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

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 Fn, also known as scalar frontogenesis) or by rotating the direction of the temperature gradient relative to the flow (the rotational component Fs). The dominant processes acting on Fn for the diffluence and confluence cases are tilting and deformation frontogenesis, respectively. The dominant process acting on Fs 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 cold advection. Thus, the rotational frontogenesis component explains the rotation of the isentropes that is responsible for the differing thermal-advection patterns. Diagnosis of these cases supports the results from the climatology indicating a strong relationship between the synoptic environment and the upper-tropospheric thermal-advection pattern. Nevertheless, current conceptual models of upper-level frontogenesis do not fully explain the variety of these features in the real atmosphere. In particular, mobile trough genesis and its associated upper-level frontogenesis can occur in weak 500-hPa thermaladvection patterns, in contrast to the confluence and cold advection that have been previously identified as important to upper-level frontal intensification. This result provides further support for the possibility that generation and intensification of mobile troughs can occur by barotropic processes.