NUMERICAL SIMULATIONS OF CONVECTIVE SNOW CLOUDS OVER THE SEA OF JAPAN- 2-DIMENSIONAL SIMULATIONS OF MIXED-LAYER DEVELOPMENT AND CONVECTIVESNOW CLOUD FORMATION

The development of mixed layers and the formation of convective snow c louds over the Sea of Japan were simulated by using a triply-nested tw o-dimensional dynamic cloud model with a recently developed microphysi cal parameterization. Two case studies were made using this model. One is the convective snow storm associated with a typical cold airmass o utbreak during the period of Feb. 2-4, 1989, that occurred during the period of an intensive field experiment. A detailed microphysical comp arison with the observational results was possible for this case. The other is the snow storm which accompanied an extremely cold airmass ou tbreak during the period of January 24-26, 1990. Through a comparison between these two cases, the effect of temperature contrast on the mix ed-layer development and snow cloud formation was investigated. The mo del simulated mixed-layer development through heat and moisture supply (total heat fluxes of 439 W/m 2 and 895 W/m2 for the 1989 and 1990 ca ses) from the warm sea surface and subsequent convective transport. Th e simulations showed good agreement with observational results in term s of cloud fetch distance, cloud top and base heights and air temperat ure increase in the mixed layer. From the viewpoint of cloud dynamics and microphysics, the model simulated observed updraft velocity, cloud water content and snow water content well. The model also simulated t he drizzle formation in snow clouds and high concentrations of ice cry stals in supercooled cloud water regions. A major defect of the simula tions was the underestimation of the number concentrations of ice crys tals by a factor of 6. In general, clouds form 50-150 km leeward of th e continental coast (depending on the air-sea temperature contrast) an d gradually develop in height and convective activity. Over Japan, sno w clouds strengthen around 30 km off the coast and then gradually weak en because of negligible heat and moisture supply over the land. In cl ouds, ice crystals first appear through the freezing of cloud droplets and grow through vapor deposition and accretion of cloud droplets. Th e dominant precipitation type changes from graupel (over the sea and t he coastal area) to snow (over the mountain area). Over mountain slope s, a marked seeder-feeder mechanism operates between upper decaying sn ow clouds and lower clouds that form due to terrain-induced updrafts. Similar, but less intense microphysical interaction ('natural seeding' ) occurs between snow clouds at different stages over the ocean. With colder air outbreaks, convective activities are stronger and mixed lay ers deeper. Higher concentrations of ice crystals are produced through deposition/sorption nucleation and the ratio of graupel/snow decrease s because of competitive consumption of cloud water among the crystals .