Cloud-resolving simulation of heavy snowfalls in Japan for late December 2005: application of ocean data assimilation to a snow disaster

We applied eddy-resolving ocean data assimilation to a cloud-resolving atmospheric simulation of a snow disaster and investigated the effects of mesoscale eddies on a heavy snowfall event in late December 2005. Ocean circulation model (OCM) data assimilation reproduces mesoscale sea surface temperature (SST) structures, which are smoothed out by optimum interpolation. This difference between OCM-assimilation and optimum-interpolation SSTs affects the atmospheric boundary layers over oceanic mesoscale eddies. The atmospheric response to the SST difference is complex at the cold tongue in the central Sea of Japan. Although the horizontal wind and turbulent mixing are quickly and locally affected by the low SST, the atmospheric temperature and water amounts are greatly affected by the upstream high SST via the northwesterly advection. In the heavy snowfall areas, the OCM assimilation greatly affects 10-day accumulated precipitation, though it does not largely influence 10-day mean vertical structures of wind, temperature and water vapor. Thus, we should recognize the significance of oceanic mesoscale eddies for heavy snowfall.