Reply to “Comments on ‘The Usefulness of Piecewise Potential Vorticity Inversion’”

Abstract

Piecewise potential vorticity inversion (PPVI), applied to Ertel’s potential vorticity (PV), was first introduced about 20 years ago (Davis and Emanuel 1991; Davis 1992) and has become an important tool in diagnosing midlatitude synoptic-scale weather developments (e.g., Stoelinga 1996; Plant et al. 2003; McInnes et al. 2009). It has also been used in studies of tropical cyclones (Wu and Emanuel 1995a,b; Kieu and Zhang 2010) and polar lows (Bracegirdle and Gray 2009; Fore et al. 2011). A few years ago, PPVI came under criticism by Egger (2008, hereafter E2008), who, based on a few examples from quasigeostrophic theory, questioned its usefulness as a diagnostic tool. In Rosting and Kristjansson (2012, hereafter RK12) we argued that the methodology used by E2008 is incomplete, giving a wrong impression of the usefulness of PPVI. We further presented two additional cases—one idealized, the other from a real weather situation—to clarify how PPVI works, not only in the context of quasi-geostrophy but also in the more general Ertel’s nonlinear PV. Recently, Egger (2012, hereafter E2012) criticized RK12, claiming that 1) E2008 had in several instances been incorrectly cited in RK12 and 2) ‘‘RK12 promote a novel interpretation of PPVI,’’ in which ‘‘a triviality . . . is announced as a success of PPVI’’ (p. 6). Below, we address these comments on RK12 in E2012. 2. Citations of E2008 in RK12