Case Study of the Coupling Mechanism of High- and Low-Level Jets of the Rainstorm Process in West China

High- and low-level jets have a substantial influence on heavy rainfall events in China; however, detailed studies on their effects on heavy rainfall in arid and semiarid West China and their coupling processes and mechanisms are lacking. In this study, the effects of the coupling effect and mechanisms of high- and low-level jets on a severe rainstorm process in western China were investigated by conventional observations, encrypted automatic meteorological station, and NCEP/NCAR reanalysis data. It was revealed that (1) heavy rain is generated under the favorable background of the southwest airflow on the west side of the western Pacific subtropical high, with cold air moving to the east and westward and northward by the cold trough low trough carrying mid-high latitudes. Thus, the coupling of high- and low-level jets is beneficial for rainstorm development. (2) High- and low-level jets are coupled with momentum transmission through secondary circulation. First, the high-altitude strong zonal wind speed in the north side of the high-level jet strengthens the divergence of the upper layers and creates a sinking movement, and the low-level strong zonal wind speed in the northwest side of the low-altitude jet strengthens the low-level convergence, which creates an upward movement under the action of the terrain uplift. A secondary circulation is formed between the high- and low-level jet streams. The sinking air flow of the secondary circulation guides the high-level potential vortices from the high level to the low level to propagate and move downward. The down flow is connected with the high potential vorticity region in the middle layer, and the coupling channel of high- and low-level jet flow is cleared. (3) The coupling of high- and low-level jets in the middle layer strengthens the accumulation of conditional symmetric instability energy in the middle layer, which promotes the development of low-level jets. Consequently, the intensity of convergence is improved, as well as the upward movement, to enhance the precipitation intensity. After conducting the sensitivity experiments of increasing and decreasing high- and low-level wind speeds, it was observed that the increase or decrease in high- and low-level wind speeds can change the maximum potential vorticity enhancement stage in the middle and low layers, which correspond to the period of heavy precipitation and the potential vorticity value. The larger the precipitation, the stronger the intensity. (4) The intensity of high- and low-level jets together has an important effect on precipitation intensity. The sensitivity test of the numerical model exhibited that when the intensity of high- and low-level jets is simultaneously increased, the precipitation intensity would be enhanced, and only the high- or low-level jets are increased, the precipitation intensity changes little, but when the intensity of high- or low-level jets is weakened, the precipitation intensity weakens.