The development of mixed layers and the formation of convective snow clouds over the Sea of Japan were simulated by using a triply-nested two-dimensional dynamic cloud model with a recently developed microphysical parameterization. Two case studies were made using this model. One is the convective snow storm associated with a typical cold airmass outbreak during the period of Feb. 2-4, 1989, that occurred during the period of an intensive field experiment. A detailed microphysical comparison with the observational results was possible for this case. The other is the snow storm which accompanied an extremely cold airmass outbreak during the period of January 24-26, 1990. Through a comparison between these two cases, the effect of temperature contrast on the mixed-layer development and snow cloud formation was investigated.
The model simulated mixed-layer development through heat and moisture supply (total heat fluxes of 439W/m² and 895W/m² for the 1989 and 1990 cases) from the warm sea surface and subsequent convective transport. The simulations showed good agreement with observational results in terms of cloud fetch distance, cloud top and base heights and air temperature 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 the drizzle formation in snow clouds and high concentrations of ice crystals in supercooled cloud water regions. A major defect of the simulations was the underestimation of the number concentrations of ice crystals by a factor of 6.
In general, clouds form 50-150km leeward of the continental coast (depending on the air-sea temperature contrast) and gradually develop in height and convective activity. Over Japan, snow clouds strengthen around 30km off the coast and then gradually weaken because of negligible heat and moisture supply over the land. In clouds, ice crystals first appear through the freezing of cloud droplets and grow through vapor deposition and accretion of cloud droplets. The dominant precipitation type changes from graupel (over the sea and the coastal area) to snow (over the mountain area). Over mountain slopes, a marked seeder-feeder mechanism operates between upper decaying snow 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 layers deeper. Higher concentrations of ice crystals are produced through deposition/sorption nucleation and the ratio of graupel/snow decreases because of competitive consumption of cloud water among the crystals.