Characteristics of snow-depth distribution in Japan during heavy snowfall of 2005-2006 winter

平成18年豪雪における積雪深分布を詳細に明らかにすることを目的として，2006年2月6日9:00に多機関で計測された積雪深を全国的に収集した．1917地点のデータを基に日本列島の積雪深分布図の作成を行い，その地域的特徴について調べた．その結果，3mを超える豪雪エリアは日本海側地域の内陸山間部を中心に広がり，なかでも越後三山周辺では5mに達するエリアがみられた．日本海側地域における積雪深の分布には高度依存性がみられたが，標高の増大に伴う積雪深の増加率は南北で異なっていた．山形県および新潟県の積雪深は，標高300m以上の地点で横ばいに分布する傾向がみられ，高標高側ほど根雪期間が長く積雪荷重が大きいため積雪の圧密が促進されることが関与していた．特に新潟県ではこの傾向が他の豪雪年でもみられることから，平成18年豪雪に限定された現象ではないと推察された．同時期の気象庁データ（315地点）を基に作成した積雪深分布図との比較を行ったところ，気象庁データでは山間部の積雪深が1m以上過小評価される傾向がみられ，内陸山間部に偏在化した積雪深分布を正確に把握することが出来ない可能性が示唆された．In this paper, we perform a detailed evaluation of the snow-depth distribution in Japan caused by heavy snowfall of 2005-2006 winter. The snow-depth data were investigated according to the values measured all over Japan on February 6, 2006; consequently, 1917 points were collected. Heavy snow areas (depth of over 3m) were found mainly in the inland mountainous regions bordering the Sea of Japan. The highest snow-depth value was 5m around Echigo-Sanzan. The snow depth uniformly increased with an increase in the altitude in all the regions bordering the Sea of Japan; however, the ratios of the increases in the snow depth to the altitude were different. The snow depths in Yamagata and Niigata prefectures remained unchanged at over 300m a.s.l. These results can be attributed to the consolidation of the snowpack as a result of both the high snowfall loading and long duration of snow covering. In the Niigata prefecture, a pattern of peak values was observed at higher altitudes during other heavy snowfall occurrences as well, which implies that such altitudinal distribution of the snow depth was not limited only during the 2005-2006 winter. In the heavy snowfall area located in inland mountainous region, the snow-depth distribution at 315 points (measured by the Japan Meteorological Agency) was estimated to be smaller than the values in our data for a ddepth of above 1m; therefore, it might be impossible to exactly understand the snow-depth distribution including the maldistribution of the snowpacks in inland mountainous regions