双参数微物理方案在WRF单柱模式中的模拟检验和对比研究

Evaluation and Comparison of Two Double-Moment Bulk Microphysics Schemes Using WRF Single-Column Model

利用耦合Milbrandt 2-mon(MY)和Morrison 2-mon(MOR)两种双参数微物理方案的WRF中的单柱模式,对TWP-ICE(Tropical Warm Pool International Cloud Experiment)试验期间的个例进行数值模拟。通过与观测资料和云分辨率模式的模拟结果进行对比发现:两种双参数微物理方案能较好地模拟出TWP-ICE期间热带云系的宏观和微观的特征。模拟的降水率、地表向下长波辐射和大气顶向外长波辐射的量级、时间演变趋势与观测相一致;总的液相和冰相水凝物的垂直分布以及随时间的演变特征总体与观测以及云分辨率模式的结果也较接近。在整个时期,两种方案水云中的雨滴宏观和微观特征差异较小,而云滴混合比在两种方案之间的差别显著;冰晶对冰云的贡献在MY方案中占据主导地位,而MOR方案中雪在冰云中扮演的角色相比于在MY方案中更为重要。微观上与MY方案相比,MOR方案中的云滴是由数量更大的小云滴构成,但冰晶却是由数量较少的大冰晶粒子构成。微物理过程转换率的区别是造成两种方案冰云宏观分布特征差异的主要原因。与冰晶和雪有关的微物理过程转换分析表明:活跃期两种方案中与冰晶有关的主要微物理转换项有冰晶的凝华增长、冰晶向雪的自动转化、冰晶被雪碰并以及冰晶的沉降过程。而雪主要的转换项包含沉降和凝华过程等,其中MY方案中雪的主要转换项更为丰富。该时期两种方案冰晶和雪的主要微物理转换项的垂直分布以及量级特征的差异同冰云的宏观分布相一致。季风抑制期,两种方案中冰晶主要的源汇项包括凝华增长和沉降过程。MY方案中凝华凝冻核化也是主要的源汇项之一。抑制期MOR方案中高空的雪发展较强,参与的微物理过程较MY方案更为丰富,主要转换项比MY方案高出约一个量级。

Two double- moment Bulk Microphysics Schemes,Milbrandt 2- mon(MY)and Morrison 2- mon(MORR),were compared using the WRF single-column model during the period of the Tropical Warm Pool International Cloud Experiment. Results from the control simulations with the default settings of the two microphysics schemes in the Weather Research and Forecasting(WRF)model were able to reasonably reproduce the characteristics of the rain rate,the liquid water content,and the frozen water content,as compared with observations and cloud resolving model(CRM)results. The surface downward longwave radiation and outgoing longwave radiation were very close to observations as well. There is little difference in the macro- and micro-physical properties of the raindrops but a large divergence in the mixing ratio of cloud droplets between two schemes throughout the whole period. In the MY scheme, Ice crystals are dominant in ice clouds while snowparticles make more contributions to the ice clouds in the MOR scheme. On a micro level, in the MOR scheme, water clouds contain more smaller cloud droplets while ice clouds are made up of less bigger ice crystals than those in the MY scheme. The distribution differences in ice clouds between two schemes are closely related with conversion rates of microphysical. Analyses of the conversion terms of microphysical processes suggest that ice crystals in both schemes are mainly related to processes of deposition growth, autoconversion of ice crystals to snow, collection by snowand sedimentation during the active monsoon period. The dominating conversion terms of snowin MY scheme are more various than those in the MOR scheme with deposition growth and sedimentation processes included in two schemes. The differences in vertical distribution and magnitude of the main conversion terms are pretty consistent with the composition features of ice clouds simulated by two schemes during the active monsoon period. In the depressed monsoon period,ice crystals in both schemes are dominated by deposition growth and sedimentation processes with ice nucleation in deposition mode and condensation freezing mode playing an equally important role only in the MY scheme. Snowclouds in the MOR scheme develop stronger with more kinds of primary conversion terms about one order of magnitude higher than the MY ones during the depressed monsoon period.