Storm track representation and verification

Abstract

A vortex representation of highs and lows is presented which mathematically describes individual local features by a small number of parameters representing magnitude, location, size, and shape. Parameter values for a representation of a particular data set are determined by minimizing the integral of the squared difference between the mathematical representation and the data set. Various objective verification measures can be based on these parameters by fitting vortex representations to both forecasts and corresponding verifying analyses. The differences in values of the parameters provide measures of errors in magnitude, location, size, and shape of individual features. An example applying this representation to a 10-day forecast by ECMWF and to the corresponding NMC verifying analysis is discussed. Representations of storm tracks in the forecast and analysis are defined by mapping the envelope of individual features with time, i.e. by determining the volume swept out by the storm. Such a two-dimensional horizontal field or map indicates the path of each storm as well as its strength and size along its track during the period included in the mapping. The usefulness of such mapping as a forecast product as well as a means for verification is illustrated.