JP1.10 The role of non-quasigeostrophic forcing in the Southern Hemisphere blocking onsets

Monday, 8 June 2009
Stowe Room (Stoweflake Resort and Confernce Center)
Li Dong, WindLogics Inc., Grand Rapids, MN; and S. J. Colucci

A non-quasigeostrophic diagnostic model for the geostrophic zonal wind tendency is developed by combining the frictionless, isobaric vorticity equation with the adiabatic, isobaric thermodynamic equation to diagnose blocking onset which is associated with the weakening of geostrophic westerlies. Fourteen SH blocking events are selected to diagnose contributions to the geostrophic zonal wind tendency from various non-quasigeostrophic forcing terms. Consistent with previous numerous studies, all non-quasigeostrophic forcing terms are found to be secondary compared to the quasigeostrophic forcing (the vorticity and thermal advection forcing). Among non-quasigeostrophic forcing, the forcing of the advection of the ageostrophic vorticity by the geostrophic wind turns out to be the largest forcing generally. Two blocking events are discussed in detail with each featuring opposing contributions of the advection of ageostrophic vorticity forcing to blocking onset. In addition, the contribution of the horizontal variations of three-dimensional varying static stability is found to be small. In general, the non-quasigeostrophic diagnostic model outperforms the quasigeostrophic counterpart, especially in stratosphere.
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