Page 128 - 《高原气象》2022年第6期

6 期 郭大梅等：关中北部一次暴雨水汽条件及不稳定分析 1491
cation 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 north‐
ern 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 gra‐
dient between the western Pacific subtropical high and the rainstorm area will strengthen the lower-level south‐
westerly jet and drives it northeastwards. The enhanced lower-level jet will converge and rise in front of it，induc‐
ing rainstorm，which in due cause depressurizes on mid-low levels，further increases the pressure gradient be‐
tween 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 Guan‐
zhong 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 be‐
tween the rainstorm area and the western Pacific subtropical high，inducing a lower-level jet over southern Guan‐
zhong 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 lev‐
el. 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 sit‐
uation 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 in‐
fluence 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 pro‐
cess，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 mech‐
anism of possible conditional symmetric instability，combined with numerical model output，come to a compre‐
hensive conclusion.
Key words：Rainstorm；southerly wind；coupling of upper-lower-level jets；latent heat of condensation；condi‐
tional symmetric instability