Diagnosis of a Heavy Rainfall Event in 1988 Caused by Easterly Wave
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Abstract:
The heavy rainfall process occurred on 30 July 1988 around the southeastern coast of Zhejiang Province is investigated by 05° NCEP Climate Forecast System Reanalysis data. It is found that the heavy rainfall was mainly caused by the merging of two low level east wind perturbations from south and north, respectively, and its interaction with upper level jets. The south and north branches of the easterly wave originated from the abrupt westerly, southerly evolution of the subtropical anticyclone, and the development of convective perturbations triggered by the typhoon inverted trough, respectively. The east west oriented upper level jet was formed under the combined forcing of the upper level cold vortex and the Huabei anticyclone system. The upper level jet possessed an obvious downward transmission feature in momentum, which would enhance the easterly wind component at middle troposphere and form a deep easterly wave system. The convective available potential energy (CAPE) in the heavy rainfall event of 30 July 1988 was mainly caused by the advection of air mass on the sea. The convergence in low pressure converted trough could accumulate water content in air and form a deep level with high equivalent potential temperature. This high equivalent potential temperature level may moved westward along with the easterly wave system and concentrate CAPE to convection development. There existed cold advection at the upper troposphere which could make the value of CAPE maintain steady during the westerly moving of easterly wave. The potential possibility of heavy rainfall caused by moving easterly wave could not be predicted by the single point sounding on the ground. The storm relative helicity matched with the easterly wave system very well and from it the emerging process of the south and north branches of east wind perturbations could be reflected clearly. The moving speed of the storm relative helicity was a little faster than that of the easterly wave system and vertical vorticity, and therefore the storm relative helicity can be used to predict the falling area of the easterly wave heavy rainfall in advance.
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Supported by:Beijing E-Tiller Technology Development Co., Ltd.
Supported by:Beijing E-Tiller Technology Development Co., Ltd.