Upper-Level Frontogenesis Associated with the Birth of Mobile Troughs in Northwesterly Flow

Abstract

Previous studies have shown that 500-hPa mobile trough births (or genesis) occur preferentially in northwesterly flow during upper-level frontogenesis, and that cold advection assists in, and is a product of, mobile trough intensification. This study focuses on the synoptic environments and thermal-advection patterns of upperlevel fronts associated with mobile trough births in northwesterly flow. A climatology of 186 such events, derived from an earlier study by Sanders, shows that most births tend to occur within uniform or diffluent flow and that most tend to be associated with relatively weaker 500-hPa thermal advection. Most mobile trough births in diffluence, however, tend to be associated with increasing 500-hPa cold advection, typically indicated by a cyclonic rotation of isentropes, whereas, most mobile trough births in confluence tend to be associated with weaker 500-hPa thermal advection. Two cases of upper-level frontogenesis associated with mobile trough genesis-one in diffluence and one in confluence-are compared to determine the processes acting to produce the differing thermal-advection patterns at 500 hPa. A thermal-advection tendency equation is developed and shows that the magnitude of the temperature advection can be changed by accelerating the advecting wind speed or by changing the temperature gradient (i.e., vector frontogenesis). The latter can be accomplished either by changing the magnitude of the temperature gradient (the frontogenetical component F n , also known as scalar frontogenesis) or by rotating the direction of the temperature gradient relative to the flow (the rotational component F s). The dominant processes acting on F n for the diffluence and confluence cases are tilting and deformation frontogenesis, respectively. The dominant process acting on F s for the diffluence case is rotation of the isentropes due to the vorticity term, whereas rotation of the isentropes due to the vorticity and tilting terms are both important for the confluence case. The rotational component of frontogenesis is cyclonic downstream of the vorticity maximum for both cases, favoring increasing cold advection downstream of the vorticity maximum. For both cases, the rate of rotation of the isentropes at a point due to horizontal advection is large and that due to vertical advection is negligible. Since advection can only transport the existing isentrope angle and cannot change the isentrope angle, the rotational component of frontogenesis normalized by the temperature gradient is the only term that can increase the isentrope angle following the flow. This term dominates in the diffluence case but is small in the confluence case. This diagnosis suggests the following reasoning. In diffluent flow, the vorticity associated with the incipient trough is compacted into a more circular shape and intensifies. The potent vorticity maximum leads to robust isentrope rotation. In confluent flow, however, the vorticity is deformed into an elongated maximum, inhibiting both strong isentrope rotation and increasing c...

Baroclinic development within zonally‐varying flows

“…As the upper-level front rounds the base of the trough, frontolysis due to both the deformation and the tilting terms weakens the eastern end of the front (Figure 9 (b), (d) and (f)). This case is similar to the northwesterly-flow case in diffluence presented by Schultz and Sanders (2002, figure 7), where tilting dominates over deformation. It differs from the cases presented by Bosart (1970Bosart ( , 2003 because frontolysis on the eastern side of the trough in DIFF is due to both deformation and tilting, whereas the cases discussed by Bosart (1970Bosart ( , 2003 feature frontolysis due to tilting but frontogenesis due to deformation.…”

A Five-Year Radar-Based Climatology of Tropopause Folds and Deep Convection over Wales, United Kingdom

“…They showed that upper-level fronts could form in northwesterly flow (as had been typically described in the literature) or in southwesterly flow. Fronts forming in northwesterly flow tended to form in diffluence, whereas fronts forming in southwesterly flow tended to form in confluence (Schultz and Doswell 1999;Schultz and Sanders 2002;Schultz and Zhang 2007). Of the 183 tropopause folds in this study, 128 (70%) occurred in northwesterly flow and 55 (30%) occurred in southwesterly flow.…”

“…1) when the fold is observed for the last time. This classification scheme is reminiscent of that for upper-level frontogenesis proposed by Schultz and Doswell (1999). They showed that upper-level fronts could form in northwesterly flow (as had been typically described in the literature) or in southwesterly flow.…”

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

“…They were thus led to emphasize the role of horizontal winds in the rotation of isentropes and in the establishment of cold air advection near the base of the thermal trough. (A similar conclusion with regard to this question, based upon examination of a thermal advection tendency equation, was reached by Schultz and Sanders (2002).) Though vorticity rotates every vector field equally, and therefore cannot promote the differential rotation of ∇θ relative to ∇φ required to initiate along-flow geostrophic cold air advection, SD's analysis nonetheless suggested a primary role for rotational frontogenesis in the upper-frontal life cycle.…”