A Numerical Study on the Effects of Environmental Flow on Tropical Storm Genesis

Abstract

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