Isobaric to Isentropic Interpolation Errors and Implication to Potential Vorticity Analysis

Abstract

Different common interpolation methods have been used for transformation of atmospheric variables from pressure levels into isentropic levels. The accuracy of the derived variables depends both on the specific interpolation method chosen and on the vertical resolution of the pressure levels. A most sensitive field is ??/?p, the stability factor, in which error largely determines also that of the isentropic potential vorticity. Five interpolation methods are compared using radiosonde observations. They are examined through their capability to resolve isentropic profiles of the stability factor as well as the pressure based on data from arbitrary pressure surfaces. Comparison of the results with full resolution data reveals, not unexpectedly, that fine vertical patterns are not resolvable. The methods differ in their ability to resolve patterns found with 10-pressure-level data when applied to only seven levels. However, the method based on the linear ?-p relation is shown to yield larger errors than other methods, such as that based on the linear T-lnp relation. In addition, the advantage of the latter method is clearly demonstrated through isentropic and vertical potential velocity distribution with ECMWF data.