Cumulus Group Activity over the Japan Sea in Wintertime in Relation to the Water Vapor Convergence in Subcloud Layer

[摘要] Statistical analyses on the activity of the cumulus clouds over the Japan Sea in wintertime are made in relation to the water vapor convergence in the subcloud layer and the evaporation from the sea surface by using radar and aerophotographic cloud observations.The convective transfer of water vapor across the base of cloud layer is defined in the water vapor budget in the subcloud layer. High correlation between the magnitude of convective transfer and the connective activity is verified.Over the zone of convergence of large-scale or mesoscale disturbance, thehorizontally converged water vapor can not be compensated by the upward transfer due to the mean vertical motion. Thus the upward convective transfer is required to balance the water vapor budget. The convective transfer of 10mm, day-1 is expected only over the zone of convergence whose order is of 10-4sec-1, a characteristic value of the convergence field of mesoscale systems. This is a reason why active cumulus groups exist only over the mesocsale convergence zone.When the distribution of mean coverage of convective cells observed on PPI radar scope is obtained by a time average for longer time interval, the effect of evaporation from the sea surface upon the cumulus activity becomes observable. It is shown that only a few mm. day-1 difference of the amount of evaporation remarkably control the activity of cumulus coluds.Some statistical features of cumulus clouds such as the height-spacing relation and the area ratio of the core of the upward motion are also discussed.

[学科分类] 大气科学