Tropical cyclone genesis from the merger of mesoscale vortices embedded in African easterly wave: A case study using a nested regional climate model

The potential role of meso-beta scale vortices in hurricane genesis has been widely discussed in the literature, but has not previously been observed in climate simulations. Here we report on an extended simulation of the entire 2005 North Atlantic hurricane season using the NCAR NRCM (Nested Regional Climate Model) two-way nested at 12km and 36km model grid resolutions. Several tropical cyclone genesis cases occurred in which the merger of mesoscale vortices played a significant role. The high-resolution model output revealed interactions of meso-beta-scale vortices embedded in African Easterly Wave prior a tropical cyclone genesis. These vortices subsequently merged to form a tropical cyclone. The process of vortex interaction appears to be enhanced by dynamical and thermodynamical processes at meso-alpha-scale, which will be discussed in detail. Comparing the high resolution run with one at 36 km revealed that the coarser resolution could not resolve the mesoscale vortices, with the consequence that genesis occurred more slowly. The implications of this for predicting genesis in global and climate models will be discussed.