Effects of the Prevailing wind with Vertical Shear on the Convective Cloud Accompanied with Heavy Rainfall

Abstract

Under the assumption that a convective system with an updraft in the downshear side and a downdraft in the upshear side is already formed in the conditionally unstable atmosphere, the effects of the prevailing wind with vertical shear on the system are studied by numerical integration of hydrodynamic and thermodynamic equations in which the formation, the evaporation and the fall of water drops are taken into consideration.
From computations of two cases which are different only in the intensity of vertical
shear, following results are obtained. In the case with stronger vertical shear, larger amount of raindrops evaporates in broader region on the way of their falls than in the case with weaker shear and the cooling due to the evaporation is more intensive in the whole atmosphere. However, the downdraft in the former case cannot develop so much as in the latter case mainly because the cooling is not concentrated in the main part of the downdraft. Strong vertical shear weakens the cold-front-like action of the air diverging from the downdraft on the warm air in low level, as it prohibits the development of the downdraft and it causes streamlines in the downdraft region to be unfavorably inclined. Further, it inhibits also the development of the updraft. These prohibiting effects of strong vertical shear are also made clear from the view-point of energy-conversion. Following conclusions can be deduced from these results, the previous work of the author (1965) being taken into account ; that is, vertical wind shear does not intensify a connective system itself, but organizes the release of convective instability-energy by maintaining the convective system and causing it to propagate. There is a criterion in the intensity of vertical shear which is favorable for the maintenance of the convective system. This criterion will be dependent upon the degree of the development of the convective system.