Observational study on formation of a localized rainfall on a basin with heat and aridity on days of weak synoptic disturbance in summer

Abstract

To elucidate the formation of localized rainfall in a basin with heat and aridity under a weak synoptic disturbance in summer, we described the characteristics of atmospheric conditions on the Kofu Basin preceding the appearance of primary precipitating cells from 23 localized rainfall events on the Kofu Basin on days of weak synoptic disturbance at the surface from 1 June to 30 September in 2012 to 2014. Furthermore, on the basis of a case study of an event on 25 July 2014, the formation of the atmospheric conditions was described from the standpoint of moisture behavior. Owing to the thermal contrast between the Kofu Basin with heat and aridity and the outside environment, the south-component wind blowing in the valley connecting to the coastal region of Suruga Bay and the eastcomponent wind blowing in the valley connecting to the Kanto Plain entered the basin as southwesterly wind and southeasterly wind, respectively, which caused an increase in the water vapor mixing ratio and a slight decrease in temperature at the surface. Thereafter, the amount of precipitable water vapor derived from the global navigation satellite system observations (GNSS-PWV) in the central region of the Kofu Basin increased abruptly after the moderate increase in GNSS-PWV at all the observation points in the basin. Finally, a cloud appeared over the local region between the southwesterly wind and the southeasterly wind and precipitating cells appeared at that location at 3.25 to 6.25 km above sea level. We discussed the moisture transport into the Kofu Basin, the concentration of that moisture in a local region, and the appearance of precipitating cells. This is an example of the formation of atmospheric conditions leading to localized rainfall in a basin with heat and aridity.