两次暴雨过程模拟对陆面参数化方案的敏感性研究

Sensitivity of heavy rainfall events simulation to land surface parameterization scheme as simulated via the two cases

选取发生在江西和福建境内的两次暴雨个例,利用NCEP再分析资料在对暴雨发生前、后的环境场和物理量场进行诊断和对比分析的基础上,采用中尺度模式WRF V3.3,通过数值模拟探讨了陆面过程对两次暴雨过程的可能影响及其相关的物理过程。结果表明,2012年5月12日江西大暴雨主要受大尺度环流和中尺度天气系统影响,具有范围大、持续时间长等特点,属于大尺度降水为主的暴雨;而2011年8月23日福建暴雨发生在副热带高压控制下的午后,局地下垫面强烈的感热和潜热通量使低层大气不稳定性增强,触发了此次对流性降水为主的暴雨。通过资料诊断分析,可以判断陆面过程对福建暴雨个例的影响程度明显强于江西暴雨个例。通过关闭地表通量试验发现,陆面过程对暴雨模拟十分重要,尤其是对于该个例中对流性降水的发生起到关键性的作用。通过陆面参数化方案的敏感性试验发现,两次暴雨过程对陆面参数化方案均较为敏感。江西暴雨对陆面过程的敏感性主要体现在对流降水的模拟上,而福建暴雨则体现在大尺度降水的模拟方面,即福建暴雨对陆面参数化方案的敏感性强于江西暴雨。敏感性产生机制与降水类型关系紧密,大尺度降水对陆面过程的敏感性主要来源于不同参数化模拟的中高空对流系统的差异,而对流降水的敏感性则与不同参数化模拟的地表通量的差异有关。通过陆面参数的扰动试验进一步发现,相比于地表粗糙度和最小叶孔阻抗,土壤孔隙度和地表反照率则是影响对流降水对陆面过程敏感的关键因子,这在本质上与地表通量是否受到扰动有关。地表通量较风场而言,受扰动引起变化的空间范围广、时间响应快,变化具有明显规律性。所得结果可为深入理解陆面过程影响暴雨等天气过程和改进数值模式对暴雨的模拟能力提供一定的参考。

Two heavy rainfall events occurred in the provinces of Jiangxi and Fujian,are investigated by comparing their envi-ronmental conditions and physical variable fields using the NCEP reanalysis data.Then the possible effects of land surface process on the heavy rainfall as well as its related physical mechanisms are explored by using the meso-scale model WRF V3 .3 . The results show that the Jiangxi heavy rainstorm event taking place on 12 May 2012 is dominated by large-scale precipitation in nature,which is featured by its large spatial scope and long lasting time.Another heavy rainstorm event occurring in Fujian Province afternoon on 23 August,2011,is mainly under the control of the Subtropical High.The local sensible heat and latent heat fluxes evidently strengthen the instability of the lower atmosphere,which triggers the thermal convective precipitation. From the diagnostic results,it is concluded that the land-surface process has more important impacts on the heavy rainstorm e-vent in Fujian compared to that on the heavy rainstorm in Jiangxi.After closing land-surface fluxes in the numerical experi-ments,it is found that land-surface process affect the heavy rain significantly,especially on the occurrence of the convective heavy rainfall in Fujian.Furthermore,several sensitivity experiments are performed by using the four land-surface parameter-ization schemes （LSPs）and the results suggest that the two heavy rainfall cases simulated are sensitive to the LSPs.The con-vective precipitation is more sensitive to the LSPs in the Jiangxi case,while the large-scale precipitation is more sensitive in Fu-jian case.On the whole,the heavy rainfall case in Fujian is more sensitive to the LSPs than that in Jiangxi.The mechanism re-sponsible for the sensitivity is precipitation-type dependent.For large-scale rainfall,the sensitivity originates from the different responses of the convective system in the middle and upper troposphere to various LSPs.However,the sensitivity is mainly re-lated to the different responses of the land surface-atmosphere flux to LSPs in the convective heavy rainfall case.Disturbance tests of the land-surface parameters are also carried out,and it is found that soil porosity and surface albedo play more impor-tant roles in the sensitivity of the convective precipitation to land-surface process compared with the surface roughness and the minimal stomatal impedance.The sensitivity of the model results to the land surface parameters disturbances strongly depends on the disturbance of the land surface-atmosphere fluxes in the nature.The responses of land surface-atmosphere fluxes to the land surface parameters are usually faster,larger in the spatial scale and more regular than that of the wind field.This study can provide a reference for our better understanding on the impacts of land surface process on the weather processes （such as torrential rainfall）and improvement of the performance of the numerical model in simulating the heavy rainfall events.