The Role of Unparameterized Boundary Layer Processes on Tropical Cyclone Intensity Prediction

Numerical simulations and forecasts of tropical cyclones (TC) predict the storm track relatively well, however intensity predictions remain problematic. The standard assumption is that there are physical processes in nature, which play an important role in intensifying the storms that are unresolved, incorrectly parameterized or unparameterized in the numerical models. A variety of mechanisms such as dynamical interactions between TC eyewalls and rainbands, injection of high entropy air from the eye into the eyewall and deficiencies in the boundary layer parameterizations have been proposed as likely candidates for the intensity problem. Because it is unlikely that any single process is solely responsible, detailed study of each candidate is warranted. The primary motivations for our focus on TC boundary layer (TCBL) parameterization are that standard theoretical models for maximum hurricane intensity are highly sensitive to the ratio of the surface fluxes of enthalpy and momentum and because the TCBL is the link between the storm body and the ocean. Recent studies indicate that the mean structure of the TCBL is significantly different from the assumptions that have been used to develop standard BL parameterizations. The most striking examples are the prevalence of intense km-scale roll vortices and smaller scale near-surface streaks in the TCBL. However, while little is currently known about how these coherent structures affect the air-sea exchange of momentum and enthalpy and the transfer of these quantities across the TCBL, preliminary indications are that the rolls and streaks strongly modify both the surface fluxes and the transport across the TCBL in a manner that cannot be modeled using standard BL parameterization techniques.