Sensitivity testing of extratropical transitions using potential vorticityinversions to modify initial conditions: Hurricane Earl case study

This study uses the Mesoscale Compressible Community model to simulate the extratropical transition and reintensification of Hurricane Earl (1998) for the purposes of testing sensitivity to modification of the model's initial conditions. Though relatively strong "classical'' cyclogenetic forcings we re present in this case, operational forecasts seriously underpredicted the severity of the reintensification. Employing a piecewise potential vortici ty (PV) inversion, the authors remove localized PV anomaly (PV') maxima fro m the initial conditions and rebalance the fields for input to the model. S everal PV' structures in an upstream trough, and the PV' associated with th e hurricane, are removed individually and the model is rerun. Comparison of the resulting output with that of the control integration allows for a qua ntification of the impact of each PV anomaly on the regeneration of Earl. I t is found that the existence of an upstream trough is of primary importanc e to the storm's reintensification, while the presence of the low-level cir culation associated with the decaying hurricane plays only a secondary role .