关键词: 超低空急流, 重力波, 下坡风暴, 临界层, 层结稳定度

Abstract: There is a northwest-southeast-oriented canyon in the middle of the Tianshan Mountains, Xinjiang, China, the length of the canyon is about 80 km, the altitude at the bottom of the valley is about 1 200 m, and the height of the mountains is about 3 000 m. Urumqi is located in the northwest opening downhill of the canyon, the Turpan Basin is located at the southeast opening. This paper uses the mesoscale WRF model to simulate the strongest southeastern gale in the past 10 years in Urumqi City which occurred on March 30, 2012.The simulated output of high temporal-spatial resolution is analyzed. With the simulation and the diagnostic analysis we concluded that the southeast gale in Urumqi is a strong downslope windstorm triggered by the combination of gravity wave and ultra-low level jet which was caused by the special landform in Tianshan canyon. The formation mechanism is summarized as follows: (1) The cold air at the bottom of the Mongolian high pressure flows back into the basin from the eastern part of the Tarim Basin, accumulates and forms a ground high pressure on the south side of the Tianshan Mountains. On the north side of the Tianshan Mountains, the front decompression occurs due to the intrusion of cold front, and thus pressure field on both sides of the Tianshan Mountains is high in the South and low in the North. Under the action of the pressure-gradient force, the cold air enters the Tianshan Gorge, and the airflow gradually accelerates due to the funneling effects. In the middle of the canyon, the wind is southeast and the wind speed exceeds 8 at a distance of 300 meters from the ground. (2) During the process of cold air at the southern end of the canyon climbing into the canyon, gravity waves are formed by the forced lifting due to the terrain. As the pressure gradient on both sides of the canyon increases, the amplitude of the gravity wave increases and is coupled with the ultra-low-level jet, the gravity wave forms a leeward wave hydraulic jump at the northern end of the canyon, triggering a strong downhill storm on the ground, that is the southeast gale in Urumqi. (3) There are two factors complementing each other and thus enhancing the ultra-low-level jet energy. One of them is the critical layer of the shear of southeast and southwest winds above the ultra-low-level jet stream absorbs the upper-layer wave energy and transmits it downward to the ultra-low-level jet stream; the other is the gravity wave breaking turbulence exacerbates the downward transfer of energy in the rapids. In this process, the stable atmospheric stratification of the lower troposphere over the leeward slope exacerbates the sinking motion of lee wave and the energy transmission caused by it.

Key words: ultra-low level jet, gravity wave, downhill storm, critical layer, stratification stability