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

Monday, 30 July 2001
Extratropical Transitions: Forecasted vs. Observed Evolution
Eyad H. Atallah, SUNY, Albany, NY; and L. F. Bosart
Poster PDF (4.1 MB)
Several recent cases (Danny 1997, Dennis, Floyd, and Irene 1999) have shown that precipitation distribution and intensity can be drastically altered by interactions with mid-latitude troughs and jet streaks during extratropical transitions. The performance of the current operational models in replicating the features associated with these transitions will be examined as these interactions can produce catastrophic rainfalls as illustrated by hurricane Floyd in September 1999.

Preliminary results suggest that regions of strong baroclinicity are often created as the cold-core anomaly associated with the mid-latitude trough approaches the warm-core anomaly associated with the tropical system. The resulting baroclinic zone can resemble a coastal front, with the significant exception that this baroclinic zone is a very deep feature, often extending throughout the troposphere. As the extratropical transition takes place, cool dry air is wrapped around the southern extent of the cyclone, resulting in a marked decrease in the precipitation on the east side of the cyclone. Meanwhile, a significant increase in both the aerial extent and rate of the precipitation occurs in the northwest quadrant of the cyclone in response to deep ascent associated with warm-air advection and differential cyclonic vorticity advection (positive potential-vorticity (PV) advection). While this evolution was somewhat captured by the models, the elevation of the dynamic tropopause over the strong tropical convection is often underestimated in current operational (Eta/AVN) models, causing the strength of the tropospheric-temperature gradient to be underestimated. Furthermore, the erosion of the mid-latitude trough at higher latitudes by the tropical convection can often lead to a negatively tilted trough structure. The combination of these factors can produce a model QPF field which is decidedly lacking.

Results comparing the evolution of the potential vorticity in the forecasted vs. observed fields will be shown. These results indicate that the Eta and AVN models failed to capture the effect of the tropical PV anomaly on the midlatitude system, resulting in a dynamically less favorable environment for precipitation.

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