Diagnosis of the Initial and Forecast Errors in the Numerical Simulation of the Rapid Intensification of Hurricane Emily (2005)

Abstract

A diagnostic study is conducted to examine the initial and forecast errors in a short-range numerical simulation of Hurricane Emily?s (2005) early rapid intensification. The initial conditions and the simulated hurricane vortices using high-resolution grids (1 and 3 km), generated from the Advanced Research version of the Weather Research and Forecasting (ARW) model and its three-dimensional variational data assimilation (3DVAR) systems, are compared with the flight-level data acquired from the U.S. Air Force C-130J aircraft data. Numerical simulation results show that the model fails at predicting the actual rapid intensification of the hurricane, although the initial intensity of the vortex matches the observed intensity. Comparing the model results with aircraft flight-level data, unrealistic thermal and convective structures of the storm eyewall are found in the initial conditions. In addition, the simulated eyewall does not contract rapidly enough during the model simulation. Increasing the model?s horizontal resolution from 3 to 1 km can help the model to produce a deeper storm and also a more realistic eye structure. However, even at 1 km the model is still not able to fully resolve the inner-core structures. To provide additional insight, a set of mesoscale reanalyses is generated through the assimilation of available satellite and aircraft dropsonde data into the ARW model throughout the whole simulation period at a 6-h interval. It is found that the short-range numerical simulation of the hurricane has been greatly improved by the mesoscale reanalysis; the data assimilation helps the model to reproduce stronger wind, thermal, and convective structures of the storm, and a more realistic eyewall contraction and eye structure. Results from this study suggest that a more accurate representation of the hurricane vortex, especially the inner-core structures in the initial conditions, is necessary for a more accurate forecast of hurricane rapid intensification.