The role of microphysics in tropical cyclone rapid intensification

An axisymmetric version of the Bryan Cloud Model (CM1) is used to determine how quickly a coherent and relatively stable tropical cyclone (TC) circulation can form from an initial vortex of nominal intensity. A suite of microphysics parameterizations (MPs) are employed, including several based on the simplest MP, the Kessler (K) scheme. The Kessler variants include NR (in which rain is prevented from forming), NCec (which excludes evaporation cooling of cloud droplets only) and NCR (in which all condensate is immediately removed from the domain) [Fovell, 2004]. Numerous studies have demonstrated that cloud microphysical assumptions can dramatically influence tropical cyclone (TC) intensity [e.g., Wang, 2002; Zhu and Zhang, 2006]. The focus of this study is not on final intensity but rather on the rapidity of development.

The model TCs differ markedly in two respects: how long it takes for the storm to organize, and how quickly organization can take place. The supplied figure shows time series of sea-level pressure (SLP) for simulations K, NCR, NR and NCec. All four TCs eventually reach the same minimum central pressure of about 950 mb after about 4 days. The control K storm starts intensifying after 48 h, which is the slowest of the group. In contrast, NCec organizes and intensifies the quickest, overshooting its final intensity by a considerable margin. How and why the microphysical assumptions influence the timing and speed of intensification will be examined, including implications for rapid intensification of TCs.