The effect of latent cooling processes in tropical cyclone simulations

In order to estimate the role of latent cooling processes for tropical cyclones, we have performed sensitivity simulations with both the axisymmetric cloud-resolving model HURMOD and the fully three-dimensional COSMO model of the German Weather Service. As an idealised initial state, a convectively unstable vortex with superimposed entropy anomalies has been assumed. While (growth rate and maximum wind speeds become unnaturally large in simulations without latent cooling processes, the presence of these processes proves to be necessary to maintain convection outside the eyewall and turns out to be responsible for a delayed development of the tropical cyclone. The latter result is in accordance with the findings of Gray, based on observations. Furthermore, intensity fluctuations of the mature tropical cyclone only occur in HURMOD when latent cooling processes are simulated. 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. HURMOD does not exhibit a threshold amplitude but it sea-surface temperature threshold for tropical cyclogenesis, while the latter only occurs with latent cooling processes. However. a threshold depending on the initial vortex amplitude, together with a stronger response to latent cooling processes, is found in the COSMO model simulations. This seems to be caused by non-axisymmetric disturbances which may circumvent cyclogenesis for a small initial vortex amplitude. Copyright (C) 2009 Royal Meteorological Society