LAUSR

A recent idealized numerical study of tropical cyclogenesis and subsequent intensification using warm-rain-only microphysics is extended to examine the modifications brought about by a representation of ice processes. It is found that the time taken to reach cyclogenesis is more than twice that in the equivalent warm-rain-only simulation. The subtle reasons for the difference in the length of the gestation period are discussed. A mid-level vortex forms during the early gestation period when ice processes are present, but not when warm-rain-only processes are present. Axisymmetric balance calculations show that the spin-up of this mid-level vortex is related to the different spatial distribution of diabatic heating rate in the presence of ice, which leads to a system-scale radial influx of absolute vorticity in the middle troposphere. The tropical-cyclone vortex that forms in the simulation with ice is similar to that in the warm-rain-only simulation, with the strengthening frictional boundary layer exerting a progressively important role in focusing inner-core deep convection. This vortex develops in situ on a much smaller scale than the mid-level vortex and there is no evidence that it is a result of the mid-level vortex being somehow carried downwards, as has been suggested previously by some researchers. Some implications of the results in relation to previous theories of tropical cyclogenesis are discussed.