A Revised 4D-Var Algorithm for Increased Efficiency and Improved Accuracy

Friday, May 3, 2002 - 11:30am - 12:30pm
Keller 3-180
Yannick Tremolet (European Cte for Medium-Range Weather Forecast)
4D-Var has been operational at ECMWF since November 1997. In the near future, data assimilation algorithms will have to cope with the cost of higher resolution and increased volumes of data, in particular high density satellite data. In addition to the number of observations, it is expected that new types of data such as cloud and rain will be assimilated. This will require an improved agreement between the inner and outer loop to allow for the analysis of small scale phenomena and humidity.

We will start by describing the main characteristics of the operational algorithm including the incremental formulation, the used data types and the main approximations involved. Some limitations of the current system will be pointed out.

Then, a revised algorithm will be introduced which includes the modification of the cost function in the inner loop to make it quadratic, the use of a conjugate gradient minimisation and a new preconditioning, a new interpolated trajectory and a multi-incremental configuration.

Joint work with Mike Fisher, Lars Isaksen and Erik Andersson, ECMWF.