It has been known that tropical cyclogenesis does not occur in the region where wind shear is large. This observational fact is understood as evidence that vertical shear ventilates the warm core which is developed by cumulus convection in the initial vortex of tropical cyclone. On the other hand, Bister and Emanuel(1997) reports that cooling associated with the evaporation of stratiform precipitation creates the initial vortex of Hurricane Guillermo(1991), and suggest that such initial vortex spin-up mechanism may be fragile in vertically sheared environmental wind.

In this article, a dry three-dimensional nonhydrostatic model is used to investigate the influence of vertical shear on the initial vortex spin-up by evaporational cooling. Total of 36 parameter experiments are performed. In each of cases, a localized cylindrical region of 4.75km height is cooled at a constant rate representing the effect of stratiform rain evaporation. The cooling rate is one of 5K/day, 10K/day and 20K/day, and the horizontal radius of the cooled region is one of 50km, 100km, and 200km. Linearly sheared envirormental wind is introduced as an initial condition with its magnitude increasing from zero at the ground surface to 0m/s(no shear cases), 1m/s, 3m/s, or 6m/s at 7.25km. The cooling region is migrated at the wind speed at the 5km height.

In case without wind shear, a cyclonic vortex develops at the top of the cooling region and extends straight to the ground. In case with weak shear, vortex extends slantwise from the top of the cooling region, but still it can reach the ground surface. In case with strong wind shear, however, background wind blows off the vortex completely, so that the vortex cannot reach the ground. The effect of wind shear on cooling induced vortex can be diagnosed by a non-dimensional number, SI, defined as SI= SD² s/ 2CL( L:the horizontal extent of the cooled region, D:the vertical extent of the cooled region, s:static stability C:the intensity of the cooling, S:the magnitude of vertical shear). The results of numerical experiments show that the vortex is blown off if SI is larger than 2-3.

With typical parameters in the real tropical atmosphere (rate, radius, and depth of evaporational cooling are 10K/day, 100km, and, 5km, respectively ), vertical shear of 4m/s/7km gives SI=2, suggesting that even relatively weak wind shear can easily prevent the genesis of initial vortex. This implies a new interpretation of the observed sensitivity of tropical cyclogenesis to vertical wind shear.