基于兰州市城市冠层模式的一次降水过程的数值模拟

Numerical simulation of a precipitation process in Lanzhou based on the urban canopy model

基于耦合了城市冠层模式(UCM)的中尺度天气预报模式(WRF)的数值模型,模拟分析了兰州市一次对流降水天气过程.探讨了不同云-微物理参数方案和积云对流参数化方案的组合对此次降水过程模拟的影响,对比分析了WRF模式在耦合单层城市冠层前后对城市降水的影响.在不同的参数化方案组合中,此次降水过程风险评分最高的组合是WSM-6方案和G-F方案,其对小雨、中雨和大到暴雨的评分分别为0.50、0.36、0.33.选择此参数化方案,利用耦合UCM的WRF模式模拟降水,结果表明,耦合UCM模式后的降水范围变大,降水量的趋势更接近实况.降水过程中,与单独的WRF模式模拟结果相比,耦合UCM的WRF模式模拟的2 m温度平均偏低1.7℃、2 m高度水汽混合比偏高0.7 g/kg,对流有效位能偏高5 J/kg,增强了降水的有利条件.对比发现城市冠层能够激发出更强的大气对流运动,影响近地面水汽的分布和城市下风向降水的分布.

Based on the weather research and forecasting model(WRF)coupled with the urban canopy model(UCM),a convective precipitation process in Lanzhou was simulated.The effects of different cloud-microphysical parameter schemes and cumulus convective parameterization schemes on the simulation of this precipitation process were discussed.The influence of WRF mode with and without coupling a single-layer urban canopy on urban precipitation was compared and analyzed.Of the different parameterization scheme combinations,the highest threat score for this precipitation process was the WSM-6 scheme and G-F scheme,and the scores for light rain,moderate rain and heavy rainstorm were 0.5,0.36,0.33,respectively.Selecting this parameterization scheme and simulating this precipitation using the WRF mode coupled to UCM.The results showed that the precipitation range after coupling UCM mode became larger,and the trend of precipitation was closer to the actual situation.In the precipitation process,compared with the WRF mode simulation results,the WRF mode coupled with UCM simulated the average 2 m temperature by making it 1.7℃lower,the water vapor mixing ratio at a height of 2 m was 0.7 g/kg higher,and the convective available potential energy value was 5 J/kg higher,which promoted the production of precipitation.The urban canopy could stimulate a stronger convective movement,and this convection would affect the distribution of near-surface water vapor and the distribution of urban downwind precipitation.