Climatological study on mesoscale extreme high temperature events in the inland of the Tokyo Metropolitan Area, Japan, during the past 22 years

Over the past 22 years (1990-2011), the inland part of the Tokyo metropolitan area has had 75 days with an extreme high temperature (EHT) higher than 37.2 degrees C in Kumagaya, Saitama Prefecture. To determine the synoptic-scale and mesoscale conditions for such EHT events, a statistical analysis was conducted using observational and objective analysis data. The results show that all the EHT events satisfy the conditions of a daily minimum surface air temperature above 21.4 degrees C on Kumagaya and an air temperature at 850 hPa above 18.8 degrees C on Tsukuba. The EHT days satisfying these conditions were categorized into 27 types according to different combinations of synoptic-pressure patterns, surface wind patterns, and the number of consecutive preceding clear-sky days. The most frequent type occurred eight times (10.7% of days) and was the WHALE' (tail of a whale) pressure pattern with a southeast (SE) surface wind, with at least four (4+) consecutive preceding clear-sky days. However, the type with the highest average maximum surface air temperature had a different wind direction, which was WHALE with a northwest (NW) surface wind and 4+ consecutive preceding clear-sky days. This type appeared only twice but produced the average maximum temperature of 39.7 degrees C.