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Sensitivity Testing of Extratropical Transitions Using Potential Vorticity Inversions to Modify Initial Conditions: Hurricane Earl Case Study

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  • 1 Department of Atmospheric and Oceanic Sciences, McGill University Montreal, Quebec, Canada
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Abstract

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 were present in this case, operational forecasts seriously underpredicted the severity of the reintensification. Employing a piecewise potential vorticity (PV) inversion, the authors remove localized PV anomaly (PV′) maxima from the initial conditions and rebalance the fields for input to the model. Several PV′ structures in an upstream trough, and the PV′ associated with the hurricane, are removed individually and the model is rerun. Comparison of the resulting output with that of the control integration allows for a quantification of the impact of each PV anomaly on the regeneration of Earl. It is found that the existence of an upstream trough is of primary importance to the storm's reintensification, while the presence of the low-level circulation associated with the decaying hurricane plays only a secondary role.

Corresponding author address: Ron McTaggart-Cowan, Department of Atmospheric and Oceanic Sciences, McGill University, 805 Sherbrooke Street West, Montreal, QC H3A 2K6, Canada. Email: rmctc@zephyr.meteo.mcgill.ca

Abstract

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 were present in this case, operational forecasts seriously underpredicted the severity of the reintensification. Employing a piecewise potential vorticity (PV) inversion, the authors remove localized PV anomaly (PV′) maxima from the initial conditions and rebalance the fields for input to the model. Several PV′ structures in an upstream trough, and the PV′ associated with the hurricane, are removed individually and the model is rerun. Comparison of the resulting output with that of the control integration allows for a quantification of the impact of each PV anomaly on the regeneration of Earl. It is found that the existence of an upstream trough is of primary importance to the storm's reintensification, while the presence of the low-level circulation associated with the decaying hurricane plays only a secondary role.

Corresponding author address: Ron McTaggart-Cowan, Department of Atmospheric and Oceanic Sciences, McGill University, 805 Sherbrooke Street West, Montreal, QC H3A 2K6, Canada. Email: rmctc@zephyr.meteo.mcgill.ca

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