Analysis on the Moist Potential Vorticity and Conditional Symmetric Instability of a Rainstorm

[Objective] The research aimed to analyze the moist potential vorticity (MPV) and conditional symmetric instability (CSI) of a rainstorm in Guangxi in the prior flood season. [Method] Based on the conventional observation data, precipitation data from the automatic station and 1°×1° six-hourly reanalyzed data from NCEP, MPV and CSI in a rainstorm process which occurred in the prior flood season in Guangxi were analyzed. The characteristics of MPV component and role of CSI before and after the rainstorm occurrence were discussed. [Result] The dense belt of isoline in the southeast of MPV1 positive-value area (MPV2 negative-value area) at 925 hPa, the underneath of sloping MPV1 positive-value column (MPV2 negative-value column) on the profile map and the underneath of dense belt of θe isoline had the good corresponding relationships with the falling zone of strong precipitation. After the system developed strongly, the enhancements of gradient and intensity of MPV1 and MPV2 predicted the rainstorm increase. In the beginning period of strong precipitation occurrence, the convective instability at 850-650 hPa co-existed with CSI. The convective instability and vertical movement played the main role. In the middle and latter periods of strong precipitation, the high-altitude weak cold air further invaded southward. It triggered the release of unstable energy, and compelled that the original sloping θe dense belt became steep. It was favorable for the development of sloping vorticity, and the atmosphere gradually turned into the neutral state of convection. CSI and ramp motion played the major role. Then, the rainstorm continued. [Conclusion] The research provided the theory basis for the application of MPV and CSI into the local routine business.

Study on Moist Potential Vorticity and Symmetric Instabilityduring a Heavy Rain Event Occurred inthe Jiang-Huai Valleys

In the light of the theory on moist potential vorticity (MPV) investigation was undertaken of the 700 hPa vertical (horizontal) component MP1 (MPV2) for the heavy rain event occurring in July 5–6, 1991. Results show that the distribution features of the two components were closely related to the development of a mesoscale cyclone as a rainstorm-causing weather system in the lower troposphere in such a way that the ambient atmosphere of which MPV1 > 0 and MPV2 < 0 with |MPV1| ≥ |MPV2| favored the genesis of conditional symmetric instability (CSI) and that, as indicated by calculations, a CSI sector was really existent in the lower troposphere during the heavy rain happening and contributed greatly to its development.

Development of an Integrated Vertical-Slantwise Convective Parameterization Scheme and Its Associated Numerical Experiments

An integrated vertical-slantwise convective parameterization scheme, based on the vertical Kuo-Anthes and the slantwise Nordeng convective parameterization schemes, is introduced into the MM5 model. By employing the MM5 model with the proposed scheme, numerical simulations of a snowstorm event that occurred over southern China on 28-29 January 2008 and of Typhoon Haitang （2005） are conducted. The results indicate that during the snowstorm event, the atmosphere was convectively stable in the vertical direction but with conditional symmetric instability （CSI） in the lower troposphere, and when the area of CSI developed and extended to upper levels, strong rising motion occurred and triggered the release of large amount of energy, producing enhanced convective precipitation with the total precipitation much closer to the observation. The development and strengthening of CSI corresponded to changes in the intensity of snowfall, convergence, and ascending motions of air, revealing that CSI was responsible for the initiation and growth of the snowstorm. The results from a 72-h explicit simulation of Typhoon Haitang indicate that CSI occurred mainly at lower levels with a well-defined spiral structure, and it tended to have a larger impact on the intensity of typhoon than on its track. The minimum pressure at the typhoon center for the 72-h runs with the integrated vertical-slantwise convective parameterization scheme was on average 3 hPa （maximum 8 hPa） lower than that from the runs with only the vertical cumulus parameterization scheme. Introducing the influence of CSI into the model has improved the warm core structure at the middle and upper levels of the typhoon, with stronger and persistent upward motions causing increased precipitation, and the latent heat released through convection in turn made the typhoon develop further.