Numerical Study On Influences Of Orographic Dynamics On Clouds And Precipitation Physics During A Heavy Rainfall Process In North China

Combining both dynamics and cloud microphysics is necessary and important to comprehensively understand the basic physical mechanism of formation and development of clouds and precipitation. First of all, a heavy rainfall happened on 22-24 July 2005 in North China is selected as a research case in this paper. Synoptic background is analyzed by using 1o×1o NCEP/NCAR reanalysis data and TBB data. On this basis, we successfully reappear this heavy rainfall process by using ARPS model. The influences of orographic dynamics on cloud and precipitation are analyzed by using high spatial and temporal model outputs data.Analysis results show that this heavy rainfall is produced by mesoscale convective cloud clusters, which develop under good synoptic conditions such as reasonable distribution structure of high-level and low-level jet, water vapor transport and convergence in North China and atmospheric instability. The results show that wind divergence that is corresponding to the rain area forms at the windward side. After horizontal flow gets over the windward side, vertical circulation flow of climbing-ascending-descending form above the mountaintop. The ice crystal locates in the upper level, and below it there is snow and graupel, the cloud water and rain water is in the low level of the cloud. Ice phase particles and supercooled water exist in the same area above 0℃level. Cold cloud processes play an important role in rainfall and growth of ice particles. Initial multiplication is important to ice accretion. Snow grows by collecting supercooled water, and rain water is converted into graupel by collection of snow, and graupel melting is the main source of rain water.Premising that model initial fields keep unchanged, sensitivity experiments of increasing or decreasing terrain height in selected area are carried out by using ARPS model, and the influences of dynamic variation on physical processes of cloud and precipitation are further analyzed. Sensitivity results show that dynamic variation induced by change of terrain height can not alter the vertical distribution structure of cloud particles, and cold cloud process make more contribution to total rainfall.Terrain heightened can enhance blocking effect, which lead to strengthen water vapor flux divergence and vertical convection, and cloud water increases but centralizes in the middle-low level. This change intensify accretion of snow by collecting more supercooled cloud water, and rain water converts into graupel by collecting more snow, and dry growth of graupel increase at the same time. Rain water augment because of more graupel melting, and rain water continue to increase by collecting cloud water in the low level. So, cold-cloud process is further enhanced.Blocking effects become smaller when mountain height is decreased. In the low level, wind shear pattern is unfavorable to vertical convection development. The base of vertical range of updraft and water vapor flux convergence rise, while convergence intensity of water vapor flux decrease. So, both maximum cloud water content and its appearing height decrease. All of the main source microphysics processes of water particles decrease. Thus, surface precipitation decrease accordingly.