Mm5 Is In The Establishment And Numerical Simulation Of The New Explicit Cloud Physics Program

A complex mixed-phase explicit microphysical parameterization scheme, which also be known as Reisner 2 explicit scheme was developed for use in the NCAR/Penn State Mesoscale Model Versions (MM5). The single-moment schemes has additional variables for graupel and ice number concentration, in which the mixing ratio of cloud water, rain water, cloud ice, snow and graupel are predicted and number concentration specified. A diagnostic equation for N0s, the Y-intercept of the assumed exponential snow distribution, is allowed to vary with snow mixing ratio. The scheme assumes a Marshall-Palmer distribution law for rain, snow and graupel with a constant intercept parameter N0. In its essence, the scheme predicts only the mixing ratios (third-order moments) of drops.Based on the dynamic frame of MM5 and Reisner 2 explicit cloud scheme, a new double-moment microphysical scheme was developed, in which both the mixing ratios and number concentrations of cloud water, rain water, cloud ice, snow and graupel were predicted. Besides its impact in the formation of the first raindrops by autoconversion, the prediction of cloud-droplet number concentration is especially important in cloud chemistry and radiative transfer. The new scheme uses the more reasonable gamma law as basis functions for the hydrometeor drop size distribution. Corresponding to the gamma size distribution, microphysical equations in the new scheme are rewritten.The number concentration of the activated cloud condensation nuclei (CCN) is described with the hypergeometric function. The collision and coalescence of cloud droplets to form raindrops has been parameterized using the formula of Grabowski (1999). The influence of the cloud droplet spectrum character and the spectrum growth and change is considered, which does not use a cut-off value for conversion from cloud water into rain water process. In the continuous coalescence equation, the particles fall velocity difference is not moved out from the integral, but is integrated in the equation as the function of diameter D to avoid the error of using particles average fall velocity. In the new scheme, generation rate of graupel due to the collection of snow by graupel and the collection of cloud ice by graupel are included. By adding the calculation and output program for new prognostic variables, the new scheme is incorporated in the three-dimensional non-hydrostatic model MM5 and becomes a new option in its explicitmicrophysical scheme, which will be so-called as the new scheme in the paper.By comparing model simulation with an observed precipitation formed by stratus cloud system, the new scheme shows the improvement of prediction precipitation. The output of the new scheme could give reasonable microstructure of cloud and indicate some characteristics of stratus cloud, which enhances the capability of MM5 to study the cloud microphysical process. The numerical simulation reveals the possible mechanism of the rainfall by one station microphysical process analysis. These results suggest that the new scheme will provide some valuable information on macro and micro structure characteristic of stratus cloud, physical process of precipitation and weather modification research.