COMPARATIVE-ANALYSIS OF LOW-LEVEL COLD FRONTS - WAVELET, FOURIER, ANDEMPIRICAL ORTHOGONAL FUNCTION DECOMPOSITIONS

Atmospheric cold fronts observed in the boundary layer represent relat ively sharp transition zones between air masses of disparate physical characteristics. Further, wavelike features and/or eddy structures are often observed in conjunction with the passage of a frontal zone. The relative merits of using both global and local (with respect to the s pan of a basis element) transforms to depict cold-frontal features are explored. The data represent both tower and aircraft observations of cold fronts. An antisymmetric wavelet basis set is shown to resolve th e characteristics of the transition zone, and associated wave and/or e ddy activity, with a relatively small number of members of the basis s et. In contrast, the Fourier transformation assigns a significant ampl itude to a large number of members of the basis set to resolve a front al-type feature. In principle, empirical orthogonal functions provide an optimal decomposition of the variance. The observed transition zone , however, has to be phase aligned and centered to yield optimal resul ts, and variance may not be the optimum norm to depict a front. It is concluded that the wavelet or local transform provides a superior repr esentation of frontal phenomena when compared with global transform me thods. Further, the local transform offers the potential to provide so me physical insight into wave and/or eddy structures revealed by the d ata.