Formation of a Cold Air Lake during the Warm Season in a Small Hollow in Mt. Inako-dake, Yatsugatake Range

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  • 八ヶ岳連峰稲子岳の凹地内における暖候期の冷気湖形成
  • ヤツガタケ レンポウ イナコダケ ノ オウジナイ ニ オケル ダンコウキ ノ

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The present study focuses on the warm season temperature conditions in a small hollow of Mt. Inako-dake, Yatsugatake Range, central Japan, where an interesting vertical structure of vegetation is observed. Here, alpine plants (Pinus pumila community) are located at the bottom (ca. 2, 300m a.s.l.) and a subalpine forest is formed above them (so-called vegetation inversion). An observational study was made of the frequency, intensity, and duration of the cold air lake during the warm sea-son in order to reveal the climate environment related to the vertical structure of vegetation.<br> Air temperature observations were carried out along the east-west section crossing the hollow from June 28 to November 7, 1995. A cold air lake was defined as that which has the maximum differ-ence (referred to as “maximum inversion” in this study) exceeding 2°C in night temperature between the summit of Mt. Inako-lake (the highest point; 2, 380m) and the bottom of the hollow (the lowest point; 2, 230m). Seventy-eight days were identified as having cold air lake formation.<br> Cold air lake formation was substantially influenced by synoptic-scale pressure patterns. Cold air lakes were manifested when this area was under the influence of the North Pacific anticyclone during summer and the migratory anticyclone during autumn. Under the migratory high during au-tumn, the maximum inversion reached 19.5°C (October 13).<br> Based on the synoptic pressure patterns, we categorized cold air lakes into two types: summer and autumn types that are formed under the North Pacific high and the migratory high, respectively. The summer and autumn types were identified for 19 days and 31 days, respectively. These types have respective average maximum inversions of 6.1°C and 8.7°C, and average periods of cold air lake formation of 6.3 hours and 8.7 hours, respectively. The cold air lakes of the summer type developed after midnight (23 h to 1 h), whereas those of the autumn type were formed earlier in the early evening (16 h) and maintained until morning (9 h). The radiative cooling that causes the cold air Takes of the summer type was likely weakened due to the increased downward long-wave radiation from clouds that persist until midnight and/or moist air. On the other hand, the dry air mass under the migratory high strengthened radiative cooling overnight for the autumn type.

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