.

scholarly journals Evolution of Pre- and Postconvective Environmental Profiles from Mesoscale Convective Systems during PECAN

2019 ◽  
Vol 147 (7) ◽  
pp. 2329-2354 ◽  
Author(s):  
Stacey M. Hitchcock ◽  
Russ S. Schumacher ◽  
Gregory R. Herman ◽  
Michael C. Coniglio ◽  
Matthew D. Parker ◽  
...  
Keyword(s):  
Potential Temperature ◽  
Cold Pool ◽  
Temperature Perturbation ◽  
Convective System ◽  
Equivalent Potential Temperature ◽  
Wind Maximum ◽  
Low Level ◽  
Equivalent Potential ◽  
Mesoscale Convective ◽  
Low Levels

Abstract During the Plains Elevated Convection at Night (PECAN) field campaign, 15 mesoscale convective system (MCS) environments were sampled by an array of instruments including radiosondes launched by three mobile sounding teams. Additional soundings were collected by fixed and mobile PECAN integrated sounding array (PISA) groups for a number of cases. Cluster analysis of observed vertical profiles established three primary preconvective categories: 1) those with an elevated maximum in equivalent potential temperature below a layer of potential instability; 2) those that maintain a daytime-like planetary boundary layer (PBL) and nearly potentially neutral low levels, sometimes even well after sunset despite the existence of a southerly low-level wind maximum; and 3) those that are potentially neutral at low levels, but have very weak or no southerly low-level winds. Profiles of equivalent potential temperature in elevated instability cases tend to evolve rapidly in time, while cases in the potentially neutral categories do not. Analysis of composite Rapid Refresh (RAP) environments indicate greater moisture content and moisture advection in an elevated layer in the elevated instability cases than in their potentially neutral counterparts. Postconvective soundings demonstrate significantly more variability, but cold pools were observed in nearly every PECAN MCS case. Following convection, perturbations range between −1.9 and −9.1 K over depths between 150 m and 4.35 km, but stronger, deeper stable layers lead to structures where the largest cold pool temperature perturbation is observed above the surface.

scholarly journals Impacts of Evaporation of Rainwater on Tropical Cyclone Structure and Intensity—A Revisit

2015 ◽  
Vol 72 (4) ◽  
pp. 1323-1345 ◽  
Author(s):  
Qingqing Li ◽  
Yuqing Wang ◽  
Yihong Duan
Keyword(s):  
Tropical Cyclone ◽  
Inner Core ◽  
Potential Temperature ◽  
Outer Core ◽  
Cold Pool ◽  
Equivalent Potential Temperature ◽  
Core Size ◽  
Size Change ◽  
Equivalent Potential ◽  
The Impact

Abstract The impact of evaporation of rainwater on tropical cyclone (TC) intensity and structure is revisited in this study. Evaporative cooling can result in strong downdrafts and produce low–equivalent potential temperature air in the inflow boundary layer, particularly in the region outside the eyewall, significantly suppressing eyewall convection and reducing the final intensity of a TC. Different from earlier findings, results from this study show that outer rainbands still form but are short lived in the absence of evaporation. Evaporation of rainwater is shown to facilitate the formation of outer rainbands indirectly by reducing the cooling due to melting of ice particles outside the inner core, not by the cold-pool dynamics, as previously believed. Only exclusion of evaporation in the eyewall region or the rapid filamentation zone has a very weak effect on the inner-core size change of a TC, whereas how evaporation in the outer core affects the inner-core size depends on how active the inner rainbands are. More (less) active inner rainbands may lead to an increase (a decrease) in the inner-core size.

What Is the Representation of the Moisture–Tropopause Relationship in CMIP5 Models?*

2015 ◽  
Vol 28 (12) ◽  
pp. 4877-4889 ◽  
Author(s):  
Yutian Wu ◽  
Olivier Pauluis
Keyword(s):  
Annual Cycle ◽  
Seasonal Cycle ◽  
Climate Models ◽  
Potential Temperature ◽  
Specific Humidity ◽  
Cmip5 Models ◽  
Equivalent Potential Temperature ◽  
Near Surface ◽  
Equivalent Potential ◽  
Extratropical Tropopause

Abstract A dynamical relationship that connects the extratropical tropopause potential temperature and the near-surface distribution of equivalent potential temperature was proposed in a previous study and was found to work successfully in capturing the annual cycle of the extratropical tropopause in reanalyses. This study extends the diagnosis of the moisture–tropopause relationship to an ensemble of CMIP5 models. It is found that, in general, CMIP5 multimodel averages are able to produce the one-to-one moisture–tropopause relationship. However, a few biases are observed as compared to reanalyses. First of all, “cold biases” are seen at both the upper and lower levels of the troposphere, which are universal for all seasons, both hemispheres, and almost all CMIP5 models. This has been known as the “general coldness of climate models” since 1990 but the mechanisms remain elusive. It is shown that, for Northern Hemisphere annual averages, the upper- and lower-level “cold” biases are, in fact, correlated across CMIP5 models, which supports the dynamical linkage. Second, a large intermodel spread is found and nearly half of the models underestimate the annual cycle of the tropopause potential temperature as compared to that of the near-surface equivalent potential temperature fluctuation. This implies the incapability of the models to propagate the surface seasonal cycle to the upper levels. Finally, while reanalyses exhibit a pronounced asymmetry in tropopause potential temperature between the northern and southern summers, only a few CMIP5 models are able to capture this aspect of the seasonal cycle because of the too dry specific humidity in northern summer.

What are the Best Thermodynamic Quantity and Function to Define a Front in Gridded Model Output?

2019 ◽  
Vol 100 (5) ◽  
pp. 873-895 ◽  
Author(s):  
Carl M. Thomas ◽  
David M. Schultz
Keyword(s):  
Real Time ◽  
Potential Temperature ◽  
Thermodynamic Quantity ◽  
Kinematics And Dynamics ◽  
Horizontal Gradient ◽  
Model Output ◽  
Equivalent Potential Temperature ◽  
Thermal Front ◽  
Equivalent Potential ◽  
Definition Of

AbstractFronts can be computed from gridded datasets such as numerical model output and reanalyses, resulting in automated surface frontal charts and climatologies. Defining automated fronts requires quantities (e.g., potential temperature, equivalent potential temperature, wind shifts) and kinematic functions (e.g., gradient, thermal front parameter, and frontogenesis). Which are the most appropriate to use in different applications remains an open question. This question is investigated using two quantities (potential temperature and equivalent potential temperature) and three functions (magnitude of the horizontal gradient, thermal front parameter, and frontogenesis) from both the context of real-time surface analysis and climatologies from 38 years of reanalyses. The strengths of potential temperature to identify fronts are that it represents the thermal gradients and its direct association with the kinematics and dynamics of fronts. Although climatologies using potential temperature show features associated with extratropical cyclones in the storm tracks, climatologies using equivalent potential temperature include moisture gradients within air masses, most notably at low latitudes that are unrelated to the traditional definition of a front, but may be representative of a broader definition of an airmass boundary. These results help to explain previously published frontal climatologies featuring maxima of fronts in the subtropics and tropics. The best function depends upon the purpose of the analysis, but Petterssen frontogenesis is attractive, both for real-time analysis and long-term climatologies, in part because of its link to the kinematics and dynamics of fronts. Finally, this study challenges the conventional definition of a front as an airmass boundary and suggests that a new, dynamically based definition would be useful for some applications.

scholarly journals GoAmazon2014/5 campaign points to deep-inflow approach to deep convection across scales

2018 ◽  
Vol 115 (18) ◽  
pp. 4577-4582 ◽  
Author(s):  
Kathleen A. Schiro ◽  
Fiaz Ahmed ◽  
Scott E. Giangrande ◽  
J. David Neelin
Keyword(s):  
Climate Models ◽  
Climate Model ◽  
Deep Convection ◽  
Potential Temperature ◽  
Strong Relationship ◽  
Field Campaign ◽  
Convective Systems ◽  
Tropical Oceans ◽  
Mesoscale Convective ◽  
Environmental Air

A substantial fraction of precipitation is associated with mesoscale convective systems (MCSs), which are currently poorly represented in climate models. Convective parameterizations are highly sensitive to the assumptions of an entraining plume model, in which high equivalent potential temperature air from the boundary layer is modified via turbulent entrainment. Here we show, using multiinstrument evidence from the Green Ocean Amazon field campaign (2014–2015; GoAmazon2014/5), that an empirically constrained weighting for inflow of environmental air based on radar wind profiler estimates of vertical velocity and mass flux yields a strong relationship between resulting buoyancy measures and precipitation statistics. This deep-inflow weighting has no free parameter for entrainment in the conventional sense, but to a leading approximation is simply a statement of the geometry of the inflow. The structure further suggests the weighting could consistently apply even for coherent inflow structures noted in field campaign studies for MCSs over tropical oceans. For radar precipitation retrievals averaged over climate model grid scales at the GoAmazon2014/5 site, the use of deep-inflow mixing yields a sharp increase in the probability and magnitude of precipitation with increasing buoyancy. Furthermore, this applies for both mesoscale and smaller-scale convection. Results from reanalysis and satellite data show that this holds more generally: Deep-inflow mixing yields a strong precipitation–buoyancy relation across the tropics. Deep-inflow mixing may thus circumvent inadequacies of current parameterizations while helping to bridge the gap toward representing mesoscale convection in climate models.

Surface Characteristics of Observed Cold Pools

2008 ◽  
Vol 136 (12) ◽  
pp. 4839-4849 ◽  
Author(s):  
Nicholas A. Engerer ◽  
David J. Stensrud ◽  
Michael C. Coniglio
Keyword(s):  
Life Cycle ◽  
Potential Temperature ◽  
Surface Characteristics ◽  
Life Cycle Stage ◽  
Cold Pool ◽  
Convective System ◽  
Cold Pools ◽  
Equivalent Potential ◽  
Life Cycle Stages ◽  
The Mean

Abstract Cold pools are a key element in the organization of precipitating convective systems, yet knowledge of their typical surface characteristics is largely anecdotal. To help to alleviate this situation, cold pools from 39 mesoscale convective system (MCS) events are sampled using Oklahoma Mesonet surface observations. In total, 1389 time series of surface observations are used to determine typical rises in surface pressure and decreases in temperature, potential temperature, and equivalent potential temperature associated with the cold pool, and the maximum wind speeds in the cold pool. The data are separated into one of four convective system life cycle stages: first storms, MCS initiation, mature MCS, and MCS dissipation. Results indicate that the mean surface pressure rises associated with cold pools increase from 3.2 hPa for the first storms’ life cycle stage to 4.5 hPa for the mature MCS stage before dropping to 3.3 hPa for the dissipation stage. In contrast, the mean temperature (potential temperature) deficits associated with cold pools decrease from 9.5 (9.8) to 5.4 K (5.6 K) from the first storms to the dissipation stage, with a decrease of approximately 1 K associated with each advance in the life cycle stage. However, the daytime and early evening observations show mean temperature deficits over 11 K. A comparison of these observed cold pool characteristics with results from idealized numerical simulations of MCSs suggests that observed cold pools likely are stronger than those found in model simulations, particularly when ice processes are neglected in the microphysics parameterization. The mean deficits in equivalent potential temperature also decrease with the MCS life cycle stage, starting at 21.6 K for first storms and dropping to 13.9 K for dissipation. Mean wind gusts are above 15 m s−1 for all life cycle stages. These results should help numerical modelers to determine whether the cold pools in high-resolution models are in reasonable agreement with the observed characteristics found herein. Thunderstorm simulations and forecasts with thin model layers near the surface are also needed to obtain better representations of cold pool surface characteristics that can be compared with observations.

scholarly journals MSE Minus CAPE is the True Conserved Variable for an Adiabatically Lifted Parcel

2015 ◽  
Vol 72 (9) ◽  
pp. 3639-3646 ◽  
Author(s):  
David M. Romps
Keyword(s):  
Potential Energy ◽  
Thermodynamic Equilibrium ◽  
Convective Available Potential Energy ◽  
Potential Temperature ◽  
Equivalent Potential Temperature ◽  
Moist Static Energy ◽  
Neutral Buoyancy ◽  
Equivalent Potential ◽  
Nonhydrostatic Pressure ◽  
Static Energy

Abstract For an adiabatic parcel convecting up or down through the atmosphere, it is often assumed that its moist static energy (MSE) is conserved. Here, it is shown that the true conserved variable for this process is MSE minus convective available potential energy (CAPE) calculated as the integral of buoyancy from the parcel’s height to its level of neutral buoyancy and that this variable is conserved even when accounting for full moist thermodynamics and nonhydrostatic pressure forces. In the calculation of a dry convecting parcel, conservation of MSE minus CAPE gives the same answer as conservation of entropy and potential temperature, while the use of MSE alone can generate large errors. For a moist parcel, entropy and equivalent potential temperature give the same answer as MSE minus CAPE only if the parcel ascends in thermodynamic equilibrium. If the parcel ascends with a nonisothermal mixed-phase stage, these methods can give significantly different answers for the parcel buoyancy because MSE minus CAPE is conserved, while entropy and equivalent potential temperature are not.

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