Effect of peripheral convection on tropical cyclone formation

The effect of peripheral convection on the formation and intensification of tropical cyclones has been studied earlier with diagnostic models and with prognostic models that do not resolve convection. In this paper, a prognostic, axisymmetric model with explicit convection is used to study the effect of peripheral convection on tropical cyclone formation from a weak mesoscale vortex. Peripheral convection becomes stronger in the model if downdrafts extinguish deep convection in the core of the mesoscale vortex. The subsequent concentration of convection in the core of the vortex and the onset of rapid intensification of the vortex occur simultaneously. In model experiments, relative humidity in the midtroposphere reaches a value of 90% within 100 km from the center of the vortex before the onset of rapid intensification occurs. Decreasing the sea surface fluxes of sensible and latent heat artificially in the outer region of the vortex decreases the amount of outer convection in the model. This results in an earlier onset of rapid intensification of the vortex into a tropical cyclone. By comparing model experiments with normal and artificially decreased sea surface fluxes in the outer region, the response to outer convection is shown to consist of an increase of tangential wind in the outer region, a decrease of tangential wind in most of the inner region in the boundary layer, and heating of the inner region. These changes are unfavorable for future inner convection. With weak inner convection, the important moistening of the inner region is retarded and the onset of rapid intensification is delayed. However, the inner convection's role in the moistening may be somewhat smaller than suggested by the model experiments if the mesoscale vortex is moistened while it forms. As the results suggest that the peripheral convection's detrimental effect for the intensification of the vortex is owing to its effect on the location of future convection, models that do not include convection as a process that responds to stability may severely underestimate the detrimental effects of peripheral convection. The effect of the Coriolis parameter on the intensification depends on the strong downdrafts that severely weaken the inner convection. If the cooling by evaporation of precipitation is prevented in the model, inner convection remains strong and development of the mesoscale vortex into a hurricane occurs in less than 40 h for the value of the Coriolis parameter of both 10 degrees and 30 degrees latitude. When cooling owing to evaporation of precipitation is allowed and Coriolis parameter is that of 10 degrees latitude, the small inertial stability in the outer region of the vortex results in little balanced response to the outer convection. The inner convection resumes relatively early and a tropical cyclone develops in 80 h from the start of the simulation. When the Coriolis parameter is that of 30 degrees latitude, balanced response to the outer convection is stronger and the development of a tropical cyclone takes twice as long as at the latitude of 10 degrees.