A Coordinate Transformation for Objective Frontal Analysis

Abstract

The analysis of a frontal discontinuity is difficult as the hypothesis of isotropy commonly assumed by inter- polation schemes is obviously erroneous in that case. Starting from the semigeostrophic theory, the authors propose a kind of flow-dependent analysis based on the use of geostrophic coordinates. The idea behind this approach is that the discontinuity should appear much more regular in geostrophic space, and then it should better fulfill the above-mentioned hypothesis of isotropy. The validity of such a scheme is first checked in the Hoskins and Bretherton dry and inviscid shear model of frontogenesis. In order to treat more realistic cases, the authors introduce a filtered geostrophic advection coordinate (FGAC) using real wind instead of geostrophic wind. Applied to simulations of the wet and viscous Eady's problem, this procedure brings a clear gain with a maximum positive impact when data spacing is in the 100?200-km range. Finally, the authors apply this method to a high-resolution dropsonde dataset collected during the FRONTS 87 experiment. Again, the FGAC transformation is shown to greatly improve the analysis, producing more consistent wind and mass fields.