Studies on the Initialization and Simulation of a Mature Hurricane Using A Variational Bogus Data Assimilation Scheme
FLORIDA STATE UNIV TALLAHASSEE DEPT OFMETEOROLOGY
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A bogus data assimilation BDA scheme is presented and used to generate the initial structure of a tropical cyclone for hurricane prediction. It was tested on Hurricane Felix 1995 in the Atlantic ocean during its mature stage. The Penn StateNCAR nonhydrostatic mesoscale model version 5 MM5 was used for both the data assimilation and prediction. It was found that a dynamically and physically consistent initial condition describing the dynamic and thermodynamic structure of a hurricane vortex can be generated by fitting the forecast model to a bogus surface low specified based on a few observed and estimated parameters. Through forecast model constraint, BDA is able to recover many of the structural features of a mature hurricane including a warm-core vortex with winds swirling in and out of the vortex center in the lower and upper troposphere respectively, the eyewall, the saturated ascent around the eye and descent or weak ascent in the eye, and the spiral cloud and rain bands. Satellite and radar data, if available, can be incorporated into the BDA procedure. We showed that satellite-derived water vapor winds have an added value for BDA to generate a more realistic initial vortex. As a result of BDA, dramatic improvements occurred in the hurricane prediction of Felix. First of all, the initial adjustment and false spin-up of the model vortex were not observed because fields of model variables describing the BDA initial vortex are well-adapted to the forecast model. Secondly, the intensity forecast was greatly improved. The model correctly simulated the slight weakening of Hurricane Felix during the initial 12 h of model integration and then maintained a central pressure near 970 hPa thereafter, in good agreement with the observational estimates of the central pressure. Thirdly, the model captured the structures of the storm reasonably well.