Mechanisms of Extreme High Temperature Phenomena in the Inland Kanto Plain

  • Yuya TAKANE, 2012: Mechanisms of Extreme High Temperature Phenomena in the Inland Kanto Plain .

Abstract

Over the past 22 years (1990-2011), the inland Kanto Plain in Japan has had 75 days with anextreme high temperature (EHT) of at least 37.2◦C on Kumagaya. To determine synoptic-scaleand mesoscale conditions for such EHT phenomena, a statistical analysis was conducted usingobservational data. The result shows that a daily minimum surface air temperature exceeding21.4◦C and an air temperature at 850 hPa exceeding 18.8◦C are necessary conditions for EHTphenomena in this area. EHT days satisfying these conditions were categorized into 27 typesaccording to different combinations of pressure, surface wind patterns, and the number of con-secutive preceding clear-sky days. The most frequent type occurred 8 times (10.7% of days) andhad the "Kujira-no-o" (WHALE) pressure pattern, a southeasterly (SE) surface wind, and atleast four (4+) consecutive preceding clear-sky days. However, the type with the highest averagemaximum surface temperature had a different wind direction. Its type was WHALE, a northwest(NW) surface wind, and 4+. This type happened only twice but produced an average temperatureof 39.7◦C.To determine the cause of two EHT phenomena, occurring on June 24, 2011 (WHALEtype, NW, 0-1 consecutive preceding clear-sky days), and August 16, 2007 (WHALE, NW, 4+),we apply multiphasic analyses using the Weather Research and Forecasting (WRF) model andobservational data.At 1420 Japan Standard Time (JST) on June 24, 2011, an EHT of 39.8◦C was observedin Kumagaya, a city located 60 km northwest of central Tokyo. In this event, surface air tem-peratures exceeding 37.0◦C were recorded in and around Kumagaya, an area just north of theconvergence line between westerly winds and a southwesterly wind. According to the heat bud-get analysis, from 0500 to 1200 JST, 101% of all sensible heat supply in the mixed layer camefrom the net heat input turbulent heat transport and the surface sensible heat. However, from1200 to 1420 JST, 97% of the net heat input came from advective heat transport. The advectivecontribution greatly increased from 1200 to 1300 JST, when a westerly wind penetrated the EHTarea. This westerly wind, according to backward trajectory analysis, Lagrangian energy bud-get analysis, and Euler forward tracer analysis, arose from a combination of two kinds of foehnflow. Specifically, the westerly wind became foehn winds caused by adiabatic heating (type 2foehn) and by diabatic heating (type 1 foehn). Such a type "hybrid" foehn wind not have beenconsidered previously as a trigger mechanism for an EHT phenomenon.At 1442 JST on August 16, 2007, an EHT of 40.9◦C was observed in Kumagaya. The anal-yses show that this EHT phenomenon was caused by a combination of two particular factors: (1)Persistent sunshine and a lack of precipitation were seen in Kumagaya during the seven consecu-tive days preceding August 16, 2007. This was the 12th longest stretch of clear-sky days in Julyand August from 1998 to 2008. Persistent clear-sky days allow the ground surface to dry out,which produces an increase in sensible heat flux from the ground surface. This contributes tothe EHT phenomenon, and its mechanism is qualitatively supported by the results of sensitivityexperiments of soil moisture on surface air temperature. (2) A foehn-like wind appears in thenumerical simulation, which is caused by diabatic heating with subgrid-scale turbulent diffusionand sensible heat flux from the ground surface when this airflow passes in the mixed layer overthe Chubu Mountains and the inland Kanto Plain. Backward trajectory analysis and Lagrangianenergy budget analysis show that the foehn-like wind plays a more important role in the EHTphenomenon than the adiabatic type 2 foehn pointed out by previous studies.

Keywords:extreme high temperature, foehn, Kanto Plain, WRF model

The following paper was based on a portion of his Doctoral's thesis was published in International Journal of Climatology(2014).» Details

The following paper was based on a portion of his Doctoral's thesis was published in J. Appl Meteor Clim.(2013).» Details