Cold and Warm Frontal Circulations in an Idealized Moist Semigeostrophic Baroclinic Wave

Abstract

Diagnoses are presented of the three-dimensional vertical circulation for a coupled cold-warm frontal system in an idealized moist semi-geostrophic (SG) baroclinic wave. The vertical circulation is computed in SG space where the solution corresponds to its quasi-geostrophic (QG) counterpart in physical space. It is shown for this QG solution that (i) the vertical motion is enhanced in the moist region due to small moist stability and strong local geostrophic forcing; (ii) the cyclonic/anti-cyclonic ageostrophic wind vorticity is induced locally by the geostrophic wind deformation/rotation; (iii) the along-flow/cross-flow ageostrophic wind divergence is associated with the along-flow curvature/speed variation of the system-relative geostrophic wind. When the solution is transformed back to physical space, the vertical motion is dramatically enhanced and concentrated along the cold and warm fronts in the moist region due to time-integrated ageostrophic wind convergence and ageostrophic feedback to the forcing. It is found that the cross-cold front circulation is strong, narrow, and deep while the cross-warm front circulation is relatively weak, broad, shallow and very slantwise with the circulation center displaced toward the cold air. But, the along-warm front circulation is stronger than the along-cold front circulation, so the vertical circulation is less two-dimensional and more complex in the warm-frontal region. Dynamical factors responsible for the differences between the cold and warm frontal circulations are examined in detail.