The influence of rotational frontogenesis and its associated shearwise vertical motions on the development of an upper‐level front

Abstract

The total quasi-geostrophic (QG) vertical motion is partitioned into transverse and shearwise couplets oriented parallel to, and along, the geostrophic vertical shear, respectively, in order to examine the role of rotational frontogenesis, and its associated shearwise circulation, in the life cycle of an upper-front/jet system in northwesterly flow. In particular the analysis emphasizes two aspects of that role: (1) the influence of shearwise ω on scalar frontogenesis, and (2) the effect of rotational frontogenesis, and its associated vertical circulation, on the initiation of along-flow cold air advection during upper frontogenesis.The case-study analysis reveals that shearwise subsidence persistently and significantly contributes to scalar frontogenesis throughout the life cycle. The transverse subsidence contribution to scalar frontogenesis is initially weak but grows more substantial after the establishment of along-flow cold air advection near the base of the thermal trough.Prior work has argued that the development of along-flow cold air advection arises from a cyclonic rotation of ∇θ promoted either by contributions from tilting or horizontal kinematics. From the QG perspective adopted in the present study, the tilting and kinematic rotations are seen as interconnected aspects of a single, underlying dynamical process: rotational frontogenesis, in which the rotation of ∇θ produced by vertical vorticity is accompanied by a discrete vertical circulation (the shearwise circulation) that provides the subsidence upshear of the vorticity maximum required to initiate along-flow cold air advection. It is further suggested that the characteristic distribution of the kinematic and tilting contributions to rotation of ∇θ in the vicinity of an upper-tropospheric vorticity maximum underlies the observed preference for the development of upper-level fronts in northwesterly flow.

On the Role of Inertial Instability in Stratosphere–Troposphere Exchange near Midlatitude Cyclones*

“…Recent studies that explore the role of latent heating during WCB ascent in creating midtropospheric positive PV anomalies and upper-tropospheric negative PV anomalies include Pomroy and Thorpe (2000), Knippertz and Wernli (2010), Lang (2011), Schemm et al (2013), and Madonna et al (2014). Cooper et al (2004) showed that penetration of stratospheric intrusions by convection can facilitate subsequent mixing into the troposphere.…”

“…Cold air advection along the UTLS jet axis has been shown to be compatible with sinking downstream of a trough at synoptic scales (Martin 2006;Lang and Martin 2010). Sawyer (1949) explored the possible role of inertial or symmetric instability in the UTLS in modifying midlatitude cyclogenesis, jet stream behavior, and meridional circulations.…”

Reply to comments on ‘The influence of rotational frontogenesis and its associated shearwise vertical motions on the development of an upper‐level front’

“…It is clear that the effective horizontal rotational frontogenesis (Figure 2a) is much smaller than that afforded by tilting (Figure 2b), suggesting a predominant role for tilting in changing the geostrophic temperature advection. Lang and Martin (2010) proposed a conceptual model, illustrated in their figure 13, which suggested that the development of along-flow geostrophic cold air advection was dependent on both kinematic rotation of ∇θ and alongflow tilting. Given the re-evaluation of the kinematics offered in this response, it is likely that rotation by deformation provides the kinematic contribution that we mistakenly attributed to the vorticity in our original paper.…”

“…The role of that vertical circulation in initiating geostrophic cold air advection along the flow is particularly at issue in Schultz (2012). Lang and Martin (2010) entered an ongoing debate between two competing hypotheses regarding the initiation of geostrophic cold air advection in the upper frontogenesis process and attempted to devise a more complete explanation that borrowed from both. Rotunno et al (1994, hereafter RSS94), using idealized baroclinic channel model simulations, examined the intensification of an upper-level front from a nearly equivalent barotropic initial state.…”

The structure and evolution of lower stratospheric frontal zones. Part 1: Examples in northwesterly and southwesterly flow

“…Sutcliffe, 1947;Trenberth, 1978), which exists along the jet axis when geostrophic cold air advection is present along an ULJF. Keyser and Pecnik (1987) showed that subsidence through the jet core is upper frontogenetic, thus the establishment of geostrophic cold air advection along the jet has been described as an important aspect of the upper front life cycle (Rotunno et al, 1994;Schultz and Doswell, 1999;Lang and Martin, 2010). An environment characterized by geostrophic warm air advection through the jet core shifts the thermally direct (indirect) circulation in the jet entrance (exit) region toward the cyclonic (anticyclonic) side, so that the jet core is conversely characterized by ascent (Figure 3(c)).…”