An Observational Study of Organized Snow Echo over the Japan Sea

Abstract

Successive observations of the radar echo which starts from random distribution of convective cells about 200km off the coast of the Hokuriku district to form longitudinal multiple echo lines, then transverse lines, and finally but less frequently, vortical echo pattern, denote typical evolution of the echo pattern of heavy snowfall on the Japan Sea side.
The present paper aims to clarify, in more detail, properties, synoptic situation and formation process of the organized snow echo associated with the mesoscale severe phenomenon such as heavy snowfall, using materials obtained by the radar.
Firstly, we measured to find that the mean spacing of the multiple echo lines was 30km, which is ten times as large as the mean height of the echo top. The observation supports Asai's theoretical result with respect to the spacing of longitudinal cloud bands.
Secondly, the synoptic situation of the mesoscale vortical echo has been specifically examined. Synoptic survey of the environmental condition of the vortical echo was performed for seven samples. Main features commonly observed in the surface pressure pattern are a sharp ridge with the radius of curvature of 100km over the mountainous region of Jadan Island, a cyclonic circulation over the Japan Sea and predominadt difference of the vertical wind shear in the lower atmosphere between northern and western parts of the mesoscale vortical disturbance.
Thirdly, the echo band nearly along the discontinuity line of the surface wind field and transverse echo lines intersecting the band were observed as a sign of the subsequent vortical echo. A model of the formation process of the vortical echo is presented in this paper.