Characteristics and Synoptic Analysis of a Regional Heavy Rain in Middle Part of Taihang Mountains Caused by "21·7" Torrential Rain System Moving Northward

Abstract

"21·7" severe torrential rain occurred in Middle-North of Henan Province from 18 to 22 July 2021， which has been studied by many scholars.Affected by the northward movement of the weather systems in “21·7” torrential rain， a regional heavy rainstorm occurred subsequently in the middle section of Taihang Mountains from 20 to 22 July.Based on 0.25°×0.25° hourly ERA5 reanalysis data and multi-source high-resolution observation data from 2508 ground dense automatic weather stations， Doppler radar， wind profile radar， L-band radiosonde， ground-based GPS water vapor and so on， the characteristics and synoptic analysis of the heavy rain in the middle section of Taihang Mountains from 20 to 22 July 2021 were analyzed.The results show that： （1） The main weather systems were subtropical high， upper-tropospheric low vortex， shear line， low-level jet， moreover the long-distance water vapor transport of double typhoon of "In-Fa" and "Cempaka" was favorable to the heavy rain.The main synoptic background was stable atmospheric circulation and long-term maintenance of low-level easterly wind.（2） The locations of surface convergence line， surface dew point temperature front area， the strong echo area with reflectivity factor greater than 30 dBz， and short-term heavy precipitation zone of hourly precipitation greater than 10 mm were nearly consistent and stably maintained at the eastern foot of Taihang Mountains with less movement and long duration.During heavy precipitation phases， the easterly component of low-level winds increased， and the maximum height of east wind decreased.Farther， the maximum easterly component was 8 to 11 m·s-1， the maximum height of east wind was 510 m.The maximum height of low-level easterly jet was 510 m to 2310 m， the maximum low-level jet index was 33.1×10-3 s-1； The maximum vertical wind shear of 0~1 km and 0~3 km were 11.9 m·s-1 and 16.9 m·s-1 respectively.The easterly wind and Taihang Mountains made a horizontal convergence on windward slope， also the obvious terrain effect induced the increase of precipitation.（3） The deep wet layer extended from the ground to 400 hPa.During the precipitation process， the condensation height was close to the surface.The convective effective potential energy in the early stage of precipitation was 1925.5 J·kg-1.In the stage of heavy precipitation， when the GPS water vapor was higher than 50 mm， the start time of high water vapor was 5~6 hours ahead of the precipitation.The large value of water vapor flux of the middle and lower layers in the western mountainous area lasted for more than 10 hours， and the water vapor convergence was significantly higher than that in the East.Heavy precipitation presented as a band shape in front of Taihang Mountains.（4） The centroid of radar echo was low， and the strong echo area with reflectivity factor greater than 30 dBz lasted for 20 hours， the characteristics of train effect and warm-cloud precipitation were obvious in the western mountainous area.

Keywords

• heavy rain
• topographic effect
• low-level jet index
• surface convergence line
• train effect