Extreme precipitation (EP) is frequently observed in southwest Zhejiang Province of China in June. The present study reveals distinct atmospheric circulations for EP in early-middle and late June. The early-middle June EP is influenced by both mid-latitude and tropical systems. It tends to occur accompanying westward extended western Pacific subtropical high, eastward extended South Asian high, enhanced East Asian jet stream, East Asian meridional wave pattern, and mid-latitude zonal wave pattern. The EP region is located to north of climatological rain band. The late June EP is mainly induced by mid-latitude systems. It is inclined to occur accompanying southward shifted East Asian jet stream, deepened East Asian trough, and mid-latitude zonal wave pattern. The EP region is located to south of climatological rain band. Anomalous ascent leading to EP is contributed by both lower-level convergence and upper-level divergence in early-middle June, but is mainly due to upper-level divergence in late June. Geopotential height anomalies switch from negative at lower troposphere to positive at upper troposphere over eastern China corresponding to early-middle June EP. This is related to positive temperature anomalies attributed to anomalous southwesterly flow. The accompanying increase of northward pressure gradient with the altitude leads to enhancement and expansion of the East Asian jet stream. Negative geopotential height anomalies increase with the altitude over eastern China corresponding to late June EP. This is associated with negative temperature anomalies attributed to anomalous northerly flows. The resultant increase of northward pressure gradient with the altitude causes southward shift of the East Asian jet stream.
Zhu et al. (Thu,) studied this question.