Two different methods to correct the initial conditions applied to the storm of 27 December 1999 over southern France

Abstract

In strong baroclinic flows, small errors in the initial conditions may grow rapidly and lead to large forecast errors even at very short ranges. It is therefore of crucial importance that these initial conditions are determined carefully. In this paper we use two different but complementary methods to improve the operational analysis of a storm that caused loss of life and much material damage in France on 27 December 1999. The first method is based on the adjoint technique: an adjoint model-run provides the sensitivity of one aspect of the forecast (the cost function) to the initial conditions. We then apply a correction whose shape is the sensitivity so that the corrected initial state better fits the observations. The second method is based on the Quasi-Geostrophic Potential Vorticity (QGPV) inversion. A misfit between the model analysis and satellite pictures (water vapour Meteosat images, for example) leads to the subjective modification of some of the features of the potential vorticity field. The analysis is modified using the wind and temperature corrections provided by the inversion of the QGPV correction. Both methods significantly improve the forecast and may provide valuable guidance for forecasters in real time.