NUMERICAL MODELING OF THE CONVECTIVE SNOW CLOUD OVER THE SEA OF JAPAN - PRECIPITATION MECHANISM AND SENSITIVITY TO ICE CRYSTAL NUCLEATION RATES

A bulk parameterization scheme of cloud microphysics which predicts not only the number concentrations of cloud ice and snow but also that of graupel, in addition to the mixing ratios of six water species (water vapor, cloud water, rain, cloud ice, snow and graupel), is developed. This scheme is applied to the 3-dimensional simulation of the convective snow cloud observed over the Sea of Japan on Feb. 4, 1989, with a cloud top temperature at - 20-degrees-C. In the simulation where conventional parameters are used for Fletcher's deposition/sorption nucleation, Bigg's freezing of cloud droplets and the Hallet-Mossop rime-splinter secondary production of ice crystals, the maximum of number concentration of ice particles is about 5 x 10(4)/m3, 1/4 of the observed counterpart (2 x 10(5)/m3). Radar reflectivity is larger by 10 dBZ than observation. Other aspects appear to be well reproduced by the model at least qualitatively. Precipitation formation in the simulated cloud is examined, and it is found to be consistent with observational studies. Sensitivity experiments to the amplification of various ice nucleation rates are conducted to simulate the cloud more realistically and to see the effects of the increase of the number concentration of ice crystals on the precipitation formation of the convective cloud. The increase in the number concentration of ice crystals to the value of 10(5) approximately 10(6)/m3 results in the increase of snow in mass and number and the decrease of radar reflectivity. The further increase in the number concentration of ice crystals to the value (10(7)/m3) results in suppression of precipitation, glaciation of cloud with little cloud water and suspension of abundant snow in the air. These experiments give some support to the possibility of suppressing the precipitation by overseeding. Some merits of predicting number concentrations of ice particles are discussed. In the simulation, the intercept parameters of the inverse exponential size distribution functions for snow and graupel, Nos and Nog, are found to be more dependent on the stage of the cloud development than on the precipitation rate. This feature cannot be reproduced by the model with the prescribed Nos and Nog. For a more realistic simulation of clouds, a more reliable knowledge about ice nucleation is crucial.