Observational analyses of topographic effects on convective systems in an extreme rainfall event in Northern China

Abstract

An extreme rainfall event occurred in northern China between 18 and 21 July 2016, with the strongest precipitation of 783.4 mm occurring within the rainfall zone of Taihang Mountain. In this study, mesoscale convective systems (MCSs) resulting in heavy rainfall are studied using high-resolution surface observations, soundings and radar data. The topographic effect in addition to shear zone cyclonic circulation is mainly responsible for the heavy rainfall. The synoptic low-pressure environmental circulation promotes mesocyclone and convection development in northern China, where a strong and moist low-level jet is observed. The interaction between the mesocyclone and mesoscale topography produces two strong, separate precipitation centers, which are clearly the direct results of three MCSs. Orographic blocking plays a crucial role in the development of MCSs. Diabatic cooling due to raindrop evaporation in association with MCS1 forms a cold pool on the plain east of the central Taihang Mountain area. Convective cell initiation and development in MCS2 is due to the impingement of southeasterly flow near the surface of the hills east of Linzhou basin. Convective cells that are successively generated and shifted along a mesoscale shear line comprise the MCS2 linear distribution and a northwestward rain belt. The northern extreme rainfall center is caused by MCS3 along the southern slopes of an eastward-opening valley. Convective cells are triggered along a shear line, which is affected by a combination of topography, a cold pool in the boundary layer and mesocyclone outer flow. Together, the orographic forcing and cold pool enhance the Taihang Mountain precipitation event.