大阪府柏原市における雨滴粒径分布の時間変動特性

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  • オオサカフ カシワラシ ニ オケル ウテキ リュウケイ ブンプ ノ ジカン ヘンドウ トクセイ

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大阪府柏原市における雨滴粒径分布の時間変動特性を明らかにすることを目的として,2015年5月から2016年9月まで光学式雨滴粒径分布計を用いて雨滴粒径分布の観測を行った。単位面積の水平面を10分間に通過するすべての粒径の雨滴数を総雨滴数と定義し,降雨強度と総雨滴数の関係を月ごとに調べた。その結果,解析した15ヶ月間のうち降雨時間数が少なかった2ヶ月を除けば,概ねどの月においても降雨強度数 mm h〓〓以下の雨では,ある降雨強度における総雨滴数のばらつきが大きかった。総雨滴数のばらつきが最も大きかった2016年6月を対象に,降雨強度と総雨滴数の関係について詳細な事例解析を行った。12個の降雨事例のうち,総雨滴数のばらつきが小さい事例(タイプA )が9事例あり,大きい事例(タイプB)が3事例あった。タイプBの事例では,事例前半の約半日の期間において,総雨滴数が比較的少ないタイプAの事例と類似した雨が降っていた。これに対して,事例後半の約半日〜約1日の期間では,総雨滴数が比較的多い,降雨強度3 mm h〓〓未満の弱い雨が降っていた。タイプAの事例およびタイプBの事例前半の期間における雨滴粒径分布は,Marshall-Palmer分布と良く一致した。一方,タイプBの事例後半の期間における雨滴粒径分布は,Marshall-Palmer分布と比べて直径1mm以上の雨滴の数密度が少なく,直径0.5mm未満の微小な雨滴の数密度が非常に多かった。したがって,約半日から約1日にわたって降り続く3事例の霧雨が,雨滴粒径分布の時間変動を特徴づけていたと結論づける。

Observations of raindrop size distribution (DSD) were conducted using an optical disdrometer from May 2015 to September 2016 to clarify the characteristics of time variations in DSD in Kashiwara, Osaka, Japan. The authors defined the number of raindrops of all particle sizes passing through the horizontal plane of the unit area over 10 minutes as the total number of raindrops, and examined the relation between rainfall intensity and total number of raindrops on a monthly basis. The results showed that (except for the two months in which the number of rainfalls was small among the analyzed 15-month period) for a rainfall intensity of less than a few mm h〓〓, the total number of raindrops at a certain rainfall intensity showed a large variation in any given month. A detailed analysis was conducted on the relation between rainfall intensity and total number of raindrops in June 2016, when the variations in the total number of raindrops were the largest. Of the 12 rainfall cases analyzed, there were nine cases in which the variations in the total number of raindrops were small (Type A), and three cases in which they were large (Type B). In the Type B cases, the rainfall was similar to the Type A cases, which had a relatively small total number of raindrops during the half-day period in the first half of the case. On the other hand, during the period from around half a day to about one day in the latter half of the case, there was weak rain with a relatively large total number of raindrops and rainfall intensity of less than 3 mm h〓〓. The DSDs in the Type A cases and during the first-half period of the Type B cases agreed well with the Marshall-Palmer distribution. Conversely, the DSDs in the latter half of the Type B cases had a smaller number density of raindrops with a diameter of 1 mm in comparison with the Marshall-Palmer distribution, and the number density of minute raindrops with a diameter of less than 0.5 mm was very large. The authors concluded, therefore, that the three cases of drizzle that continued from around half a day to about one day characterized the time variations in DSD.

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