The effect of 300 mb divergence on surface cyclogenesis
Sauer, Erika Lauren.
Pauley, Patricia M.
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Quantitative predictions of surface extratropical cyclone development are correlated to the degree of upper-level forcing from 500 mb vorticity advection and 300 mb divergence. Although it is difficult to obtain an accurate analysis of upper-level divergence, modern models and data assimilation procedures do produce consistent divergence fields. The divergence field partitioned into the longitudinal (alongstream) and transverse (cross-stream) components approximately estimates the effect of curvature and jet streaks on cyclone intensification. Verification of the Eta model indicated not significant difference in the mean central pressure to a confidence level of 95%, and verification of the 300 mb divergence was very similar to the 500 mb vorticity advection. The presence of subgeostrophic flow in the trough and supergeostrophic flow in the ridge was noted with the longitudinal divergence. Large variability in longitudinal divergence reflected some sensitivity to small-scale perturbations in the height field. Transverse divergence showed greater consistency and projected a similar wind pattern to the classic jet streak model, however, modifications were seen as ageostrophic winds may also be directed toward regions of height falls. The composition of the total divergence from the longitudinal and transverse divergence is about 50:50. However, with very large total divergence values, the dominant contributor is the transverse divergence.
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