Midlatitude Tropopause and Low-Level Moisture

2014 ◽  
Vol 71 (3) ◽  
pp. 1187-1200 ◽  
Author(s):  
Yutian Wu ◽  
Olivier Pauluis
Keyword(s):  
Interannual Variability ◽  
Northern Hemisphere ◽  
Annual Cycle ◽  
Gaussian Approximation ◽  
Potential Temperature ◽  
Department Of Energy ◽  
Equivalent Potential Temperature ◽  
Surface Distribution ◽  
Equivalent Potential

Abstract A new relationship between the surface distribution of equivalent potential temperature and the potential temperature at the tropopause is proposed. Using a Gaussian approximation for the distribution of equivalent potential temperature, the authors argue that the tropopause potential temperature is approximately given by the mean equivalent potential temperature at the surface plus twice its standard derivation. This relationship is motivated by the comparison of the meridional circulation on dry and moist isentropes. It is further tested using four reanalysis datasets: the Interim ECMWF Re-Analysis (ERA-Interim); the NCEP–Department of Energy (DOE) Reanalysis II; the NCEP Climate Forecast System Reanalysis; and the Twentieth-Century Reanalysis (20CR), version 2. The proposed relationship successfully captures the annual cycle of the tropopause for both hemispheres. The results are robust among different reanalysis datasets, albeit the 20CR tends to overestimate the tropopause potential temperature. Furthermore, the proposed mechanism also works well in obtaining the interannual variability (with climatological annual cycle removed) for Northern Hemisphere summer with an above-0.6 correlation across different reanalyses. On the contrary, this mechanism is rather weak in explaining the interannual variability in the Southern Hemisphere and no longer works for Northern Hemisphere wintertime. This work suggests the important role of the moist dynamics in determining the midlatitude tropopause.

Interannual Variability of Monsoon Precipitation and Local Subcloud Equivalent Potential Temperature

2013 ◽  
Vol 26 (23) ◽  
pp. 9507-9527 ◽  
Author(s):  
John V. Hurley ◽  
William R. Boos
Keyword(s):  
Interannual Variability ◽  
Southern Oscillation ◽  
Potential Temperature ◽  
Specific Humidity ◽  
Quasi Equilibrium ◽  
Equivalent Potential Temperature ◽  
Monsoon Precipitation ◽  
Relevant Variable ◽  
Near Surface ◽  
Equivalent Potential

The interannual variability of monsoon precipitation is described in the context of a convective quasi-equilibrium framework. Using two reanalysis products and two global precipitation datasets, the authors examine linear relationships between seasonal anomalies of precipitation and subcloud equivalent potential temperature (θeb) local to six monsoon regions. This approach provides a single near-surface thermodynamically relevant variable over both land and ocean, extending previous studies of interannual monsoon variability that emphasized ocean surface temperatures. After removing the variability linearly associated with an index of the El Niño–Southern Oscillation, positive monsoon precipitation anomalies are shown to be associated with enhanced θeb local to and slightly poleward of the climatological θeb maximum. The variations in continental θeb local to the monsoon precipitation maxima are mainly due to variations in subcloud specific humidity, with changes in subcloud temperature having the opposite sign. Motivated by the fact that some of these subcloud humidity anomalies occur over deserts poleward of monsoon regions, the relationship of 700-hPa flow with precipitation is examined, and enhanced precipitation in several regions is found to covary with the properties of shallow meridional circulations. The implications of these results for the understanding of monsoon interannual variability are discussed.

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 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.

scholarly journals Thermodynamic Analysis of Supercell Rear-Flank Downdrafts from Project ANSWERS

2007 ◽  
Vol 135 (1) ◽  
pp. 240-246 ◽  
Author(s):  
Matthew L. Grzych ◽  
Bruce D. Lee ◽  
Catherine A. Finley
Keyword(s):  
Potential Temperature ◽  
Surface Wind ◽  
Thermodynamic Characteristics ◽  
General Concept ◽  
Equivalent Potential Temperature ◽  
Near Surface ◽  
Project Analysis ◽  
Equivalent Potential ◽  
Virtual Potential Temperature ◽  
Convective Inhibition

Abstract Data collected during Project Analysis of the Near-Surface Wind and Environment along the Rear-flank of Supercells (ANSWERS) provided an opportunity to test recently published associations between rear-flank downdraft (RFD) thermodynamic characteristics and supercell tornadic activity on a set of 10 events from the northern plains. On average, RFDs associated with tornadic supercells had surface equivalent potential temperature and virtual potential temperature values only slightly lower than storm inflow values. RFDs associated with nontornadic supercells had mean group equivalent potential temperature and virtual potential temperature values that were colder relative to storm inflow values than their respective tornadic counterparts. Additionally, the analysis revealed that RFDs associated with tornadic supercells had higher CAPE and lower convective inhibition than the RFDs of nontornadic supercells, on average. The results of this study provide further support for the general concept that a thermodynamic delineation generally exists between the RFDs of tornadic and nontornadic supercells.

The Global Atmospheric Circulation in Moist Isentropic Coordinates

2010 ◽  
Vol 23 (11) ◽  
pp. 3077-3093 ◽  
Author(s):  
Olivier Pauluis ◽  
Arnaud Czaja ◽  
Robert Korty
Keyword(s):  
Mass Transport ◽  
Total Mass ◽  
Potential Temperature ◽  
Equivalent Potential Temperature ◽  
Moist Air ◽  
Low Level ◽  
Equivalent Potential ◽  
Entropy Transport ◽  
Mean Circulation ◽  
The Tropics

Abstract Differential heating of the earth’s atmosphere drives a global circulation that transports energy from the tropical regions to higher latitudes. Because of the turbulent nature of the flow, any description of a “mean circulation” or “mean parcel trajectories” is tied to the specific averaging method and coordinate system. In this paper, the NCEP–NCAR reanalysis data spanning 1970–2004 are used to compare the mean circulation obtained by averaging the flow on surfaces of constant liquid water potential temperature, or dry isentropes, and on surfaces of constant equivalent potential temperature, or moist isentropes. While the two circulations are qualitatively similar, they differ in intensity. In the tropics, the total mass transport on dry isentropes is larger than the circulation on moist isentropes. In contrast, in midlatitudes, the total mass transport on moist isentropes is between 1.5 and 3 times larger than the mass transport on dry isentropes. It is shown here that the differences between the two circulations can be explained by the atmospheric transport of water vapor. In particular, the enhanced mass transport on moist isentropes corresponds to a poleward flow of warm moist air near the earth’s surface in midlatitudes. This low-level poleward flow does not appear in the zonally averaged circulation on dry isentropes, as it is hidden by the presence of a larger equatorward flow of drier air at same potential temperature. However, as the equivalent potential temperature in this low-level poleward flow is close to the potential temperature of the air near the tropopause, it is included in the total circulation on moist isentropes. In the tropics, the situation is reversed: the Hadley circulation transports warm moist air toward the equator, and in the opposite direction to the flow at upper levels, and the circulation on dry isentropes is larger than that on moist isentropes. The relationship between circulation and entropy transport is also analyzed. A gross stratification is defined as the ratio of the entropy transport to the net transport on isentropic surfaces. It is found that in midlatitudes the gross stability for moist entropy is approximately the same as that for dry entropy. The gross stratification in the midlatitude circulation differs from what one would expect for either an overturning circulation or horizontal mixing; rather, it confirms that warm moist subtropical air ascends into the upper troposphere within the storm tracks.

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