Structure of a Land Breeze and Snowfall Enhancement at the Leading Edge

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.

A Doppler Radar Observation on Wave-like Echoes Generated in a Strong Vertical Shear

“…Takayabu (1992) reported the roll cloud caused by K-H instability. Wakimoto et al (1992) observed K-H waves in a warm frontal rain band using a airborne Doppler radar, and Tsuboki et al (1989) analyzed K-H waves along the interface of land breeze and prevailing wind within snow clouds using a Doppler radar. Wang et al (1983) discussed the Ri in the vicinity of the rainband, where the atmosphere is in a saturated or near-saturated condition.…”

Diurnal Variations of Snow Precipitation in Wakasa Bay during Winter

“…This convergence enhances the development of clouds during night and early morning hours. Tsuboki et al (1989) also reported that the land breeze strongly influenced the precipitation distribution and snowfall could be enhanced due to the land breeze. On the contrary, when winds blow from the sea during daytime, the reverse situation occurs, which causes precipitation to decrease in the bay area.…”

“…During winter in Japan, precipitation (including snowfall) distribution changes largely by synopticscale and mesoscale circulations, although topographical features also play an important role in varying the characteristics (Tachibana 1995). Ishihara et al (1989) showed the development of mesoscale snow clouds associated with land breeze and winter monsoon flow, and Tsuboki et al (1989) studied a winter land breeze and its effect on snow clouds. However, the land-sea breeze in the above studies is described as a more seasonal feature than a diurnal one driven by the seasonal land-sea temperature contrast, and can be observed at any time of the day.…”

Doppler Radar Observation of Convergence Band Cloud Formed on the West Coast of Hokkaido Island. II: Cold Frontal Type

“…Simpson et al (1977) suggested that the mixing is due to Kelvin-Helmholtz instability on the sea breeze fronts. Fujiyoshi et al (1988) and Tsuboki et al (1989) have reported that KelvinHelmholtz instability waves occure along a landbreeze frontal surface and that the wavelength is *1 km. In the case of a cold frontal type, the RHV display showed the occurrence of Kelvin-Helmholtz instability waves whose wavelength was 1-2km.…”