The Uses and Limitations of Contour Advection as a Technique for Examining Arctic Vortex Dynamics

Abstract

A contour advection technique, contour advection with surgery (CAS), is applied to the Northern Hemisphere Arctic vortex during several dynamically active periods in midwinter and at several different levels in the stratosphere. The ability of the technique to accurately depict vortex evolution is assessed. Isentropic potential vorticity (PV) is used as the dynamic tracer, and observed winds on isentropic surfaces are used to advect the PV contours. Results of the current study show that while it may provide a continuous view of vortex evolution that is spatially and temporally consistent, quantitative information regarding vortex area change derived from CAS is of limited utility. The results are shown to be somewhat sensitive to subtle differences in the wind and PV fields for quantities (such as area) that develop slowly in time. Increasing the temporal resolution of the advecting wind field does not appear to improve the agreement between CAS results and model or observed data. The poor correspondence between the area changes from CAS and those derived from analyzed data suggests that diabatic effects during most of these events are important. Percentage differences between the CAS and the analyzed PV contours are more pronounced above the lower region of the stratosphere, and a large part of the differences are acquired in the first day of a CAS calculation. Contour length parameters such as e-folding time are relatively insensitive to uncertainties in the initial conditions, suggesting that quantities that change rapidly (as length does) are better determined by CAS; however, the filamentary structure contributes little to vortex area results.