Potential Vorticity Diagnosis in the Quasigeostrophic and Nonlinear Balance Systems

Abstract

Quantitative diagnosis of low-Rossby-number flows using potential vorticity (PV) includes using elements of PV advection to deduce instantaneous tendencies of the balanced atmospheric state, most commonly the geopotential field. This technique, piecewise tendency diagnosis (PTD), is here applied with the prognostic balance equations (Bal-PTD) to obtain a quantitative dynamical diagnosis that in principle may be much more accurate than similar diagnoses using the quasigeostrophic (QG) equations. When both are applied systematically to a case of rapid oceanic cyclogenesis, differences are found to arise owing to a variety of factors. The dominant factor is differences in the vertical influence of PV anomalies, which affects the partitioning between local and remote processes. QG overestimates the effect of lower-level PV, including surface potential temperature, in amplifying and controlling the motion of the upper-level system. Other differences are found, but overall the QG diagnosis gives results that are qualitatively similar to the nonlinear balance diagnosis. Quantitative accuracy requires the use of Bal-PTD.