Journal Article Synoptic-scale fire weather conditions in Alaska

10 ( 3 )  , pp.217 - 226 , 2016-09
Recent concurrent widespread fires in Alaska are evaluated to assess their associated synoptic-scale weather conditions. Several periods of high fire activity from 2003 to 2015 were identified using Moderate Resolution Imaging Spectroradiometer (MODIS) hotspot data by considering the number of daily hotspots and their continuity. Fire weather conditions during the top six periods of high fire activity in the fire years of 2004, 2005, 2009, and 2015 were analyzed using upper level (500 hPa) and near surface level (1000 hPa) atmospheric reanalysis data. The top four fire-periods occurred under similar unique high-pressure fire weather conditions related to Rossby wave breaking (RWB). Following the ignition of wildfires, fire weather conditions related to RWB events typically result in two hotspot peaks occurring before and after high-pressure systems move from south to north across Alaska. A ridge in the Gulf of Alaska resulted in southwesterly wind during the first hotspot peak. After the high-pressure system moved north under RWB conditions, the Beaufort Sea High developed and resulted in relatively strong easterly wind in Interior Alaska and a second (largest) hotspot peak during each fire period. Low-pressure-related fire weather conditions occurring under cyclogenesis in the Arctic also resulted in high fire activity under southwesterly wind with a single large hot-spot peak.
アラスカの活発な火災の総観規模での気象条件を明確にした。活発な火災活動期間は、2003〜2015年のMODISホットスポット(HS)のデータを使って、日HS数(>300)とその継続性で抽出した。総HS数の大きな7大火災期間中の気象条件は、大気の高層レベル(500 hPa)と低層レベル(1000 hPa)での大気再解析データなどを用いて分析した。上位4位までの火災期間では、ロスビー波の砕波(RWB)現象が共通して発生していた。高層ではブロッキング高気圧がアラスカ上空で形成され、低層ではアラスカ南側の高気圧が北側に移動していた。この高気圧の移動に伴い、最初はアラスカ南部の高気圧から南西風が、次にアラスカ北部の高気圧から北東風がアラスカ内陸部に吹き込んで、2段階での活発な火災期間が生じていた。特に、北側に移動した高気圧がボーフォート海で発達し、比較的強い北東風が継続して吹き続けることで、大火災が生じることを明らかにした。

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