A new 4D variational assimilation of multiple spaceborne datasets for regional water and energy budgets. An application to the Bret Hurricane

For some years many satellites were launched in order to bring new and/or more information on meteorological circulations and associated cloud systems over oceanic surfaces. To process these new data, new methods, in particular methods of data assimilation allowing the processing and merging of these new various kinds of data are needed. In this context MANDOPAS4D, 4D variational data assimilation technique to retrieve 4D fields from space-borne data such as wind, specific humidity and related quantities (pressure, temperature, vorticity potential, apparent heat source, apparent moisture sink, evaporation rate, condensation rate), has been developed. The present work illustrates this method on a real data set devoted to the study of the hurricane Bret. This hurricane was located over the Gulf of Mexico from the 18th to the 23rd of August 1999.

The first objective of this MANDOPAS4D application deals with the estimate of the latent heat release. Indeed the knowledge of the space-time evolution of diabatic heating distributions and thus latent heat, since the energy necessary to the functioning of hurricane is essentially supplied under this form, is crucial for storm diagnostics and forecasting applications.

The second objective deals with the need to improve initial humidity fields used in mesoscale modelling of these hurricanes. Indeed, various studies have shown the necessity to use better initial humidity fields to improve the simulation of vortex tracks, structure and intensity. Currently, the need to use all available sources of data in the numerical prediction of hurricanes has been recognized. Many techniques were proposed. Atmospheric data with a great resolution (dropsonde, radar …) have a large potential but up to now have not been assimilated. In this context MANDOPAS4D can help as shown by the present application on the Bret hurricane.

This cyclone appears as one element of a periodic structure of large scale ascending motions and potential vorticity anomalies. The surface precipitation diagnosed by the analysis is shown to be in good agreement with the one deduced from TRMM observations. Strong evaporations are evidenced in north-western side of the vortex. Retrieved pressure and temperature fields show that the cyclone propagates along a pre-existing baroclinic zone. During the presentation a detail description of these fields will be given focusing on the processes involved on the time evolution of the apparent moisture sink (Q2), the apparent heat source (Q1) and the heating latent profiles during the intensification phase of Bret hurricane on the 21st.