Tropical Storm Development and Decay: Sensitivity to Surface Boundary Conditions

Abstract

Hurricane models have rarely been used to investigate the observational fact that tropical disturbances seldom form, develop, or intensify over land. Furthermore, rather ad hoc assumptions have been made when modeling landfall. The general consensus is that energy supplied primarily through surface fluxes is necessary for tropical cyclone development and maintenance. In the past, rather a priori assumptions have been made such as the elimination of surface sensible and latent heat fluxes over land or the reduction of surface land temperature. By incorporating an improved version of the Geophysical Fluid Dynamics Laboratory (GFDL) tropical cyclone model with diurnal radiation and a bulk subsurface layer with explicit prediction of land temperature, a series of experiments was performed to test the sensitivity of surface boundary conditions to tropical cyclone development and decay at landfall. A triply nested version of the GFDL model was used in an idealized setting in which a tropical disturbance, taken from the incipient stage of Gloria (1985), was superposed on a uniform easterly flow of 5 m s?1. A control case was performed for ocean conditions of fixed 302-K SST in which the initial disturbance of about 998 hPa developed to a quasi-steady state of 955 hPa after one day of integration. Using identical atmospheric conditions, a series of experiments was performed in which the underlying land surface was specified with different values of thermal property, roughness, and wetness. By systematically changing the thermal property (i.e., heat capacity and conductivity) of the subsurface from values typical of a mixed-layer ocean to those of land, a progressively weaker tropical system was observed. It was found that the initial disturbance over land failed to intensify below 985 hPa, even when evaporation was specified at the potential rate. The reduction of evaporation over land, caused primarily by the reduction of surface land temperature near the storm core, was responsible for the inability of the tropical disturbance to develop to any large extent. Under land conditions, the known positive feedback between storm surface winds and surface evaporation was severely disrupted. In sensitivity experiments analogous to the all-land cases, a series of landfall simulations were performed in which land conditions were specified for a region of the domain so that a strong mature tropical cyclone similar to the ocean control case encountered land. Again as in the all-land case, the demise of the landfalling storm takes place due to the suppression of the potential evaporation and the associated reduction of surface temperatures beneath the landfalling cyclone. Even when evaporation was prescribed at the potential rate, a realistic rapid filling (36 hPa in 12 h) ensued despite the idealized nature of the simulations. Although not critical for decay, it was found that surface roughness and reduced relative wetness do enhance decay at landfall.