A Case Study on Summer Precipitation Process in the Central Tianshan Area Using Multi-radar Observation and Model Simulation

The fine vertical structure and evolution of orographic precipitation in the middle Tianshan area was analyzed using measurements taken from a micro rain radar, Ka-band cloud radar, and microwave radiometer. In addition, a high-resolution simulation is conducted to analyze the thermal instability and wind shear layer influence on cloud generation. The observations reveal that the precipitation was generated owing to the convergence between the southwesterly wind flying across the mountain ridge and the northerly wind generated by the thermal forcing in the terrain. Because the observed convective updraft was not strong enough, the low-level northerly flow turned southward as it approached the high-level southwesterly wind, resulting in strong wind shear. Following the precipitation, the low-level convective clouds were constrained to remain below the wind shear layer, and the cloud tops were generally flat and low, indicating that the wind shear layer significantly inhibited the convection. The model simulation suggests that the influence of wind shear on developing precipitation clouds and the change in the thermal instability on the northern slope of the Central Tianshan Mountains during this precipitation is crucial. The advective transport of equivalent potential temperature, caused by the action of upper-level southerly winds, was responsible for the layer above the wind shear layer becoming thermally unstable and the layer underneath it becoming thermally stable, thereby suppressing low-level convection and promoting upper-level convection. If low-level convective updrafts were not strong enough to break through the stable laminar junction caused by vertical wind shear, convection would be constrained to remain below the vertical wind shear layer, preventing intense precipitation.