Influence of Multiscale Orography on the Initiation and Maintenance of a Precipitating Convective System in North China: A Case Study

Using convection-resolving Weather Research and Forecasting simulation, this work studies a heavy rainfall event that occurred in the Shanxi Plateau, North China on 21-22 May 2017. Of particular interest is the influence of multiscale orography on the initiation and maintenance of the precipitating convective system. Results showed that the initial convection was triggered over the small-scale Taiyue Mountain in the Shanxi Plateau. In the nighttime, with a stable boundary layer, orographic gravity waves were generated by the Taiyue Mountain. The gravity wave-induced lifting cooled and saturated the lower troposphere adiabatically, thereby producing a moist absolutely unstable layer favorable for subsequent convection initiation. In the daytime, a mountain-plains solenoid (MPS) was established due to the differential solar heating between the Taiyue Mountain and Changzhi Basin. The MPS upslope wind increased the low-level convergence above the mountain, which eventually promoted the convection. This convection moved eastward and grew into a quasi-stationary, quasi-linear convective system in the eastern Shanxi Plateau, which was significantly impacted by the daytime MPS caused by the Shanxi Plateau and North China Plain. The low-level easterlies associated with the plateau-scale MPS hindered the eastward propagation of the precipitating system and enhanced the low-level vertical wind shear ahead of the quasi-linear convective system, resulting in an overall balance with the system cold pool circulation. These findings provide some new insights into the development of a precipitating convective system under the influence of multiscale orography in a relatively pure environment of weak synoptic forcing and convective available potential energy.