The signature of cyclonic PV filaments in the lower stratosphere

The perturbation temperature and wind fields induced by a given zonally inv ariant potential vorticity (PV) filament in an isothermal atmosphere with u niform westerly background flow is investigated using a two-dimensional ise ntropic semigeostrophic model. Such PV filaments are commonplace in the low er midlatitude and polar stratosphere during winter and spring. The model i s formulated in isentropic geostrophic coordinates because this formulation greatly simplifies the equations of motion describing the system. Given th e potential vorticity distribution, the dynamics of the filament problem ar e obtained by inverting a single nonlinear elliptic equation. The model dia gnoses relatively cool air below and relatively warm air above cyclonic fil aments. In the case of anticyclonic filaments, air above the filament is co oler and air below the filament is warmer than the ambient air. The thermal perturbations in an isothermal atmosphere are sensitive to vertical gradie nts in PV, while the zonal along-filament flow associated with PV filaments is sensitive to quasi-horizontal PV gradients. If the PV anomaly marking t he stratospheric filament is weak compared to ambient PV values, the magnit ude of the thermal perturbations in the isothermal atmosphere is a linear f unction of the magnitude of the PV anomaly. In principle, it is possible to remotely detect the thermal signature of PV filaments using a Microwave Te mperature Profiler (MTP), for example. This instrument, mounted aboard an a ircraft flying in the lower stratosphere, can measure vertical profiles of the thermal structure below and above the flight track.