The effect of mid-level moistening on tropical cyclogenesis

A high resolution nonhydrostatic cloud model is used to investigate the importance of mid-level moistening on the process of tropical cyclogenesis. The model is initialized with a weak mid-tropospheric cyclonic vortex over a warm ocean surface. The simulation shows the development of deep cumulonimbus towers that possess intense cyclonic vorticity in their cores. The transition to a tropical depression takes approximately four days. The pathway to this stage is by a series of intense convective pulses that consume CAPE, followed by periods of regeneration of CAPE. At the same time the mid-levels of the vortex core gradually moistens due to venting of moist near surface air aloft by the "vortical" hot towers. Prior to rapid development the vortex core is close to saturation at mid-levels and the mid-level circulation has also intensified significantly. Deep convection within this near saturated air close to the center of the vortex leads to a rapid strengthening of the low-level circulation. The system continues to intensify to hurricane strength and develops an eye and sloping eye wall. In order to ascertain the importance of mid-level moistening an identical simulation is carried out, but with the initial vortex moistened to be close to saturation.