Transient dynamics and tropical cyclone genesis in a nested regional climate model

Climatological conditions suitable for tropical cyclone genesis have been known for several decades. They are frequently met over the tropical North Atlantic during the hurricane season, yet the multitude of cloud clusters that do not develop into tropical cyclones suggests these conditions are not sufficient. Here we examine the importance of transient dynamics, in the form of interactions of African Easterly Waves (AEW) with the background flow in driving tropical cyclone genesis. The evolution of the dynamical environment prior to hurricane genesis is examined using a high-resolution simulation (12km horizontal grid spacing) of the tropical North Atlantic region for the entire 2005 hurricane season using the NCAR Nested Regional Climate Model (NRCM). This simulation produced the observed number of tropical cyclones, together with realistic statistics of genesis locations and cyclone tracks. A composite dynamical environment leading up to genesis time, based on the seven tropical cyclones that formed over the tropical Atlantic, differs significantly from the mean environment. Specifically, we find coherent regions in which easterly winds increase towards the east and vertically, which change the kinematic character of the AEWs resulting in regional accumulation of wave energy and a reduction in the longitudinal and vertical scale of AEWs, thereby increasing relative vorticity and the likelihood of genesis.