Gaoyuan qixiang (Dec 2022)
Water Vapor Condition and Instability Analysis of a Rainstorm in Northern Guanzhong
Abstract
Conventional observation and EC-thin data are used to analyze the forecasting difficulties of a severe rainstorm event on 14 September 2019 in northern Guanzhong.Results show that the rainstorm occurred in the process of trough a loft moving eastwards and the Western Pacific subtropical high receding southeastwards.At initial stage of rainstorm， northern Guanzhong lies in the cold high 240~300 km behind the cold front on surface， on the north side of the transverse shear on 700 hPa level.The sounding graph demonstrates a stable stratification of frontal temperature inversion on lower-middle level with a deep cold pad below， lack of water vapor on lower level， and low energy on surface.The 1-hourly peak rainfall accumulation is 7 mm during the entire rain process obtained by national observation station， which doesn’t fall in convective torrential rain， but frontal rain caused by the cold front continuously for a certain time.Analyzing water vapor in northern Guanzhong， it is found that a large value area of water vapor is concentrated on 700 hPa level during the heavy rainfall episode.The southerly wind on 700 hPa level strengthened over time brings it with plenty of water vapor， and drives a large value area of specific humidity and the shear line northwards to northern Guanzhong， which provides strong water vapor convergence conditions for the rainstorm.Therefore， whether the southerly wind over northern Guanzhong can increase over time is the key to the occurrence of the rainstorm.The coupling of upper-lower level jets and the release of latent heat of condensation are two important factors affecting southerly wind over northern Guanzhong.The ascending branch of the secondary circulation formed by the coupling of upper-lower level jets is favorable for depressurization on mid-low levels over the rainstorm area.The increase of pressure gradient between the western Pacific subtropical high and the rainstorm area will strengthen the lower-level southwesterly jet and drives it northeastwards.The enhanced lower-level jet will converge and rise in front of it， inducing rainstorm， which in due cause depressurizes on mid-low levels， further increases the pressure gradient between the western Pacific subtropical high and the rainstorm area， and the lower-level jet is strengthened.This forms a positive feedback of the lower-level jet and rainstorm， resulting in lower-level jet over southern Guanzhong and enhanced southerly wind over northern Guanzhong.The release of latent heat over the rainstorm area is favorable for upward movement， depressurizes on mid-low levels， which also enlarges pressure gradient between the rainstorm area and the western Pacific subtropical high， inducing a lower-level jet over southern Guanzhong and enhancing the southerly wind over northern Guanzhong.A strong vertical and a northward transverse geostrophic wind shear exists on 700~600 hPa level over the rainstorm area in northern Guanzhong， while the southerly wind transports water vapor and energy， which induces conditional symmetrical instability on this level.The southerly wind ascends along the front over the rainstorm area， converges up to the area of symmetrical instability， where produces a slant convection， which causes the rainstorm in northern Guanzhong.Under the situation of the western Pacific subtropical high retreating southeastwards and a cold high dominating on surface， most often， forecasters will consider that water vapor and energy are sufficient in southerly area， and miss the influence of southerly wind enhanced by the coupling of upper and lower jet streams and the release of latent heat of condensation， which leads to a weaker precipitation forecast in northern area.For this kind of rainstorm process， forecast should focus on the trend of the western Pacific subtropical high， the coupling of upper- lower jets， the influence of release of condensation latent heat on the lower-level jet and the southerly wind， and mechanism of possible conditional symmetric instability， combined with numerical model output， come to a comprehensive conclusion.
Keywords
