The modification process of a cold airmass crossing over the Japan Sea and its coastal region on 29 January 1993 was simulated using a three-dimensional compressible non-hydrostatic cloud model with detailed microphysics coupling with a simple first-order boundary layer parameterized model. The microphysical characteristics of the shallow convective snow clouds formed in association with the cold airmass as well as the heat and moisture transfer processes between air and sea were investigated.
The simulation of the cold airmass was initiated based on the rawinsonde soundings taken from middle of Japan Sea (42N, 135E) at 00Z on 29 Jan 1993. The domain size for the simulation was 36 x 36 km in the horizontal and 19 km in the vertical. The grid intervals were *x=*y=1 km, *z=0.5 km. A cold pool placed in the center of model domain with size of 8 x 8 km in the horizontal and 2 km in the vertical was used to initiate convection in the cold airmass (Tao et al., 1990). The peak temperature perturbation in the center of thermal bubble was 0.5 K. is assumed to be homogeneous in the horizontal and keep constant during the simulation time. The maximum lifting velocity of is taken as 0.1 ms-1. The 10℃ and 0℃ of temperatures are used in the simulation for the sea and land surface, respectively.
The simulation results show that the simulated clouds experience three pronounced phases while crossing the Japan Sea and its coastal region, which is the same as that observed. The first stage is shown as a rapid intensification of the simulated radar echo due to the formation of a considerable number of snow aggregate particles in the clouds over the warm sea. In the second stage the snow clouds appear as a rapid decrease of radar echo due to the heavy snowfall along the coastal region. The third stage shows that the snow clouds become gradually intensive again due to the production of a limited number of graupel particles after landing the Japan Islands. The echo structure is changed from the isolated type to the diffused type during landing process. |
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