TROPICAL STORM DEVELOPMENT AND DECAY - SENSITIVITY TO SURFACE BOUNDARY-CONDITIONS

Hurricane models have rarely been used to investigate the observationa l fact that tropical disturbances seldom form, develop, or intensify o ver land. Furthermore, rather ad hoc assumptions have been made when m odeling landfall. The general consensus is that energy supplied primar ily through surface fluxes is necessary for tropical cyclone developme nt and maintenance. In the past, rather a priori assumptions have been made such as the elimination of surface sensible and latent heat flux es over land or the reduction of surface land temperature. By incorpor ating an improved version of the Geophysical Fluid Dynamics Laboratory (GFDL) tropical cyclone model with diurnal radiation and a bulk subsu rface 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 tr iply nested version of the GFDL model was used in an idealized setting in which a tropical disturbance, taken from the incipient stage of Gl oria (1985), was superposed on a uniform easterly flow of 5 m s-1. A c ontrol case was performed for ocean conditions of fixed 302-K SST in w hich the initial disturbance of about 998 hPa developed to a quasi-ste ady state of 955 hPa after one day of integration. Using identical atm ospheric conditions, a series of experiments was performed in which th e underlying land surface was specified with different values of therm al property, roughness, and wetness. By systematically changing the th ermal property (i.e., heat capacity and conductivity) of the subsurfac e from values typical of a mixed-layer ocean to those of land, a progr essively weaker tropical system was observed. It was found that the in itial disturbance over land failed to intensify below 985 hPa, even wh en evaporation was specified at the potential rate. The reduction of e vaporation over land, caused primarily by the reduction of surface lan d temperature near the storm core, was responsible for the inability o f the tropical disturbance to develop to any large extent. Under land conditions, the known positive feedback between storm surface winds an d surface evaporation was severely disrupted. In sensitivity experimen ts analogous to the all-land cases, a series of landfall simulations w ere performed in which land conditions were specified for a region of the domain so that a strong mature tropical cyclone similar to the oce an control case encountered land. Again as in the all-land case, the d emise of the landfalling storm takes place due to the suppression of t he potential evaporation and the associated reduction of surface tempe ratures beneath the landfalling cyclone. Even when evaporation was pre scribed at the potential rate, a realistic rapid filling (36 hPa in 12 h) ensued despite the idealized nature of the simulations. Although n ot critical for decay, it was found that surface roughness and reduced relative wetness do enhance decay at landfall.