Wednesday, 25 January 2017
Hurricane Joaquin was a powerful and costly category 4 hurricane that developed from an upper-level low over the western Atlantic in late September 2015. It initially moved southwest towards the Bahamas and then rapidly intensified before making a sharp `U-turn' back to a northeasterly trajectory. Few model forecasts gave an accurate prediction of the intensification and track, even at short lead times ahead of the main turn. This resulted in the National Hurricane Center issuing a forecast for landfall in the mid-Atlantic United States, while in reality the storm moved offshore. In this work we use the NASA GEOS-5 and Navy COAMPS adjoint models to investigate how errors in the initial conditions of the model could have contributed to the problematic forecasts. The adjoint allows for the backward in time propagation of sensitivity and reveals the regions where small errors in the initial conditions can have a significant impact on the storm track and intensity. We demonstrate the strong interaction between intensity and active steering winds using adjoint derived perturbations. It is shown that small changes even far to the east of the storm and at relatively low heights can completely alter the foretasted track by fairly minimal changes to the steering winds. This demonstrates the sometimes very large sensitivity to the steering environment and the need to produce accurate initial conditions at all scales. It has been hypothesized that intensification of Joaquin could have been facilitated by outflow from the east Pacific storm Marty. We use the adjoint to investigate this hypothesis by computing sensitivity to upper level potential temperature.
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