Permalink : http://hdl.handle.net/2115/91746

JaLCDOI : 10.14943/lowtemsci.82.183

KEYWORDS : 筋状降雪雲;日周期変動;領域大気モデル;snowbands;diurnal variation;regional atmospheric model

衛星やレーダーの画像を調べると，シベリアからの寒気吹き出し時に発生する日本海北東部の筋状降雪雲は，寒気吹き出しに対し直交方向に半日周期で振動することが確認できる．一方，日本海の北西部では，降雪雲の半日周期の振動は不明瞭で，1日周期の振動が卓越する．本研究では，筋状降雪雲の半日周期振動のメカニズムを，領域大気モデルを用いた寒気吹き出し事例の数値シミュレーションと，境界層大気の変動を表現する線形浅水波モデルによる数値実験により調べた．降雪雲の振動をもたらす海上の風速擾乱の励起には，大陸上の境界層乱流の日変動に伴う加熱と摩擦両方の変動が重要であることがわかった．また、沿海州のシホテアリニ山脈は，大きな１日周期気温変動を伴う内陸の寒気の流出を妨げることで，筋雲の半日振動を際立たせる働きを持つと考えられる．
During wintertime cold-air outbreaks from Siberia, the sea-effect wind-parallel snowbands over the northeastern part of the Sea of Japan often exhibit pronounced semidiurnal oscillations in the direction normal to the prevailing wind. On the other hand, in the northwestern part of the Sea of Japan, semidiurnal oscillations are less noticeable and diurnal oscillations are predominant. In this study, the mechanisms for the semidiurnal oscillation of snowbands are investigated through the analysis of realistic numerical simulations of cold-air outbreaks using a regional atmospheric model and idealized numerical experiments using a linear reduced-gravity shallow water model that models the inversion-capped boundary layer flow. It is shown that the diurnal variations of both the oundary-layer heating and friction due to the boundary-layer turbulence over the windward coastal region are important in generating the wind perturbations responsible for the oscillations of snowbands. It is argued that the Sikhote-Alin mountain region acts to highlight the semidiurnal oscillations of snowbands by blocking the outbreak of inland cold air, which is accompanied by a large amplitude of diurnal temperature variation.