Observational Analysis on a Cold Front Heavy Rain Using Doppler Radar Data

Forced by cold front and low-level jet, a regionally torrential rainfall occurs in the east of Hubei Province during June 22—23, 2003. On 22 June, a synoptic-scale cyclone is developing over the reaches of the Changjiang and Huaihe River. Leaded by the deepening cyclone, the ground cold front moves southward and triggers meso-scale convective system (MCS) when it confronts with southwest low-level jet. The MCS perches on the tail of the comma-pattern cloud system in infrared weather satellite image. The MCS moves southward, strengthens and results in heavy rain in the east of Hubei Province.Wuhan Doppler radar collects the data set during the heavy rain period. The radial velocity and spectrum width features of the cold front and the low-level jet on Doppler radar are analyzed in detail using base velocity, velocity cross section, base spectrum width data collected by Wuhan Doppler radar every six minutes. Also, combining analysis on radar base reflectivity and surface rain-gages data, the synoptic relationships between the spread of the MCS and the cold front, low-level jet are discussed briefly. The results are as follows: ① There is a convergence line corresponding to cold front on base velocity product. Because Doppler radar only detects radial velocity of the wind, the convergence line gets short gradually when it moves towards radar station. Spectrum width is high near the cold front for relatively high vertical wind shear in low level of the troposphere, which helps to determine the position of the cold front. ② The salient features of the meso-scale low-level jet on base velocity product include big radial velocity and big tangent gradient of the radial velocity, and the farther the gale core is apart from radar station, the clearer the big tangent gradient behaves, which is called "great gale core" detected by Doppler radar many times. ③ The MCS that brings regional torrential rainfall is jointly resulted from cold front and low-level jet. At the intense developing stage of the MCS, the strong low-level jet climbs along cold surface front quickly and steeply. At weakening stage of the MCS, rainfall intensity decreases because of lessened gradient of the cold front though there is a divergence flow in troposphere aloft. In addition, the heavy rain occurs near the join point of the cold front and low-level jet and on the left of the jet. As the direction and position of the low-level jet is fixed, the join point can shift to west when cold front moved to south. Accordingly, MCS strengthens in the west of the join point for horizontally cyclonic vortex on the left of the jet, and weakens in the east of it for horizontally anti-cyclonic vortex on the right of it.