Investigation of a recent extreme high-temperature event in the Tokyo metropolitan area using numerical simulations: the potential role of a 'hybrid' foehn wind

A record-breaking high temperature of 39.8 degrees C for June in Japan was observed at 1420 Japan Standard Time on 24 June 2011 60 km northwest of central Tokyo. In this extreme high-temperature (EHT) event, surface air temperatures above 37.0 degrees C were recorded in and around Kumagaya, an area just north of the convergence line between westerly winds from the Chubu Mountains and a southwesterly wind from the Pacific Ocean. To determine the mechanism of this EHT event, we applied various analyses using the Weather Research and Forecasting (WRF) model and observational data. According to the heat budget analysis obtained from the WRF model, during the morning most of the sensible heat supply to the mixed layer came from the net heat input, due to surface sensible heat transported by subgrid-scale turbulent diffusion. However, most of the net heat input came from advective heat transport after noon, when the westerly wind penetrated the EHT area. This westerly wind, according to backward trajectory, Lagrangian energy budget and Eulerian forward tracer analyses, arose from a combination of two kinds of foehn flow (hybrid'-type foehn wind). Specifically, the westerly wind became a foehn wind that was caused by dry-adiabatic heating and wet-diabatic heating with water vapour condensation. This hybrid' foehn wind was an important factor in causing the present EHT event.