KEYSERSYMP Using the Nondivergent Wind to Diagnose Mesoscale Circulation Systems in Convection-Allowing Models

Mesoscale circulation systems modulate when and where deep, moist convection occurs. Diagnosis of mesoscale ascent associated with these systems is an important component of the research and forecast process as it provides insight on the underlying dynamics driving the mesoscale ascent. Quasigeostrophic (QG) theory is a useful framework for diagnosing synoptic-scale ascent. The forcing terms on the right-hand-side of the traditional, Trenberth, and Q forms of the QG omega equation and the Petterssen frontogenesis equation (i.e., the geostrophic “forcing”) are easily computed from model grids in real time and provide key insights on the underlying dynamics driving vertical motion (i.e., the ageostrophic “response”). Visualizing and interpreting these forcing terms become difficult, however, as the horizontal grid spacing of the model grid decreases and finer-scale motions are represented. While increases in model resolution have resulted in the improved representation of mesoscale circulation systems, these increases also have resulted in noisier diagnostic signatures of mesoscale ascent using the geostrophic wind. Nielsen-Gammon and Gold (2008) argued that substituting the nondivergent wind for the geostrophic wind is “fully consistent with quasigeostrophic theory” and allows for diagnosis in the balanced framework with reduced noisiness by fine-scale features. Galarneau and Keyser (2008) and Kenyon et al. (2020) extended the utility of the nondivergent wind for diagnosis of mesoscale snowbands in extratropical cyclones.

Keyser et al. (1988) introduced vector frontogenesis (F-vectors), where the components of F in natural coordinates represent the across- (F_n) and along-isentrope (F_s) directions. The component F_n is the Lagrangian rate of change of |𝛁𝜃| (increase in |𝛁𝜃| when F_n points in the −𝒏 direction toward warmer air) and F_s is the Lagrangian rate of change of the direction of 𝛁𝜃 (counterclockwise rotation of 𝛁𝜃 when F_s points in the 𝒔 direction) (see also Keyser et al. 1992). Keyser et al. (1988) noted that Q is the QG analog to F, where the Lagrangian derivative is defined by the geostrophic flow. Davies-Jones (1991) introduced the “alternative balance” diagnostic model for vertical motion, where the geostrophic wind in Q is replaced by the nondivergent wind resulting in F introduced by Keyser et al. (1988). The alternative balance model may be more representative of the diagnosed vertical motion for subsynoptic-scale flows and perhaps represents a framework in which cleaner diagnostic signatures for ascent can be visualized from high-resolution convection-allowing model output.

The aim of this presentation is to compare diagnostic signatures of vertical motion and frontogenesis derived from Q and F for cases of convection initiation with warm season mesoscale circulation systems. The diagnostics considered here are divergence of Q and F, including their across- and along-isentrope components. The cases of convection initiation that will be examined include the Texas dryline supercells on 20 May 2019 and the Iowa derecho of 10 August 2020. Results from these cases show that use of the nondivergent wind (as shown by F) in a balanced dynamical framework produces cleaner and more coherent diagnostic signatures of mesoscale ascent and frontogenesis with less spatial smoothing than the geostrophic wind (as shown by Q). It is recommended that the nondivergent wind (and streamfunction) be included as standard output for realtime regional and global numerical model forecast grids for use by researchers and forecasters.