A COORDINATE CHANGE FOR DATA ASSIMILATION IN SPHERICAL GEOMETRY OF FRONTAL STRUCTURES

In this note, it is discussed how a coordinate transformation based on semigeostrophic theory can be introduced in a global data assimilatio n scheme in order to improve the analysis of frontal structures. An ex pression of the coordinate change on the sphere is proposed and an ite rative procedure is defined to perform this transformation. This algor ithm also includes an iterative filtering of the wind field used for t he coordinate change. This filtering is equivalent to a filtering of t he wind in the transformed space that nearly keeps unchanged the corre sponding high cyclonic vorticity values in physical space associated w ith cold fronts. It is shown that the transformation can be built for a real meteorological case. The main impact of this transformation in the framework of data assimilation is that it enables the use of aniso tropic forecast correlations that are flow dependent. As expected, the correlations in the vicinity of a front are found to be contrasted in the horizontal but also vertically tilted.