The Research On Numerical Simulation Of The Impact Of Urban Underlying Surface Characteristics On The Thunderstorm In Nanjing

With the rapid development of the city, its activity and influence range is very significant, which changed the character of city underlying surface, the natural surface covered by original plant is replaced by hard dense, impermeable and drying materials. The unique properties of the urban underlying surface has a strong climatologically influence on regional thunderstorms. As the capital city of Jiangsu Province, China, Nanjing has experienced a rapid urbanization in recent years, such urban expansion, with increasing built-up areas and human activities, results in a huge change of the underlying surface characteristics. A specific thunderstorm case on23July2011is examined through using the meso-scale numerical model: Weather Research and Forecasting Model (WRF v3.3), coupled with the Noah land surface model, using the NCEP/FNL data updated every six hours with horizontal resolution of1°×1°as the initial fields and the lateral boundary conditions, to study the impact of urban underlying surface and urban canopy parameterization on the regional convective rainfall events in Nanjing.Three land use tests are performed in the study firstly, including a control experiment (CNTL), which uses MODIS30" static data to represent the current land use situation, and two sensitivity tests:(1) CROP experiment, removing the urban region and replacing it with the dominant rural land use category:crop.(2) URBAN experiment, removing crop and replacing it with urban. In comparison with the automatic weather stations’observation rainfall data, CNTL simulation results predict reasonably well. The contrasts within these tests suggest that the occurrence and the development process of the thunderstorm were closely related to urban underlying surface in this region:first of all, the urban heat island effect of Nanjing was evident before the thunderstorm. Secondly, sensible heat flux over the urban regions is much higher than the surroundings, combine with the effect of urban heat island circulation caused by surface thermal differences between urban and rural area, will enhance the convergence over the urban areas and provide uplift motion, so that plays a more direct role in convection initiation and development of the thunderstorm. The expansion of urban underlying surface helps to larger boundary layer thickness and the vertical mixing height, which contributed to cloud development. In addition, the perturbations potential temperature of the lower atmosphere also enhances affected by urban and built up land use, offering unstable stratification conditions for the thunderstorm. Finally, when crop region is replaced with urban, the storm precipitation increases significantly along the windward side in the city, the low level wind is blocked by the dynamic effect of land due to a larger surface roughness over urban region possibly, which will strengthen vertical motion and water vapor accumulation, as well as change the rainfall pattern and amount of the storm.Based on the foundation work before, WRF with two different urban canopy parameterizations are conducted to study the impact on the simulation results of thunderstorm case on23July2011in Nanjing, the tests are (1) UCM experiment, and (2) BULK experiment. Through the comparison of the two experiments in the near surface meteorological elements、heat flux、PBL height、TKE、CAPE and the temperature stratification shows that the simulations of thunderstorm is sensitive to the urban canopy parameterization. Differences varies in the two experiments on the simulation of2m air temperature and10m wind speed. Different schemes have large impact on the rainfall intensity and areas of the convection. UCM experiment describes the rainfall intensity better than BULK experiment, which also reflects the the nocturnal variation of the sensible heating well. The latent heating release of BULK is lower due to its uniform medium of urban underlying surface. In addition, UCM model reflects the city building distribution and geometric effect reasonably, and is capable to reflect the city larger roughness underlying surface, results the simulated PBL height TKE before the storm and the CAPE of the early precipitation process of the main urban area in UCM lower than BULK before the storm, which helps to reflect thunderstorm process appropriately.