Analysis Of Two Cold-air Outbreak Snowstorm Events Controlled By Different Meso-scale Systems In Shandong Peninsula

Two severe cold-air outbreak snowstorms occurred in Shandong Peninsula on 4-5 and 20-21 December 2008 and the local economy suffered very much. During the two events, a Bohai Sea convergence zone and a meso-scale cyclone play very important roles in the event of 4-5 December and 20-21 December respectively. In the paper, several kinds of observations, including satellite and radar data, and numerical model are used to analyze mechanisms of the Bohai convergence zone and the meso-scale cyclone. The impacts of these two meso-scale systems on the cold-air outbreak snowstorm are also analyzed.On 4-5 December 2008. a meso-scale convergence zone oriented in northwest-southeast formed in Bohai Sea and resulted in a snowband oriented in northwest-southeast centered in Yantai, Muping and Wendeng. When northwestly winds flowed around Taihang Mount, a convergence zone formed because of the blocking of Taihang Mount, at the same time, a warm ridge existed in the lower troposphere, so a northwest-southeast convergence zone formed in Bohai Sea. Under the influence of horizontal advection of northwestly wind and ageostrophic velocity, the Bohai convergence zone moved northeastward, at the same time, a mesoscale cold trough extended from the northwest coast of Bohai to Bohai Sea in the layer of 700-400hPa and intensified regularly. The cold trough not only made the lower troposphere convectively instable. but also intensified the Bohai convergence zone in the lower troposphere by intensifying the vertical circulation. When northly winds flow over the northwest coast of Bohai Sea, the northwest part of the Bohai convergence zone moved southward due to the horizontal advection of northly wind, but the southeast part of the Bohai convergence zone continued to move northeastward due to the horizontal advection of northwestly wind and the force of ageostrophic winds and merged with the convergence zone caused by coastal front in north of Shandong Peninsula. It is the different moving directions of the two part of Bohai convergence zone that make the undulation in the convergence zone. When the Bohai convergence zone landed Shandong Peninsula, the severe snowstorm occurred.On 20-21 December 2008, a meso-scale cyclone formed behind a large-scale cold front because of a upper trough moving southeastward. Due to the blocking of Taihang Mount and the occurrence of leeside depression, the meso-scale cyclone intensified in the leeside of Taihang Mount. The surface sensible and latent heat in Bohai Sea also intensified the meso-scale cyclone. When the meso-scale cyclone passed Shandong Peninsula, a snowband parallel to the coast of Shandong Peninsula formed under the influence of meso-scale frontgenesis. Also, the horizontal and vertical advection of meso-scale or large-scale convergence by small-scale wind can intensify the local convergence.Generally, when a cold-air outbreak occurs, Taihang Mount plays an important role in the meso-scale convergence zone and cyclone by the formation of leeside depression, furthermore, these meso-scale systems couples with the coastal front near the north coast of Shandong Peninsula and intensified the snowstorms.