On 6 May 2012, a strong tornado with Fujita scale 3 struct Tsukuba city, located middle part of Japan. The analysis on environmental fields indicated that the northward inflow of low-level humid air over the Pacific Ocean was the main factor for the formation of a supercell storm to cause the tornado. The specific humidity at a height of 500 m became almost doubled from 6 g/kg during 12 hours when the low-level air traveled over the ocean south of the Japanese Islands. The increase could be mainly caused by updrafts associated with a low-level short trough travelling eastward. In this study, the accumulation processes of low-level water and the effect of warm ocean current are examined using the Japan Meteorological Agency-nonhydrostatic model with a horizontal resolution of 5 km.
The effect of warm ocean current Japan Current on the increase was examined from two sensitivity experiments with modified sea surface temperature (SST), because the route of the low-level air almost corresponded with the location of the current. In one experiment (Max20∞C) the maximum SST was set under 20 degrees of Celsius, and in the other (Minus2K) the SST around Japan Current south of the Japanese Islands was 2 Kelvin decreased. In the control experiment, about one-third of water vapor is accumulated in the low level due to the latent heat flux from the sea surface, and the other is due to kinematic processes (e.g., horizontal convergence of water vapor). In Max20∞C the contribution of kinematic processes becomes smaller in comparison with that of latent heat flux, while in Minus2K kinematic processes make about 75% contribution to accumulate low-level water vapor. This indicates that the distribution of warm ocean current significantly affect the accumulation of low-level water vapor. In the control experiment compared with Max20∞C, the pressure drop over the current accelerates low-level horizontal winds, causing the increase of latent heat flux from the ocean and the enhancement of updrafts over the short trough.