The signature of cyclonic PV filaments in the lower stratosphere

Abstract

The perturbation temperature and wind fields induced by a given zonally invariant potential vorticity (PV) filament in an isothermal atmosphere with uniform westerly background flow is investigated using a two‐dimensional isentropic semigeostrophic model. Such PV filaments are commonplace in the lower midlatitude and polar stratosphere during winter and spring. The model is formulated in isentropic geostrophic coordinates because this formulation greatly simplifies the equations of motion describing the system. Given the potential vorticity distribution, the dynamics of the filament problem are obtained by inverting a single nonlinear elliptic equation. The model diagnoses relatively cool air below and relatively warm air above cyclonic filaments. In the case of anticyclonic filaments, air above the filament is cooler and air below the filament is warmer than the ambient air. The thermal perturbations in an isothermal atmosphere are sensitive to vertical gradients in PV, while the zonal along‐filament flow associated with PV filaments is sensitive to quasi‐horizontal PV gradients. If the PV anomaly marking the stratospheric filament is weak compared to ambient PV values, the magnitude of the thermal perturbations in the isothermal atmosphere is a linear function of the magnitude of the PV anomaly. In principle, it is possible to remotely detect the thermal signature of PV filaments using a Microwave Temperature Profiler (MTP), for example. This instrument, mounted aboard an aircraft flying in the lower stratosphere, can measure vertical profiles of the thermal structure below and above the flight track.