On Snow Particles Comprising an Aggregate

Abstract

Types, degree of riming and size of snow particles comprising aggregates from snow bands were investigated. From the data obtained, the microphysical processes which control the precipitation intensity were elucidated. The results are summarized as follows: 1) Snow particles comprising aggregates were classified into three types: nonrimed or lightly rimed snow crystals; rimed or heavily rimed snow crystals having discernible snow crystals prior to riming; and heavily timed or graupel-like snow particles. 2) Nonrimed or lightly rimed snow crystals occupied about 30% of the total number of snow particles comprising an aggregate when precipitation intensity (R) was very intense (>5 mm h?1). 3) Regardless of the intensity of precipitation, there were no apparent differences in size distribution of snow particles. 4) Although the cloud depth was larger when R > 5 mm h?1 than when R < 5 mm h?1, more kinds of snow crystals were observed when R < 1 mm h?1 than when R > 5 mm h?1. 5) Graupel-like particles with nonrimed dendritic extensions and rimed stellar crystals with nonrimed dendritic extensions were found when precipitation intensity was very intense (>5 mm h?1). From these results, it was concluded that riming dominates deposition in the large part of clouds and deposition dominates riming only in layers with an air temperature of around ?15°C. In addition, the depositional growth of the graupel-like particles in this layer was suggested to be an important process leading to increased precipitation intensity. Bemuse they have many long extensions, their collection efficiency must be great and their fall velocity would be larger than that of nonrimed or rimed snow crystals, so that collision frequency between these particles must be high. Therefore, the particles can catch snow particles effectively.