On the Applicability of the Geostrophic Approximation to Upper-Level Frontal-Scale Motions

Abstract

The techniques of scale-analysis and case-study diagnosis are used to establish a theory of meteorological approximations for upper-level frontal zones. A scale-analysis of the governing equations reveals that. 1) the vertical component of motion within upper-level frontal zones is on the order of 10 cm sec?7; 2) the individual terms which comprise the horizontal acceleration are of the same order of magnitude as the Coriolis acceleration and compensate one another to the extent that their sum is one order of magnitude less than their individual contributions to that sum; 3) to the first order of approximation, the horizontal momentum equation reduces to the statement of geostrophic equilibrium; and 4) in the middle-tropospheric portions of upper-level frontal zones, the divergence and tilting terms of the vorticity equation are equal in magnitude and of opposite sign such that their sum is one order of magnitude less than their individual contributions to that sum. These results are verified in a case study of the intense upper-level frontal zone of 0000 GMT 8 December 1963.