Sensitivity testing of Hurricane Earl's extratropical transformaion using PV inversions to modify initial conditions

The problem of forecasting the extratropical transition of Atlantic hurricanes remains largely unsolved. Although "classical" cyclogenetic forcings such as upstream troughs, strong thermal gradients, latent heat release, and stratospheric potential vorticity (PV) intrusion are known to enhance the transition and reintensification processes, their relative importance is difficult to assess. This study uses the Mesoscale Compressible Community (MC2) model to simulate the extratropical reintensification of Hurricane Earl (September, 1998) for the purposes of testing sensitivity to the modification of the model's initial conditions.

The control simulation is initialized with Hurricane Earl decaying rapidly off the coast of Virginia. By 36 hours into the integration, however, the storm has redeveloped into a strong (957 hpa) extratropical system over Newfoundland while retaining its warm-core central structure. By employing a piecewise PV inversion, localized PV maxima are removed from the initial conditions and the fields rebalanced for input to the model. In this way, several PV structures in an upstream trough, and even the PV associated with the hurricane itself, can be removed individually and the model run again. Comparison of the resulting output with that of the control integration allows for a quantification of the impact of each PV anomaly on the transition and reintensification of Earl. Complete removal of the upstream trough yields a weak and disorganized storm whose deepening is delayed by almost 12 hours compared to the control. Reduction of either the depth or the amplitude of the trough results in more vigorous redevelopment than in the previous case, but the structure of the system is once again relatively disorganized. Removal of the anomalous PV associated with the remnants of Earl had less of an effect than might have been expected. In this case, a small surface cyclone moves east from Quebec and intensifies to 975 hpa in almost precisely the same location as did Earl in the control. Interpretation of these results will be presented with the goal of achieving a better understanding of the relative importance of some of the extratropical forcings that influence transition and reintensification events