Dual-Doppler Radar Observations on Factors Causing Differences in the Structure of Snow Clouds during Winter Monsoon Surges

Various processes of development in a convective cell were investigated in order to clarify the factors causing differences in the structure of snow clouds. Convective cells in snow clouds were observed by two Doppler radars over Ishikari Bay, Hokkaido, Japan, during winter monsoon surges. Seven cells that allowed us to examine structural changes were selected for the investigation. From the time series of maximum reflectivity and vertical mass flux in each cell, the analyzed cells were classified into two groups: “rapid change” and “slow change.” The former was further divided into two types according to the strength of vertical wind shear in the environment: rapid change in weak shear (Rw), and rapid change in strong shear (Rs). The structure of the Rw-type changed greatly, due to the formation and the descent of a high-reflectivity core (> 30 dBZ), and the subsequent dissipation of the updraft. Since the updraft hardly tilted and collapsed when the core descended in the updraft, loading by graupel particles was suggested to be effective to terminate the updraft. The structure of the Rs-type changed greatly due to the short lifetime of the updraft that was tilted toward the downshear side due to the strong vertical shear. In contrast, the cell of slow-change group (S-type) maintained its structure because of a persistent updraft. Since the updraft, which was tilted hardly, was sustained even when a core (≤30 dBZ) descended in the updraft region, the loading effect was thought to be small due to the small amount of graupel particles. From a comparison of the three types, it was found that the different features of the cell’s structure were caused by two factors: the tilt of the updraft, and the effect of loading by graupel particles. Based on these results, we discussed the maintenance process of a snow cloud composed of convective cells. The fact that convective cells in the past observational studies can also be classified in the same way as cells analyzed in the present study, was also discussed. The relationship between the growth of graupel particles, and the temperature in the environment, was also discussed.

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