Type B Cyclogenesis in a Zonally Varying Flow

Abstract

It is hypothesized that surface cyclogenesis in the Northern Hemisphere storm-track regions can be described by the structural modification of baroclinic wave packets traversing a zonally varying flow field. We test this hypothesis using a linear, quasigeostrophic model with a zonally varying basic state and zonally varying Ekman layer eddy viscosity. At midchannel, the basic state consists of a region of strong low-level baroclinicity and weak Ekman dissipation, surrounded by regions of weak low-level baroclinicity, strong Ekman dissipation, and enhanced low-level static stability. Eigenanalyses and initial-value integrations support this model of Type B cyclogenesis. The results can be summarized as follows:1) A disturbance initiated upstream of the midchannel baroclinic zone rapidly evolves into a wave packet with maximum amplitude near the tropopause. The wave packet undergoes a structural modification upon entering the low-level baroclinic zone, developing maximum amplitude at the surface. The storm track in this model results from the transient amplification and structural modification of wave packets passing through the midchannel baroclinic zone.2) The effective growth rate of the surface disturbance exceeds those of the most unstable mode of the zonally varying basic state, and of the most unstable mode of zonally homogeneous basic-state characteristic of the midchannel baroclinic zone.3) The transient evolution of the wave packet is a result of the superposition and interference between the many global eigenmodes with different structures and frequencies excited by the initial condition. The surface cyclogenesis can be interpreted as a local constructive interference between these eigenmodes.4) From a potential vorticity perspective, the evolution of the baroclinic wave packet is a two-stage process. Initially, the growth of upper-level disturbances results from the mutual interaction of potential vorticity anomalies near the tropopause and in the lower troposphere. After the wave packet enters the storm-track region, the growth of surface cyclones is associated with the interaction between tropospheric potential vorticity anomalies and surface-temperature anomalies.5) The addition of a simple parameterization of moist physics in the midchannel baroclinic zone does not significantly alter the initial stages of surface cyclogenesis, but results in a longer period of rapid development and a reduction in the characteristic scale of the disturbance.