The relationship between air-sea interactions and tropical cyclone intensity

The Hurricane Weather Research and Forecast (HWRF(TM)) System is used to simulate forecasts of Tropical Storm Debby (August 24 - August 26) and Hurricane Helene (September 15 - September 20) during the 2006 Atlantic Hurricane Season. The model is uncoupled and initiated with the Global Forecasting System (GFS) 1°x1° model.;The impact of air-sea interactions (latent and sensible heat fluxes) on the forecasting of tropical cyclone activity in the Atlantic Ocean is examined. In this study, simulations with varying latent and sensible heat fluxes in the HWRF model are undertaken to evaluate the hypothesis that latent and sensible heat fluxes play the primary role in tropical cyclone intensity.;The control simulations show HWRF's forecast positive biases of storm intensity. The control simulations predict maximum intensity for Tropical Storm Debby to be a Category 2 hurricane and Hurricane Helene to be a Category 5 hurricane. In the simulations where the fluxes are reduced by multiplying their equations by 0.5, the maximum intensities are reduced to a tropical storm for Debby and Category 3 hurricane for Helene, which is in line with the observations.;There are 5 experiments undertaken including the control experiment. The control experiments are examined and compared to observational data. The simulations that vary latent and sensible heat fluxes are compared to the control simulations to determine the sensitivity of the surface fluxes on the forecast of tropical cyclone intensity. Experiment 1 is the control experiment. The equations for latent and sensible heat flux given in the HWRF model are unchanged. This experiment serves as the basis for the others. Experiments 2 and 5 nullify and double both the latent heat flux and sensible heat flux equations. These are extreme values for the experiments. Experiment 3 decreases the latent heat flux by multiplying it by half, while the sensible heat flux remains as the control value. Experiment 4 multiplies both the latent and sensible heat flux equations by half.;The experiments provide a lower and upper bounds on the tropical cyclone intensity experiments as well as the linkages to sensible/latent heat fluxes. In these experiments, the reduction of surface fluxes represents what occurs when tropical cyclones encounter cool pools in the ocean and make landfall. The increase in surface fluxes represents what occurs when tropical cyclones encounter warm pools and strengthen.;The storm intensity for Tropical Storm Debby varies by 96kts and 93mb between the six experiments; the intensity for Hurricane Helene varies by 133kts and 141mb. The extreme high intensities occur during the experiment where both latent and sensible heat fluxes are doubled. The lowest intensities occur where the latent and sensible heat fluxes are zero. Multiplying the surface fluxes by 0.5 predicts values that are more realistic in comparison to the observations, thus decreasing the surface energy flux calculations produces a more accurate intensity forecast.;The following physical parameters are examined with latent and sensible heat fluxes: precipitation rates, vertical heating rates, vertical wind profile, vertical velocity, sea level pressure, and wind speed. The results show the impact of increasing and decreasing latent and sensible heat fluxes on the physical parameters that determine tropical cyclone maintenance and that latent heat flux plays a very important role in hurricane intensity with a non-linear feedback in intensity changes. The results from the experiments in the HWRF model are similar to earlier studies that examine the relationship of latent and sensible heat fluxes to tropical cyclone intensity.