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

Thursday, 2 August 2001
Diagnosis of a rapid surface cyclolysis event in the Bering Sea
Nathan Marsili, Univ. of Wisconsin, Madison, WI; and J. E. Martin
The rapid decay of a Bering Sea surface cyclone observed in February 1994 is examined using output from a successful numerical simulation of the event performed using the Penn State/NCAR MM5 model. A piecewise potential vorticity (PV) inversion of the model output is employed in order to quantify the contributions of discrete pieces of the PV to the rapid surface cyclolysis. During the period of rapid surface decay, the tropopause-level PV anomaly underwent a reduction in magnitude and scale forced by negative PV advection by the full wind at that level. This reduction forced an attendant weakening of the associated lower tropospheric geopotential height anomaly. In addition, diabatic redistribution of PV, directly associated with latent heat release, further reduced the effective length scale of the tropopause PV anomaly. Finally, strong lower tropospheric cold air advection served both to weaken the lower boundary theta perturbation as well as to decrease the penetration depth by increasing the static stability beneath the upper PV anomaly. These changes acted to further decrease the geopotential height anomalies near the surface contributing to the observed rapid surface cyclolysis.

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