Generalized Moist Potential Vorticity Research Articles

Incorporating the Effects of Moisture into a Dynamical Parameter: Moist Vorticity and Moist Divergence

Abstract Properly including moisture effects into a dynamical parameter can significantly increase the parameter’s ability to diagnose heavy rain locations. The relative humidity–based weighting approach used to extend the moist potential vorticity (MPV) to the generalized moist potential vorticity (GMPV) is analyzed and demonstrates such an improvement. Following the same approach, two new diagnostic parameters, moist vorticity (MV) and moist divergence (MD), have been proposed in this study by incorporating moisture effects into the traditional vorticity and divergence. A regional heavy rain event that occurred along the Yangtze River on 1 July 1991 is used as a case study, and 41 daily regional heavy rain events during the notorious flooding year of 1998 in eastern China are used for a systematic evaluation. Results show that after the moisture effects were properly incorporated, the improved ability of all three parameters to capture a heavy rain area is significant (statistically at the 99% confidence level): the GMPV is improved over the MPV by 194%, the MD over the divergence by 60%, and the MV over the vorticity by 34% in terms of the threat score (TS). The average TS is 0.270 for the MD, 0.262 for the MV, and 0.188 for the GMPV. Application of the MV and MD to assess heavy rain potential is not intended to replace a complete, multiscale forecasting methodology; however, the results from this study suggest that the MV and MD could be used to postprocess a model forecast to potentially improve heavy rain location predictions.

Mesoscale dynamics and its application in torrential rainfall systems in China

Progress over the past decade in understanding moisture-driven dynamics and torrential rain storms in China is reviewed in this paper. First, advances in incorporating moisture effects more realistically into theory are described, including the development of a new parameter, generalized moist potential vorticity (GMPV) and an improved moist ageostrophic Q vector (Qum). Advances in vorticity dynamics are also described, including the adoption of a “parcel dynamic” approach to investigate the development of the vertical vorticity of an air parcel; a novel theory of slantwise vorticity development, proposed because vorticity develops easily near steep isentropic surfaces; and the development of the convective vorticity vector (CVV) as an effective new tool. The significant progress in both frontal dynamics and wave dynamics is also summarized, including the geostrophic adjustment of initial unbalanced flow and the dual role of boundary layer friction in frontogenesis, as well as the interaction between topography and fronts, which indicate that topographic perturbations alter both frontogenesis and frontal structure. For atmospheric vortices, mixed wave/vortex dynamics has been extended to explain the propagation of spiral rainbands and the development of dynamical instability in tropical cyclones. Finally, we review wave and basic flow interaction in torrential rainfall, for which it was necessary to extend existing theory from large-scale flows to mesoscale fields, enriching our knowledge of mesoscale atmospheric dynamics.

PV-based wave-activity density and its application to tracing heavy precipitation

This paper is focused on the tracking of heavy precipitation by detecting strong wave activities in precipitating atmosphere. Based on the generalized moist potential vorticity (GMPV), a new wave-activity density and its wave-activity relation are first derived. The wave-activity density, which is the second-order portion of GMPV and quadratic in disturbance, is then applied to a heavy-rainfall event in North China. It is shown that the wave-activity density contains the vertical shear of wind perturbation, vertical vorticity perturbation and the spatial gradients of latent heating function perturbation associated with the moisture condensation. Due to these important characteristics of precipitating atmosphere, the wave-activity density shows strong anomalies over the precipitation region. The total GMPV and its basic-state and first-order components are also analyzed as comparisons. The result shows that the basic-state GMPV is not capable of diagnosing precipitation, while the total GMPV and the first-order GMPV, although present strong anomalies, are not capable of distinguishing precipitation and non-precipitation areas. This is likely due to the basic state information contained in GMPV and its basic-state and first-order components, which has no direct relation to the precipitation. The spatial distribution of wave-activity density further verifies its capability on detecting and tracing heavy precipitation. Moreover, the statistical result reveals the wave-activity density has a high correlation coefficient with the observational rainfall in a long time series and passes through the significance test.

Diagnostic Analyses and Applications of the Moisture Helicity and the Thermal Helicity for Two Strong “Sauna” Weather Processes in Northern China

Abstract“Sauna” weathers generally occur in high‐temperature environments during summertime. However, it is difficult to diagnose sultry “sauna” weather only based on the single temperature parameter because of its large humidity. Previous studies show that the relatively stable subtropic‐high circulation dominates the middle troposphere and weak sinking motion maintains during “sauna” weather events. Due to its special hot and humid environment and circulation conditions, it is of particular importance to find a simple and convenient parameter to characterize high‐temperature and high‐humidity “sauna” weather. Based on previous research, the IM (Ingredients‐based methodology) method, which is developed for rainfall studies in the past, is extended to the study of “sauna” weather in this paper. Several physical variables such as high temperature, large relative humidity, and sinking motion, which are used as single factors to describe “sauna” weather in the past studies, are combined into IM theory to diagnose “sauna” weather. In terms of the IM method, thermal helicity (TH), moisture helicity (MH), and the moist thermal helicity (MTH), are introduced to dynamically identify “sauna” weather. Two “sauna” weather events in North China are selected to calculate above several parameters. One is from 30 July to 4 August 2002. Another is from 7 to 9 July 2009. And the generalized moist potential vorticity (GMPV) is used to make a comparison with them. It is found that the MH anomalies accompany the “sauna” weather processes, concentrate in the lower troposphere around Beijing and its peripheral areas in North China. However, the coverage of TH anomalies is larger than our target region of the “sauna” weather events. Even for the MTH and GMPV including both temperature and humidity effects, it is also worse to use their anomalies areas to dynamically identify “sauna” weather than using MH's anomalies. It implies that humidity is a determining factor during the sultry “sauna” weather process in North China in general high‐temperature summertime. The MH, which integrates both the atmospheric circulation characteristics and humidity effect, is a possible optimization factor to dynamically identify “sauna” weather in North China. And it has great value of practical application to analysis of “sauna” weather.

PV‐based diagnostic quantities of heavy precipitation: Solenoidal vorticity and potential solenoidal vorticity

Two new diagnostic quantities called solenoidal vorticity (SV) and potential solenoidal vorticity (PSV) are defined in this paper for a moist atmosphere based on the generalized moist potential vorticity (GMPV). SV and PSV are introduced to incorporate vorticity information along the isentropic surfaces that GMPV (or PV) has lost. SV is the scalar product of vorticity and the moist solenoidal term ∇ p × ∇ α*, while PSV is the scalar product of vorticity and (∇ p × ∇ α*) × ∇ θ* (a new vector). Application of GMPV, SV, and PSV to a diagnostic study of a heavy rainfall case shows that SV reflects the coupling of vertical wind shear and the baroclinity of the moist atmosphere weighted by the vertical gradient of pressure, while PSV reflects the coupling of vertical wind shear and baroclinity, the static stability, and the vertical gradient of pressure. Although all three quantities exhibit strong signals over the observed precipitation region, SV and PSV are more capable of distinguishing between precipitation and non‐precipitation areas. The following three main factors are concluded to contribute to the very different distribution patterns of GMPV, SV, and PSV: static stability,the vertical gradient of pressure, and different ways of coupling vertical wind shear and baroclinity. These factors are responsible for the good performance of SV and PSV in tracing and predicting heavy rainfall, as demonstrated by the correlation coefficient between SV (PSV) and the rainfall amount obtained from long time series analyses. The results of this study suggest that SV and PSV are good detectors of heavy rainfall and may be used as precipitation prediction parameters.

On the Significance and Use of the Generalized Moist Potential Vorticity Equation

In this paper, the continuity and thermody-namic equations including moisture forcings were derived. Using these two equations and the basic momentum equation of local Cartesian coordinates, the bu...

Diagnostic study of generalized moist potential vorticity in a non‐uniformly saturated atmosphere with heavy precipitation

AbstractA Meiyu front accompanied by a band of heavy precipitation in East Asia is typically characterized by a much larger moisture gradient than temperature gradient. Many previous studies have suggested use of equivalent potential temperature as a thermodynamic variable under this circumstance. However, dynamic variables coupled with such a thermodynamic variable, e.g. a derived moist potential vorticity (MPV) based on equivalent potential temperature, does not provide useful dynamic insight into these systems. In this study, generalized moist potential vorticity (GMPV) is derived based on a generalized form of potential temperature. Diagnoses of numerical simulations for three typical Meiyu rainfall events show that GMPV provides remarkably accurate tracking of rainfall location, suggesting its potential use as a dynamic tracer for heavy rainfall events such as Meiyu rain bands. Copyright © 2010 Royal Meteorological Society

Physical basis of generalized potential temperature and its application to cyclone tracks in nonuniformly saturated atmosphere

This paper presents the physical basis and the conservation property of the generalized potential temperature θ* from the first law of thermodynamics. Here θ* is defined when the commonly used step function is replaced by a smooth function of relative humidity to some power. This new variable, which includes the weighting function acting as a condensation probability factor in the expression of specific humidity, may be more appropriate to describe the property of nonuniform saturation of the real atmosphere. Generalized moist potential vorticity (GMPV), defined as αζa · ∇θ* as expressed by Gao et al. (2004) (α is the specific volume and ζa is the absolute vorticity), is studied theoretically on its application to trace cyclones. It is shown through a case study that cyclone tracks can be well represented by the low negative GMPV centers. Thus the GMPV anomaly may be used as an alternative tool to the traditional use of surface pressure in cyclone tracks and to make a short‐term prediction. What is more, other potential usages of GMPV are also pointed out in this study.

Analyses of hot and humid weather in Beijing city in summer and its dynamical identification

The circulation, hygrothermal property and moisture transport character of typical hot and humid weather were analyzed in Beijing areas from July 30 to August 4, 2002. It was pointed out that, under the control of subtropical anticyclone which stretches to the west and north, downdraft suppresses the lifting of lower-troposphere moisture, which makes moisture keep in the lower troposphere. That is the direct reason causing hot and humid weather. Considering the non-uniformity saturated character in real atmosphere, generalized moist potential vorticity (GMPV) equation is derived by the introduction of generalized moist potential temperature concept. The analysis of GMPV shows that negative GMPV anomaly occurs in the lower troposphere. It has indicative sense to hot and humid weather. Thus, the GMPV anomaly can be utilized to identify this kind of weather and to make a short-term prediction.

Generation of generalized moist potential vorticity in a frictionless and moist adiabatic flow

The discontinuity of the latent heat term could occur in the thermodynamic equation in the transition area between the unsaturated and saturated air where the latent heat is released associated with the condensation in the saturated air whereas the latent heat is not released in the unsaturated air. To fix the discontinuity, the condensation probability function ()k (q and qs are specific humidity and saturated specific humidity respectively, k = 9) is introduced into the thermodynamic framework with ()kqs. The generalized moist potential vorticity (GMPV) is thus defined and its tendency equation is derived. The analysis shows that in a frictionless and moist adiabatic flow, the GMPV can be generated in the unsaturated air only and its generation rate is determined by the environmental moisture and its gradient.