Diagnostic Analyses and Application of the Moist Ageostrophic Vector Q

Considering the main thermal forcing factor, which is critical for the development of synoptic systems, the concept of the moist ageostrophic vector Q is introduced. A formula of the moist ageostrophic Q and the ageostrophic diabatic equation, in which the divergence of the moist ageostrophic Q is taken as a single forcing term, is derived. Meanwhile, the moist ageostrophic Q is applied to diagnose a torrential rain process in North China. The results suggest that the moist ageostrophic Q can clearly reveal the system development during the torrential rain process; the corresponding relationship between the divergence of the moist ageostrophic Q and the rainfall area is better than that of the vertical velocity (w) and the divergence of the dry Q; the 6-h rainfall region can be correctly drawn according to the negative area of the divergence of the moist ageostrophic Q, and its precipitation is positively correlated to the magnitude of the divergence of the moist ageostrophic Q. The research provides valuable information for improving short-term weather forecast.

A Modified Moist Ageostrophic Q Vector

The quasi-geostrophic Q vector is an important rainfall associated with large-scale weather systems diagnostic tool for studying development of surface and is calculated using data at single vertical level. When ageostrophic Q vector was introduced, it required data at two vertical levels. In this study, moist ageostrophic Q vector is modified so that it can be calculated using data at a single vertical level. The comparison study between the original and modified moist ageostrophic Q vectors is conducted using the data from 5 to 6 July 1991 during the torrential rainfall event associated with the Changjiang-Huaihe mei-yu front in China. The results reveal that divergences of original and modified moist ageostrophic Q vectors have similar horizontal distributions and their centers are almost located in the precipitation centers. This indicates that modified moist ageostrophic Q vector can be used to diagnose convective development with reasonable accuracy.

The Effects of Strong Ageostrophic Outflows on the Formation of Surface Mesoscale Pressure Systems in Squall Lines

Based on the previous study of the streamline field triggered by singularities in a two-dimensional potential flow,the wind field caused by vorticity lines in an incompressible flow is deduced in this paper.The result shows an elliptic cyclonic（anticyclonic） circulation in association with a positive（negative） vorticity line.By use of the shallow-water model,the flow fields are simulated in a weak wind background under the influence of mesoscale vorticity lines.In the case of two vorticity line,one positive and the other negative,a mesoscale vortex couplet forms in the flow.When three vorticity lines are considered,three mesoscale circulations develop,and a mesohigh and two mesolows similar to the thunderstorm high,wake low and pre-squall mesolow of a mature squall line are produced.Theoretical analysis and numerical simulations show that the formation of the surface mesoscale pressure systems in squall lines may be partly attributed to the dynamical effects of the ageostrophic outflows.The strong downdrafts under the thundercloud base of the squall line lead to surface ageostrophic outflows,and produce positive-negative-positive arranged vertical vorticity bands（VBs） along the direction normal to the squall line,then the mesoscale circulations develop and mesoscale pressure systems form or strengthen during the geostrophic adjustment.By use of the scale separation method,this dynamic mechanism is confirmed by a case study of a severe storm passing over eastern China on 17 June 1974.

Eddy Kinetic Energy Study of the Snowstorm over Southern China in January 2008

The energetics of the third stage of a snowstorm over China was analyzed using ECWMF data.The analysis of the energy budget for the Middle East trough and the western Pacific trough that developed toward China on 25-28 January 2008 showed the advection of the geopotential by the ageostrophic wind to be both a crucial source and the primary sink of the eddy kinetic energy centers associated with the troughs.The magnitudes of the energy conversion terms,interaction kinetic energy conversion and baroclinic conversion,were too small to explain the development of the energy centers and the jet streaks.The energy centers gained energy at their entrance regions via the convergence of the ageostrophic geopotential fluxes,and then lost energy at their exit regions by the same fluxes.At the entrance regions,the fluxes converged,increasing the geopotential gradient,which generated a stronger geostrophic wind and higher kinetic energy,resulting in an ascending motion in this area.When the troughs moved to China,the ascending motion caused by the convergence of the fluxes at entrance region intensified the snowstorms over central and southern China.

The Effect of the Subtropical Jet on the Rainfall over Southern China in January 2008

ABSTRACT The third precipitation episode of China＇s great snowstorms of 2008 was analyzed using station observations and ECMWF six-hourly data. The variation of the shape of the upper-level subtropical jet played an important role in the rainfall over south- ern China. With the eastward movement of the trough, the jet shape changed from two straight jets to a tilting jet over China and then it moved southward. With these variations, the south-north movement of ascending flow and precipitation area over southern China occurred.