Frontal instability generated by tropospheric potential vorticity anomalies

Abstract

The development of secondary disturbances on a two-dimensional front is described using a linear stability analysis. A two-stage process is envisaged in which, first, strong frontogenesis produces condensation at the front. This leads to the generation of a lower tropospheric zone of high potential vorticity (PV) lying along the front. As frontogenesis weakens, the second stage is entered in which an essentially steady front is susceptible to the growth of linear normal modes along the front owing to the existence of the PV anomaly or, equivalently, the frontal rainband. Thus for a typical band of width 160 km a frontal wave of some 800 km is produced with a growth rate of about 1 day−1. the dynamics of these modes is examined by considering both the realistic frontal flow as well as idealized strips of PV anomalies. the idealized case is potentially applicable to a variety of meteorological situations such as upper jet streaks. Useful insight is gained from an analysis of the energetics of the modes. This shows that for narrow frontal PV anomalies the waves main energy source is the basic kinetic energy and further they have a negative vertical heat flux. Recent observations suggest that frontal waves are indeed characterized by such a heat flux. As most frontal zones involve significant condensation, and therefore are likely to exhibit a lower tropospheric PV maximum, it is stressed that studies of frontal waves need to include this aspect of the frontal structure.