The impact of latent cooling processes on tropical cyclogenesis and intensity fluctuations

The impact of latent cooling processes on tropical cyclones is investigated by sensitivity simulations with both the axisymmetric cloud-resolving model HURMOD and the fully three-dimensional COSMO model of the German Weather Service. The models are initialized with an idealized axisymmetric cyclone embedded in a convectively unstable atmosphere at rest where convection is triggered by temperature and moisture anomalies. While growth rate and maximum wind speeds become largest in simulations without latent cooling processes, the presence of these processes turns out to be responsible for a delayed development of the tropical cyclone. The latter result is in better accordance with observations. Furthermore, intensity fluctuations of the mature tropical cyclone only occur in HURMOD with latent cooling processes. An analysis of CAPE, CIN and DCAPE reveals that convection outside the eyewall explains these intensity fluctuations by mixing low-entropy air into the boundary layer, while without latent cooling processes significant convective inhibition due to drying by absence of evaporation and cyclone-scale subsidence suppresses vertical mixing. The results by HURMOD and COSMO agree with the exception that small intensity fluctuations still occur in COSMO for the case without latent cooling. These seem to be related to deviations from axisymmetry. A theory for maximum potential intensity should focus on axisymmetric tropical cyclone simulations in which latent cooling is neglected because in this case the cyclone is nearly stationary, has a simple structure and has the largest intensity.