Intensification of tropical-cyclone-like vortices in uniform zonal background-flows

The effect of uniform zonal background-flows on the intensification of tropical-cyclone-like vortices is investigated using a numerical three-layer shallow-water model that includes parameterizations of convection, sea-surface energy-exchange, and boundary-layer friction. Calculations on an f-plane showed that during the developing stage, an initially weak vortex intensified more rapidly in the presence of a uniform zonal background-flow. Compared to an environment at rest, the uniform zonal background-flow resulted in increased boundary-layer convergence collocated with increased boundary-layer mixing ratio (as a result of surface moisture fluxes), producing stronger convection and more rapid intensification. After the vortex achieved hurricane strength in the presence of an easterly (westerly) background-flow, a region of convectively stable air formed to the south (north) of the vortex centre as a result of small values of surface moisture flux and the downward transport of relatively dry middle-layer air. This convectively stable air penetrated slowly into regions of boundary-layer convergence, thereby weakening convection and reducing the maximum intensity of the vortex compared to the case without background flow. This effect was more pronounced for stronger background-flows. Calculations on a β-plane showed that a westerly background-flow is more favourable for intensification than an easterly background-flow of the same strength. In the former, the vortex always moved into a region of convectively unstable air, whereas, in the latter, the vortex progressively moved into a region of convectively stable air produced by subsidence and drying of the bundary layer. Because of the relatively high vortex drift speed in the easterly background-flow case, there was not sufficient time for surface moisture fluxes to moisten the boundary-layer air rapidly enough to eliminate the convectively stable region ahead of the vortex, resulting in the overall weakening of convective activity and a less intense storm than in the westerly flow case. The foregoing calculations suggest that the imposed background-flow affects the development of the vortices in the shallow-water model by establishing an environment in which boundary-layer convergence and surface moisture fluxes may interact to produce stratifications that either enhance or suppress convection, the occurrence of which is necessary for the intensification of the vortices in the model.