Saturday, 13 January 2007
Comparison of CHAMP Radio Occultations with Global Model Forecasts: 2005 Hurricane Season
GPS RO technique measures the bending of radio signals as they pass through the Earth's atmosphere. From the vertical profile of bending angles, one can derive vertical soundings of refractivity. GPS RO soundings are of very high accuracy and very high vertical resolution, and of uniform global coverage. The availability of GPS RO soundings over the ocean provides a unique opportunity to verify model hurricane forecasts. This investigation primarily focused on the Global Forecasting System (GFS) model. To verify the GFS forecasts, the GFS outputs of temperature, pressure, and water vapor pressure were converted into refractivity. A mean absolute value of the fractional differences between the RO measured refractivity and the GFS refractivity was calculated as a measure of the error of GFS analysis and forecast. The mean absolute error of GFS forecasts was calculated for the cases with and without tropical cyclones. This study further analyzed cases with tropical cyclones and correlated the GFS output error to the errors in the predicted track and intensity. Results for all the storms for 2005, the 2005 Atlantic and East Pacific hurricane season showed consistent error in the GFS output. The analysis reveals that the GFS error was larger closer to the core of the storm when a tropical cyclone was in the vicinity. These results suggest that including RO data in the initial conditions of GFS models may improve the GFS forecast of hurricanes.