Impacts of Foehn for High Temperature in Hokuriku and North Kanto Districts

  • Ai KAKINUMA, 2014: Impacts of Foehn for High Temperature in Hokuriku and North Kanto Districts .

Abstract

In this study, conditions of foehn day are determined and extracted by wind direction, potential temperature and cloud cover condition. Then, impacts of foehn on night-time minimum and day-time maximum temperature (T_min, T_max) are identified for extreme high temperature days and high temperature days in Hokuriku (Nigata, Toyama) and north Kanto (Maebashi, Kumagaya) districts. In addition, the foehn days are categorized by type of foehn, using the Froude number condition and occurrence frequency of each foehn type is analyzed by each place.

First, the extreme high temperature days and high temperature days are defined and target days for investigation are set as from April 2001. Here, extreme high temperature days and high temperature days are determined by the difference between T_min, T_max and climatic normal value for the past 20 years (April 1994 - March 2014), and categorized into four different types. For the target days, days which fulfill weak foehn condition (surface and upper wind direction) are determined as possible-foehn-occurring day. And, frequency of weak foehn condition days is also analyzed. As a result, occurrence frequency of weak foehn condition days is the highest in Maebashi. This is considered due to upper prominent wind direction, the mountain wind and less influence of sea breeze. For seasonality, in north Kanto district frequency of weak foehn condition days in winter is the highest, because of seasonal wind (northwest wind). In Hokuriku district, frequency of weak foehn condition days peak at spring and autumn, it’s considered due to southerly wind such as extratropical cyclone and typhoon respectively.

Second, strong foehn condition is determined by wind direction, the difference of potential temperature between windward and leeward of mountains, and cloud cover at leeward of mountains. The days which fulfill the strong foehn condition are determined as foehn occurrence days, and frequency of strong foehn condition days is analyzed for extreme high temperature days and high temperature days. As a result, foehn frequency for all types of high temperature days is higher in north Kanto district than in Hokuriku district. Frequency of strong foehn condition days for weak foehn condition days is also higher in north Kanto district than in Hokuriku district. As for seasonality, similar to weak foehn condition, frequency of strong foehn condition days in winter is the highest in north Kanto district. Frequency of strong foehn condition days is about 50% of weak foehn condition days in Maebashi, about 80% in Kumagaya, each month. In Hokuriku district, both at T_min and T_max, frequency of strong foehn condition days was found to be higher in spring and autumn with high frequency of weak foehn condition day. In winter, frequency of strong foehn condition day is almost 0%, this is presumed due to fewer clear sky days in winter in Hokuriku district.

Third, for the days which fulfill strong foehn condition of high temperature days, foehn occurrence days are categorized by the type of foehn based on the Froude number (Fr) condition. Here, Fr high days (Fr ≥ 1.2) are as the foehn type I (wet foehn), and Fr low days (Fr < 0.8) are as the foehn type II (dry foehn). For T_min, in Hokuriku district, frequency of foehn type II is higher than that of foehn type I. In north Kanto district, frequency of the foehn type I and II are comparable level. For T_max, in Hokuriku district, frequency of foehn type II is higher than that of foehn type I. In north Kanto district, frequency of foehn type I was found to be higher than type II.

Finally, for T_max in Maebashi and Toyama, foehn occurrence day conditions and foehn type condition are verified by observation data. As a result, it was confirmed that the extraction with foehn occurrence day conditions is mostly correct and the classification with foehn type condition (Fr condition) is also mostly correct.

Key Words:foehn, high temperature, Hokuriku district, north Kanto district