Multiphasic analyses of a recent record-breaking high temperature of 39.8ºC in the Tokyo metropolitan area

A record-breaking mesoscale high surface air temperature in Japan of 39.8 ºC occurred at 1420 JST 24 June 2011, 60-km northwest of central Tokyo. In this extreme high temperature (EHT) event, surface air temperatures exceeding 37.0 ºC extended in and around Kumagaya, an area just north of the convergence line between westerly winds and southwesterly sea breeze. To determine the cause of this EHT event, we apply multiphasic analyses to the Weather Research and Forecasting (WRF) model and observational data. According to the heat budget analysis, from 0500 to 1200 JST, 101% of all sensible heat supply in the mixed layer came from the net heat input turbulent heat transport and the surface sensible heat. But from 1200 to 1420 JST, 97% of the net heat input came from advective heat transport. The advective contribution greatly increased from 1200 to 1300 JST when westerly wind penetrated the EHT area. This westerly wind, according to backward trajectory, Lagrangian energy budget, and Euler forward tracer analysis, arose from a combination of two kinds of foehn flow. Specifically, the westerly wind became a dry (dynamic) foehn wind caused by adiabatic heating combined with a wet foehn wind caused by diabatic heating. Such a “hybrid-foehn” wind may not have previously been considered as a trigger mechanism for an EHT event.