18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Monday, 30 July 2001
Evaluation of a bogus vortex technique using four-dimensional variational data assimilation
Zhao-Xia Pu, University of Maryland, Baltimore County, Greenbelt, MD; and S. Braun
The effectiveness of a four-dimensional variational data assimilation technique for creating bogus hurricane vortices in numerical simulations is evaluated. Results indicate that assimilation of both bogus surface pressure and wind information are critical. With assimilation of only bogus pressure information, the response in the wind field is contained largely within the divergent component, with strong low-level convergence leading to strong upward motion in the storm center. With assimilation of wind information only, the model response is dominated by the rotational wind component, but the adjustment to sea-level pressure is often insufficient. When both pressure and wind information are assimilated simultaneously, the model produces a vortex that more closely matches the observed intensity and structure. Including the wind information improves upon the pressure-only case because the wind information provides an approximate gradient wind balance constraint.

The impact of vortex size is also examined. When the scale of the specified bogus vortex is smaller than that which can be resolved by the model, the assimilation method may result in structures that do not completely resemble observed structures in hurricanes. In contrast, when the vortex is sufficiently large for it to be resolved on the horizontal grid, but not so large as to be unrealistic, more reasonable hurricane structures are obtained.

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