SCAPE Charts from Numerical Weather Prediction Model Fields

Abstract

Although the SCAPE concept is strictly only meaningful for quasi-two-dimensional flows it can be given a precise definition for flows satisfying the conditions governing the semigeostrophic approximation. For this type of balanced flow it is argued that SCAPE corresponds to the positive area on the tephigram when the environment curve defines the thermodynamic state along a semigeostrophic vortex line. A procedure for the calculation of SCAPE as a numerical model diagnostic is described and is applied to an operational model analysis prior to the great storm which struck southern England on 16 October 1987; values of SCAPE greater than 1000 J kg?1 are found over a large area near the embryo storm. Since the degree of slantwise stability (or instability) is widely considered to be relevant to the intensity of cyclogenesis and rainband formation, SCAPE charts may be of some utility to the bench forecaster.