Evaluating the intensification of tropical cyclones with the GFS model

Several tropical cyclones that developed in the East Pacific and the Atlantic basins were studied using the results from non-operational simulations with the GFS model, in order to understand the physical mechanisms that favor the intensification of tropical cyclones. We have analyzed the output corresponding to simulations during the period June-July 2005. The balance between the tendency of the absolute circulation, the convergence of absolute vorticity into the system at each level, the tilting and friction terms was evaluated for each cyclone, as a function of time as they evolve from tropical depression up to hurricane stage. The friction term is represented by the surface stress calculated with a bulk formula and also estimated as the residual from the other terms in the balance.

The convergence of absolute vorticity, which depends predominantly on the mass convergence, largely dominates the intensification of tropical cyclones into tropical storms and hurricanes. In contrast, the tropical storms that do not develop into hurricanes are influenced by the larger magnitude of friction compared to the convergence of absolute vorticity. Observations from TEXMEX (Raymond et al. 1998) indicate that as cyclones intensify, a coherent evolution of the vertical structure of the mass convergence emerges: initially, low level convergence is moderate in magnitude and observed throughout a deep layer of approx 6-7 km. As the cyclones intensify, the low level convergence increases in magnitude but is confined to a shallower layer (3-4 km). This evolution is reproduced reasonably well by the GFS simulations for the cases studied here.

We further investigate the evolution of the thermodynamic ventilation term, to estimate its role on the intensification of tropical cyclones. Tropical storms that do not reach hurricane stage, appear characterized by large negative ventilation, particularly at low levels, consistent with the decrease of equivalent potential temperature within their cores.