Baroclinic development within zonally‐varying flows

Abstract

Previous idealized-modelling studies have shown the importance of across-jet barotropic shear to the resulting evolution of cyclones, anticyclones, surface-based fronts, and upper-level fronts. Meanwhile, many observational studies of cyclones have shown the importance of along-jet variations in the horizontal wind speed (i.e. confluence and diffluence). This study investigates the importance of these along-jet (zonal, for zonally-oriented jets) variations in the horizontal wind speed to the resulting structures and evolutions of baroclinic waves, using idealized models of growing baroclinic waves. An idealized primitive-equation channel model is configured with growing baroclinic perturbations embedded within confluent and diffluent background flows. When the baroclinic perturbations are placed in background confluence, the lower-tropospheric frontal structure and evolution initially resemble the Shapiro-Keyser cyclone model, with a zonallyoriented cyclone, strong warm front, and bent-back warm front. Later, as the baroclinic wave is amplified in the stronger downstream baroclinicity, the warm sector of the cyclone narrows, becoming more reminiscent of the Norwegian cyclone model. The upper-level frontal structure develops with a southwest-northeast orientation, and becomes strongest at the base of the trough, where geostrophic cold advection is occurring. In contrast, when the baroclinic perturbations are placed in background diffluence, the lower-tropospheric frontal structure and evolution resemble the Norwegian cyclone model, with a meridionally-oriented cyclone, strong cold front, and occluded front. The upper-level frontal structure is initially oriented northwest-southeast on the western side of the trough, before becoming zonally oriented. Weak geostrophic temperature advection occurs along its length. These results are compared to those from previous observational and idealized-modelling studies.

Development of an idealised downstream cyclone: Eulerian and Lagrangian perspective on the kinetic energy

“…Thus, it will be useful to successively add more realism to our channel simulations by the introduction of turbulent surface fluxes, the inclusion of moisture and a convection scheme, and to assess the influence of these processes on the energetic development of the downstream cyclone. Furthermore, confluent and diffluent background flows were found to be decisive for the evolution of certain frontal structures (Schultz and Zhang, 2007). Investigations of cyclone development in zonally varying flows will help to understand the different evolutions of the Norwegian and Shapiro-Keyser type cyclones from an energetics perspective.…”

Acceleration of near‐surface strong winds in a dry, idealised extratropical cyclone

“…To this state, a potential temperature perturbation θ ′ is added (e.g. Polvani et al 2004; Schultz and Zhang, 2007). This perturbation takes the form of where A is the amplitude of the perturbation (taken to be 1 K) and L x = 4000 km.…”

Comments on ‘The influence of rotational frontogenesis and its associated shearwise vertical motions on the development of an upper‐level front’ by A. A. Lang and J. E. Martin (January A, 2010, 136: 239–252)

“…The first pitfall is extrapolating the results from a single case-study to an entire conceptual model for upperlevel frontogenesis. For example, Schultz and Zhang (2007, pp 1110-1111 show that, when an idealized baroclinic wave is placed in large-scale diffluence, the relationship between horizontal terms and the tilting term in the scalar frontogenesis equation differs from that described by Rotunno et al (1994). In another example, the climatologies discussed in section 5 of this comment indicate that cold-air advection in northwesterly flow is not the most common type of upper-level frontogenesis.…”