Tuleya, R. E., and Y. Kurihara, 1981: A numerical study on the effects of environmental flow on tropical storm genesis. Monthly Weather Review, 109 (12), 2487-2506.

Abstract: The role of the environmental wind in tropical storm genesis is studied using a numerical simulation model. The model used is an 11-level, primitive equation model covering a channel domain of 25 degrees span with open lateral boundaries at 5.5 and 30.5 degrees N. A number of experiments were integrated for 96 h in which the initial zonal mean flow was specified differently. The superposed initial wave disturbances were identical in all experiments. The role of the environmental wind in tropical storm genesis is studied using a numerical simulation model. The model used is an 11-level, primitive equation model covering a channel domain of 25 degrees span with open lateral boundaries at 5.5 and 30.5 degrees N.
The dynamic coupling between the upper-level winds and the low-level movement of the disturbance was found to be an important factor in explaining the role of the environmental wind in storm genesis. Another important factor is the impact of the low-level winds on the latent energy supply. This supply is affected by the relative inflow into a disturbance and by the transfer of momentum from aloft into the boundary layer in a large area surrounding the disturbance.
According to the model results, the storm genesis potential is definitely biased toward easterly vertical shear (easterlies increasing with height) of the environmental flow rather than westerly shear when the mean surface flow is easterly, i.e., -5 ms^-1. The initial perturbation developed into a vigorous tropical storm when an easterly vertical shear of 15 ms^-1 was specified between 150 and 850 mb. In an experiment with a specified westerly vertical wind shear of 15 ms^-1, the perturbation failed to develop beyond a weak tropical depression. In a third experiment with no vertical wind shear but with anticyclonic shear aloft, a tropical storm also developed. In analyzing the structure of the disturbances at the early wave stage it was found that the vertical shear modulated the vertical velocity and rainfall patterns relative to the trough axis.
In studies involving the horizontal wind shear of the basic flow, it was found that cyclonic shear at low levels and, to a lesser extent, anticyclonic shear at upper levels are conducive for storm genesis. The experimental results also indicate a significant change of structure of the disturbance between uniform westerly and easterly flows. Under uniform westerly environmental flow, the initial perturbation developed more and its low-level structure became more characteristic of mid-latitude cyclones.