A Simple Parameterization Scheme of Snow-particle Radiative Properties and Effect on Mesoscale Precipitation

Snow-particle, which is produced in Bergeron process, is a kind of solid particles in cloud. Snow-particle takes part in several microphysical processes, so it presents in different shapes. The multi-shape feature makes it difficult to develop a direct and detailed parameterization of the snow-particle radiative properties. At present, the water content and number concentration of snow water are predictable in newly developed dual-parameterized explicit moisture schemes, such as Reisner scheme and CAMS scheme. Consequently, the development of a simple parameterization scheme of snow-particle radiative properties becomes possible.An individual parameterization scheme, in which all snow radiative properties are determined by effective size, is set up and implemented in MM5 V3 mesoscale model. A study on the effect of snow radiation on mesoscale precipitation is carried out using a South China severe storm case on June 8th, 1998. Four numerical experiments are designed to represent no snow radiative effect, assuming snow-particle as a part of graupel and ice crystal, and to calculate snow radiation using the algorithm set up respectively.The distribution of snow water and its effect on the rainfall pattern, domain averaged integration rainfall and rain rate are investigated. The numerical experiment results show that the effect of snow radiation on mesoscale precipitation is obvious:① Snow water plays an important role in the atmospheric water substances, it has the largest water content and the second largest number concentration; ② Snow radiation distinctly modifies the local properties of precipitation, especially the rain rate and position of rainfall center, though it has little effect on overall precipitation pattern; ③ The effect of snow radiation on precipitation during the daytime is much larger than that during the nighttime; ④ Distinct difference among the different experiment results indicates the necessity of establishment of an independent snow-particle radiative properties parameterization.The discussion on the influences of snow radiation on precipitation shows that the indirect effect, which is the convective enhancement due to the relative snow radiative heating, contributes to the major rainfall variation. The radiative absorption of snow water heats the middle and upper atmosphere, causes stronger convection and produces more precipitation as a result. While the direct decreasing effect of snow radiative heating by microphysics is relatively neglectable.In general, the effect of snow-particle radiation on mesoscale precipitation is obvious and an independent snow radiative properties parameterization is much necessary for the improvement of the ability of mesoscale model on the precipitation prediction.