Mechanism of Coupled Upper Troposphere and Boundary Layer Induces Two Types of Short-Term Heavy Rainfall Along the Eastern Foothills of the Taihang Mountains

Abstract

Using 11 years of hourly merged rainfall records and ERA reanalysis data, this paper reveals two major circulation modes leading to two types of short-term heavy rainfall (STHR) along the Taihang Mountains' eastern foothills, and further explains their mechanisms. One circulation mode has a distinct warm anomaly at 300 hPa covering most areas of North China, together with the boundary-layer westerly anomaly occurring in North China and its southern region (UTWA-BLWA). UTWA-BLWA effectively contributes to the reinforcement of the upper-level divergence and the low-level moisture convergence by promoting the strengthening of upper anticyclonic and low-level southwesterly anomalies. The combined effects of low-level jet (LLJ) and topographic uplift form the central-northern STHR pattern. The other circulation structure has a 300-hPa warm anomaly located to the southeastern Russia and a prominent 300-hPa cold anomaly covering the south, together with the boundary-layer easterly anomaly occurring over the whole region of eastern China (UTCA-BLEA). The southern STHR pattern is attributed to the exit of the boundary-layer jet (BLJ) over lower elevations due to moist transport and dynamic uplift associated with the easterly anomaly. The results indicate that the locations of STHR are related to the direction, intensity, and height of the LLJ. The findings highlight that the upper-tropospheric temperature anomalies (UTTA) and boundary-layer easterly flow jointly modulate heavy rainfall. Analysis of the coupled upper troposphere and boundary layer could help understand and forecast heavy rainfall.