Mechanism Analysis Of Cold-air Snowstorms In The Shandong Peninsula

On the basis of the datasets from routine meteorological observations, radars, satellites and NCEP/NCAR reanalysis, and using numerical simulation and diagnostic analysis, this paper studies the characteristics and mechanisms of cold-air snowstorms in the Shandong Peninsula, the differences between cold-air strong snowstorms and the snowstorms induced by other factors, the cause of locally strong cold-air snowstorms in the north of Shandong Peninsula, and effects of the underlying surface on the cold-air snowstorm in Bohai rim. The innovative results are summarized as follows:1.Mechanisms of cold-air snowstorms in the Shandong Peninsula:The strong cold-air from the north influences the Shandong Peninsula in two ways. On one hand, the cold air rotates cyclonically due to heating in Bohai, which favors ascending motion; on the other hand, cold air gets large amounts of heat and moisture from ocean, so the atmosphere at lower level becomes warmer and wetter to form the stratified instability (cold-air instability). Under the influence of cold air, the land breeze, originated from the land-sea temperature difference along the coast of the Shandong Peninsula, interacts with the environmental wind (northwesterly) to produce convergence (land breeze convergence) in the northern coast and divergence (land breeze divergence) in the southern coast. The convergence in the northern coast triggers the release of unstable energy in the lower atmosphere and produces the cold-air snowstorms in the northern coast. The favorable large-scale weather background is very important. The large-scale convergent system passing the cold-air unstable regions in the boundary layer is favorable to the cold-air strong snowstorms.2. The cause of locally strong cold-air snowstorms in the north of Shandong Peninsula:The warm-water belt from Huanghai to Bohai interacts with the environmental wind (northwesterly) to produce a shear line along mid-Bohai, Shandong Peninsula, and southwestern Huanghai (i.e. warm-water shear line). The convergent system, originated from land breeze in the western coast of Liaodong Peninsula and the environmental wind, moves southward to merge with the shear line and strengthens it, and then moves downstream along the shear line. The overlapping of shear line and convergent system intensifies the local ascending motion at the merging point and lead to the locally strong cold-air snowstorms in the northern Shandong Peninsula and inhomogeneous distribution of cold-air snowstorms along the northern coast of Shandong Peninsula.3. Ocean-Iand-atmosphere components of the cold-air in the Bohai rim and their importance:Based on investigating the effects of the underlying ocean and cean-land distribution on the cold-air snowstorms in the Bohai rim, we suggest a conceptual model of the cold-air system in the Bohai rim, whose constituents are strong cold air, warm water in Huanghai and Bohai, land breeze convergence in the Shandong Peninsula and Liaodong Peninsula, shear line in the center of warm ocean, and convergent system in the lower atmosphere. Their roles are described separately in the following. The strong cold air is necessary for the unstable stratification in the low-level atmosphere, which interacts with the land breeze along the coast to form small-scale convergence and divergence. The warm water in Huanghai and Bohai provides unstable energy and water vapor for the cold-air snowstorms and is the heat source of low-level atmosphere. The land breeze convergence and divergence along the coast of Shandong (Liaodong) Peninsula produces ascending motion in the northern (western) part which is favorable to the cold-air snowstorms, and descending motion in the southern (eastern) part which is unfavorable to the cold-air snowstorms. The shear line in the center of warm ocean is also forced to oscillates eastward and westward. The shear line in the center of warm ocean produces convergence and ascending motion near the shear line and is beneficial for the cold-air snowstorms. The large-scale convergent system in the lower atmosphere favors the development of medium-and-small scale convergent systems and the release of cold-air unstable energy.