InSAR observation and numerical modeling of the water vapor signal during a heavy rain: A case study of the 2008 Seino event, central Japan

This study reports the first detection and analysis of a localized water vapor distribution obtained using interferometric synthetic aperture radar (InSAR) during the Seino heavy rain episode. The InSAR data retrieved during the ALOS/PALSAR emergency observations for the event revealed a radar line-of-sight (LOS) change of up to 130mm within 10km. Based on the signal, we estimated the three-dimensional water vapor distribution using the ray-tracing method, which indicated a column of nearly saturated water vapor within a 10km(2) area reaching from the surface to 9000m above ground level. To geophysically confirm this signal, Weather Research and Forecasting (WRF) model simulations were performed, revealing a deep convection that was initiated by orographic lift caused by the Yoro Mountains. Another simulation that did not include the Yoro Mountains did not produce a deep convection. The WRF simulation also suggested that the effect of hydrometeors can account for approximately 20% of the maximum LOS change but this effect is even more localized than the effect of water vapor.