Orographic Dependency of Heavy Rainfall Events in Upper Reach of Tokachi River Basin

Abstract

 In August 2016, when typhoon 10^th approached Hokkaido, record heavy rainfall occurred near the Hidaka Mountain. In this research, we aimed to classify and discuss the heavy rain characteristics using the calculation of Brunt-Vaisala frequency to show the stability of the atmosphere and the dimensionless number. Brunt-Vaisala frequency in dry state and wet Brunt-Vaisala frequency including wetting effect respectively. In area with high altitudes, the Brunt-Vaisala frequency was calculated to be small irrespective of the dry and wet state, suggesting that the atmosphere is unstable. We classified the heavy rain according to the magnitude of the dimensionless number which is the ratio of the kinetic energy of the atmospheric flow around the mountain area and the energy necessary to cross the mountain. It was found that the rainfall increases with the altitude rose, especially in the case where the dimensionless number was large, resulting in more localized rainfall.