Bjørn Røsting

Dynamical processes related to cyclone development near Greenland

An unusual cyclone that moved over Greenland and caused blizzard conditions over eastern Greenland and northern Iceland on 20-21 September 2003 is investigated. Numerical simulations are conducted to assess the role of Greenlands orography for the development, as well as to evaluate the significance of other factors such as latent heating and sea surface temperature (SST). The simulations reveal that the cyclone evolution was crucially dependent on an interaction between the background flow and the orography of Greenland. When the orography is removed, a deep, well organized baroclinic low develops rapidly and moves eastward at 75°N. Conversely, in the control run, which is similar to the analyses, the evolution of the (primary) baroclinic low is greatly suppressed by the orographic retardation of the warm air ahead of and the cold air behind the low. Instead, a secondary low developing off Greenlands east coast at 68°N intensifies due to a coupling between an approaching upper level PV-anomaly and a lower level PV-anomaly generated from lee effects. This secondary low, absent in the run without orography, then moves eastward and causes the extreme weather conditions that were observed. Inversion of selected potential vorticity anomalies lends support to the above explanation. Further sensitivity experiments show that latent heating contributes about half of the deepening of the low, while SST amounts contribute much less.

A Diagnostic Study of the Flateyri Avalanche Cyclone, 24–26 October 1995, Using Potential Vorticity Inversion

Abstract The evolution of a deep North Atlantic cyclone, which caused devastating avalanches in northwest (NW) Iceland in October 1995, was investigated. As the main tool for this investigation, potential vorticity analysis was used. This allows the quantification and comparison of the roles of different processes that contribute to the cyclone deepening at different stages. Interpretation of potential vorticity inversions and isentropic air trajectories yields the following picture of the cyclone development. The thermal field over the North Atlantic had acquired strong west–east gradients due to a combination of advection of cold air southeastward from a cold cyclonic gyre south of Iceland and advection of warm air northward on the westward flank of a warm anticyclonic ridge over central Europe. A low-level baroclinic wave forming just south of Ireland was rapidly reinforced due to interaction with a descending, high-value, upper-level potential vorticity anomaly and was isentropically advected from the...

The Usefulness of Piecewise Potential Vorticity Inversion

AbstractIt is today widely accepted that potential vorticity (PV) thinking is a highly useful approach for understanding important aspects of dynamic meteorology and for validation of output from state-of-the-art numerical weather prediction (NWP) models. Egger recently presented a critical view on piecewise potential vorticity inversion (PPVI). This was done by defining a PV anomaly by retaining the observed PV field in a specific region, while changing the observed PV fields to zero elsewhere. Inversion of such a modified PV field yields a flow vastly different from the observed. On the basis of this result it was argued that PPVI is useless for understanding the dynamics of the flow.The present paper argues that the results presented by Egger are incomplete in the context of PPVI, since the complementary cases were not considered and that the results also depend on the idealized model formulations. The complementary case is defined by changing the observed PV to zero in the specific region, while retai...

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

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