陸風の構造とその前線における降水の強化

Abstract

A land breeze is frequently observed along the coastal region of the Ishikari Plain during the winter season. A land-breeze front is formed offshore between colder over-land air and warmer over-sea air. In this study, we mainly used a single Doppler radar to examine the structure of the land breeze and its effect on snow clouds. The vertical profile of wind velocity was divided into three layers: the landbreeze layer (-300m in depth), the shear layer and the prevailing wind layer. The frontal surface was steep near the front, but almost horizontal at a distance from the front. Kelvin-Helmholtz instability waves were observed along the interface of the land breeze and the northwesterly monsoon wind.The land breeze strongly influenced the modification of snow-clouds. A rapid intensification of snow-cloud echo, which was often observed at the land-breeze front, might have been caused by a strong low-level convergence at the front. Behind the front, however, the echo rapidly weakened and dissipated, since the air mass of the north-westerly monsoon wind lost vapor and its lower part became stable when mixed with the land breeze. Consequently, convection was suppressed. Since the intensification and dissipation of echo recurred, the intense echo region seemed to be stagnant at the front, while each cell moved at a speed of the prevailing wind; this resulted in the localization of heavy precipitation at the front. The time-averaged distribution of echo intensity shows the concentration of precipitation in the coastal region where the front was located. The analysis of the AMeDAS data also shows that precipitation was localized along the coastal region when there was a land breeze.