Conventional observation data, radar data and EC-thin data (0 25°×0 25°) are used to analyze an autumn rainstorm behind a cold front before the ending of a consecutive rain on 27 September 2017 in Xi’an.Results show that the rainstorm locates 300~400 km back from a cold front on surface, where the temperature is rather low, unfavorable for the accumulation of warm and moist energy.Meanwhile, the northerly wind on 700 hPa and 850 hPa brings no water vapor.However, westerly trough on 500 hPa provides a favorable background circulation for the rainstorm.Through analyzing saturated pseudo equivalent potential temperature and geostrophic absolute momentum, the mechanism of this rainstorm is considered conditional symmetric instability.The cold front slopes gradually from south to north.A southerly wind on 700 hPa brings warm moisture from the periphery of the Western Pacific Subtropical High over to the south of Shaanxi.The airflow is forced upward by the cold front to the conditional symmetric unstable area above Xi’an, which results in inclined convection.In the strongly baroclinic atmosphere, a moderate vertical wind shear is favorable to the geostrophic absolute momentum with a small slope.On middle and upper level, the warm and humid airflow makes the saturation pseudo-equivalent potential temperature have a large slope, thus conditional symmetric instability are formed.The precipitation emerges as paralleling belt echoes on radar, which is parallel with vertical wind shear vector from 0 to 6 km.There is a good correspondence among the conditional symmetric unstable area, the upward sloping motion and the echo height.
Damei GUO
,
Liujie PAN
,
Yueqin SHI
,
Qiyuan HU
,
Linrong WU
,
Jiahuimin LIU
,
Jianling TAO
. Analysis of a Rare Autumn Rainstorm behind Cold Front in Xi’an[J]. Plateau Meteorology, 2020
, 39(5)
: 986
-996
.
DOI: 10.7522/j.issn.1000-0534.2019.00097
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