Frequency and Temporal Characteristics of the Thermal Belt over the Western Slope of Mt. Tsukuba

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  • HORI E. Masatake
    Department of Environmental Sciences, University of Tsukuba Meteorological Research Institute/Japan Science and Technology Agency
  • UEDA Hiroaki
    Department of Life and Environmental Sciences, University of Tsukuba
  • NOHARA Daisuke
    Terrestrial Environmental Research Center, University of Tsukuba Meteorological Research Institute/Japan Science and Technology Agency

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  • 筑波山西側斜面における斜面温暖帯の発生頻度と時間変化特性
  • ツクバサン ニシガワ シャメン ニ オケル シャメン オンダンタイ ノ ハッセイ ヒンド ト ジカン ヘンカ トクセイ

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Abstract

Utilizing spatially distributed air temperature monitors, we studied the frequency and temporal characteristics of the thermal belt over the western slope of Mt. Tsukuba, located in Ibaraki Prefecture, Japan. Automated observation at 10-minute intervals was carried out for 90 days from November. 3, 2002, to January. 31, 2003. Long-term observation of the thermal belt with such fine temporal resolution is unprecedented and has made the quantitative study of the thermal belt based on a large number of cases possible. Throughout the observation period, the nighttime minimum temperature on the slope of the mountain was higher than that on the plain at the foot of the mountain. Particularly in November, while temperatures lower than 0°C were frequently observed on the plain, the temperature on the slope was always above 0°C, which revealed a distinct bias toward warmer temperatures. While the nighttime temperature distribution over the plain showed no bias toward a temperature warmer or colder than 0°C, a distinct warm bias favoring temperatures above 0°C was observed on the slope. The criterion for the thermal belt event was defined as a temperature inversion between the slope and the plain of greater than +2°C and persisting longer than 10 hours. Thirty-seven to 47 cases of the thermal belt constituting 42-53% of the observational period were found, with no particular differences in frequency amang months. In a typical thermal belt case, the temperature on the plain cooled rapidly around sunset, leading to a temperature inversion between the slope and the plain. Subsequently, the temperature decrease over the slope became gradual, thus maintaining a sustained level of temperature inversion throughout the night. Similar results were seen in the composite temperature field for the thermal belt cases. While the temperature in the center of the thermal belt became colder toward January, the extent of the inversion increased slightly. The temperature field also revealed a zonal structure of the thermal belt centered near the northwestern slope with an altitude of 200-300m throughout the night.

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