Article ID: 2019-029
The September 2015 Kanto-Tohoku heavy rainfall occurred in a stationary linear convective system between Typhoons Kilo and Etau. We investigated the influence of sea surface temperature (SST) on the local heavy rainfall event using a regional air-sea strongly coupled data assimilation system based on the local ensemble transform Kalman filter (LETKF) and a nonhydrostatic atmosphere model (NHM) coupled with ocean-surface wave model and a multilayer ocean model together with the Advanced Microwave Scanning Radiometer 2 (AMSR2) level 2 (L2) SST product. From the validation of SST analyzed by the coupled data assimilation system with the Japanese geostationary satellite multi-functional transport satellite 2 hourly SST product and in situ observations at the moored buoy, we demonstrated that the coupled system with AMSR2 L2 SST led to improvement of the SST analysis. From the verification by using radiosonde observations and radar-raingauge rainfall analysis, the analysis of the lower-atmospheric components were improved by the air-sea coupled NHM-LETKF.
The local torrential rain occurred around 37°N in the Tochigi prefecture was embedded in a stationary linear convective system. The location of the linear convective system corresponded to the synoptic-scale convergence area between the cyclonic circulation associated with Etau and easterly lower-tropospheric winds. Strong southerly winds associated with Etau caused enhancement of local convection periodically along the convergence area on the upwind side of the linear convective system and resulted in a wave-like train of the total water content around 4-8 km altitudes on the leeward side. The improvement of SST analysis could change not only the transition of Etau to the extratropical cyclone but also lower-tropospheric wind field and thereby the location of the stationary linear convective system with embedded local torrential rain.
