A COORDINATE TRANSFORMATION FOR OBJECTIVE FRONTAL ANALYSIS

The analysis of a frontal discontinuity is difficult as the hypothesis of isotropy commonly assumed by interpolation schemes is obviously er roneous in that case. Starting from the semigeostrophic theory, the au thors propose a kind of flow-dependent analysis based on the use of ge ostrophic coordinates. The idea behind this approach is that the disco ntinuity should appear much more regular in geostrophic space, and the n it should better fulfill the above-mentioned hypothesis of isotropy. The validity of such a scheme is first checked in the Hoskins and Bre therton dry and inviscid shear model of frontogenesis. In order to tre at more realistic cases, the authors introduce a filtered geostrophic advection coordinate (FGAC) using real wind instead of geostrophic win d. 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 m ethod to a high-resolution dropsonde dataset collected during the FRON TS 87 experiment. Again, the FGAC transformation is shown to greatly i mprove the analysis, producing more consistent wind and mass fields.