A Double-Moment Explicit Scheme Mesoscale Modeling Study Of Stratiform Precipitation Formation

Based on the dynamic frame of MM5 and Reisner 2 explicit cloud scheme, a new comprehensive double-moment microphysical parameterization scheme was developed. The new scheme predicts the evolution of mixing ratios as well as number concentrations of cloud water, rain water, cloud ice, snow and graupel which uses the more reasonable gamma law than M-P law as the drop size distribution for the rain water and three ice-phase hydrometeor species (cloud ice, snow and graupel). Corresponding to the gamma size distribution, microphysical equations in the new scheme are rewritten. Since the number concentrations of all these hydrometeors are calculated explicitly, the activation of cloud condensation nuclei process has been parameterized with the hypergeometric function, which is not predicted in most state-of-the-art cloud resolving models. In addition, the scheme adds and improves some microphysical parameterization. A method for calculation radar reflectivity using Rayleigh theory is developed in the scheme to straightforward and comprehensive compare simulated and observed radar reflectivity. By adding the calculation and output program for new prognostic variables, the new scheme is completely incorporated in the MM5 and becomes a new option in its explicit microphysical scheme. The parallel computation version of the new scheme saves calculation time and is suitable for real-time weather forecasting. An observed stratiform precipitation in northwest China in September, 2002 is simulated by using the PSU/NCAR mesoscale model (MM5) with the new double-moment explicit microphysics parameterization scheme. For comparison, the first case was simulated with the new scheme and Reisner 2 scheme in domain 2 respectively. By comparing model simulation with the observed precipitation, the new scheme shows some improvement of precipitation location and intensity prediction. The output of the new scheme could give reasonable microstructure of stratus cloud field and indicate its some characteristics, which enhance the capability of MM5 to study the cloud microphysical process. These results suggest that the new scheme will provide some valuable information on the research of macro and microstructure characteristic of stratus cloud, physical process of precipitation and weather modification.In the two-moment scheme, the size distribution of each hydrometeor category is often described by gamma distribution law n ( D)= N0 Dαexp(?λD) and we generally treat intercept N0 and slopeλas prognostic parameter while holding spectral shape parameterαas constant in the model. The role of the spectral shape parameterαis investigated by examining its effects on sedimentation and microphysical source/sink terms. The first case of stratiform precipitation is simulated by using MM5 with the double-moment microphysics scheme. Various rain water spectral shape parameter,αr, is introduced for the model simulation experiments. It is found that the model simulation results are generally in agreement with the theory analysis. There are differences between surface precipitation and cloud macro and micro characteristics for various rain water spectral shape parameterαr, while the difference is not great for stratiform precipitation.A stratiform precipitation in northwest China in September, 2003 is simulated by using MM5 with double-moment scheme. The second case simulation results show that the threat score of 24h precipitation forecasting at the categories 0.1-9.9 mm is high while the threat scores of threshold greater than 10 mm are low and the predicted precipitation locations exist biased error compared to observation. The results indicate that the model has certain prediction ability for stratiform precipitation. The output result of the radar reflectivity calculation in the model is agreed with the radar echo on the RHI display at Yan'an station, which has the characteristic of stratiform radar echo and exists 0℃isotherm bright band. The third case is a stratiform precipitation associated with cyclone in northeast China in June, 2005 which has intensive rain gauge observation. The high resolution simulation with the two-moment explicit microphysics scheme outcome shows that moderate rain prediction is good and some stations precipitation forecasting is agreement with the intensive rain gauge observation. The model does not forecast the torrential rain which is related to the cumulus cloud in this case.To explain the microphysical mechanism and processes of the simulated three cases stratiform precipitation formation, a three-layer model is proposed. The first layer contains ice crystals and has no supercooled liquid water. The second layer contains supercooled liquid water and is the region for different ice phase particles growth. The boundary deck between first layer and second layer is not fixed. Below the 0℃isotherm is the third layer which contains warm cloud water. The first layer precipitates ice crystals to the second layer cloud is called"seeder"zone and the second layer supplies supercooled liquid water is called"feeder"zone which ice crystals can grow and multiply very rapidly. Once the snow and graupel which produced in the second layer fall below the freezing-level melting occur, and precipitation is formed in the third layer. At the same time, rain water continuously grows at the cloud water droplet's expense for the warm cloud processes. The second layer is called"seeder"zone with respect to the third layer. The output of the model could provide the stratiform cloud field spatial structure of three-layer model, the microphysical values profile and the different hydrometeor particles sources analysis reveal the main precipitation formation processes and explain the precipitation formation mechanism of three-layer model at different stations. The surface precipitation is mainly associated with the second layer and third layer in the three-layer model according to percentage of the precipitation rates originated in every layer. The three-layer model illustrates the entire mechanism and processes of the stratiform precipitation formation completely and comprehensively.