Orographic Effects on Storm-Scale Structures in Short-Time Heavy Rainfall Events Under Weak Synoptic Backgrounds

Abstract.

Precipitation data with high temporal and spatial resolutions provided by automatic weather stations, SA radar data, and radiosonde data over Beijing area are used in this study, together with ERA-Interim global re-analysis data with horizontal resolution of 0.25*0.25 and 37 vertical layers provided by ECWMF. Three types of orographic short-time heavy rainfall events under weak synoptic backgrounds featuring in short-time strong precipitation and overlapped rain storm centers are analyzed. Special attentions are paid to discussions of orographic effects on the triggering mechanisms, structure evolutions and propagations of convective systems under several environmental flows with different dynamic characteristics and distinct stratifications. The following results are obtained. Firstly, downslope wind formed by dry environmental flow climbing over the mountains intensifies the descending motions and evaporations in the rear part of the convective storm, benefits the formation of deep convective cold pool, and forces the inflow to lift in front of the convective system. Therefore, the convective storm grows into a supercell storm after climbing down the mountain, and leads to short-time strong precipitation. Secondly, when there is a conditionally stable layer of certain thickness and vertical shear of certain intensity existing in the middle and lower troposphere, the low level current convergence by orographic blocking together with the orographic gravity wave benefits for the formation of convective cells in front of mountainous area near the plain. When there’s no or negligible dry and cold air drawn into the convective system in the middle and upper troposphere, the cold pool near surface is not capable of preventing the lower level flow (inflow) from penetrating or bypassing the convective system. The descending current in the rear of the convective system overlays the environmental flow when it diverges backwards in the boundary layer. It intensifies the orographic lifting in the rear part of the convective system (much closer to the orography), triggers continuous convective systems much closer to the orography, and forms a multi-cell convective system which develops backwards. Therefore, in this circumstance, the short-time heavy rainfall event will last a little longer. Thirdly, in saturated lower atmosphere with negligible restraining convections, the surface flow blocked by orography leads to converging motions which are capable of triggering convective instability. Line pattern convections form alongside the orography and much closer to the plain. As the convective system intensifies, there are descending motions in its rear part and divergences in both sides near surface. The divergent outflow in the front side and the environmental flow (outflow) together strengthen the development of the convective system. This positive feedback process is the important mechanism of remaining the line pattern. Meanwhile, the strengthened divergent flow at the back side triggers new convections near the mountain by orographic lifting. “Bi-linear convective cloud lines” are gradually built up and lead to heavy rainfall with longer time durations.

Keywords: orography, gravity waves, short-time heavy rainfall event, structure of convective systems, propagations.