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The influence of PV inversion on polar‐vortex dynamics and passive‐tracer simulations in atmosphere‐like regimes
Author(s) -
Methven John
Publication year - 2003
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1256/qj.01.181
Subject(s) - potential vorticity , rossby wave , physics , geophysical fluid dynamics , polar vortex , tropopause , atmospheric sciences , vorticity , baroclinity , advection , rossby radius of deformation , mechanics , geology , vortex , geophysics , stratosphere , thermodynamics
Both the stratospheric polar‐vortex edge and the midlatitude tropopause are characterized by strong potential vorticity (PV) gradients on isentropic surfaces, a jet and persistent wave activity. Passive tracers in these regions also develop strong gradients, where contours of mixing ratio lie almost parallel to PV contours. Both sets of contours stretch and fold producing narrow filaments. This behaviour is associated with time‐dependent large‐scale straining flow. The dominance of the large spatial scales in velocity on the tracer evolution depends upon the smoothing effects of PV inversion. A quasigeostrophic shallow‐water model is used to investigate regimes under which a polar vortex with a sharp PV gradient and persistent wave activity can be maintained. In all experiments waves grow on a zonal jet in a Rossby wave critical layer forced by orography. A minimal set of parameters is varied: the Rossby radius of deformation L , orographic amplitude Z t, radiative relaxation time‐scale τr and diffusion time‐scale τd. Although the quasi‐geostrophic approximation is not strictly valid globally, it is used here because the PV inversion operator depends on a single parameter, L . Finite amplitude wave‐mean flow theory is used to diagnose a zonally symmetric background state and wave activity that can evolve only through non‐conservative processes (which act on slower time‐scales than advection). For given τr and τd, a curved band in ( L , Z t) parameter space can be identified with an ‘atmosphere‐like regime’ where both the background jet speed and wave activity are greater in the turbulent late‐time near‐equilibrium state than during the initial turnover of the critical layer. The ‘tracer scale factor’ (the ratio of the smallest resolved velocity‐scale to the narrowest trustworthy tracer scales from contour advection) is shown to fall as L is decreased across the atmosphere‐like regime, demonstrating that passive tracers become more difficult to simulate accurately as the smoothing effect of PV inversion lessens. Copyright © 2003 Royal Meteorological Society