A Comparison of Quasigeostrophic and Nonquasigeostrophic Vertical Motions for a Model-simulated Rapidly Intensifying Marine Extratropical Cyclone

Abstract

Large-scale departures from quasigeostrophic vertical motions are diagnosed for a model simulation of the QE II storm (9?11 September 1978). The simulation was performed by the Limited-Area Mesoscale Prediction System (LAMPS), initialized at 1200 UTC 9 September 1978. The model cyclone intensified from a central pressure of 1003 mb to 976 mb in 24 h, considerably short of the 59 mb (24 h)?1 observed deepening but reasonable in comparison to other model simulations of this storm. This diagnosis centers on a hydrostatic generalized omega equation, which scales to the quasigeostiophic omega equation for small Rossby number. Vertical motions were computed both from this generalized omega equation and the quasigeostrophic omega equation in order to examine the importance of nonquasigeostrophic effects. The high correlation of vertical motions from a control experiment (using most of the terms in the generalized omega equation) with the vertical motions predicted by the model establishes the validity of the method. A further comparison against satellite imagery also shows that these computed vertical motions portray a pattern similar to the satellite cloud shield. However, the pattern and magnitude of the quasigeostrophic vertical motions are quite different from those of the generalized vertical motions. An evaluation of individual terms in the generalized equation shows that although additional terms in omega placed on the left-hand side significantly affect the magnitude of the vertical motion, the greatest nonquasigeostrophic effects are provided by the diabatic term and the ageostrophic advections. Latent heating greatly enhances the upward motion in the cyclone?s cloud shield, while ageostrophic advections both suppress downward motion behind the cold front and enhance upward motion near the warm front.