Posted: 1 October 2008
On 17 September 2008 the assimilation system for the global model GME of DWD was changed from an 'Optimum Interpolation (OI)' scheme to a three-dimensional variational system. Compared to the old scheme, the new system leads to a marked improvement in forecast quality.
One of the advantages of the 3D-Var system is a better balance of the analysed prognostic variables. This results in a smaller spin-down at the start of the forecast. As a consequence, precipitation amounts during the first few forecasts hours are smaller than in the old scheme. This impacts on the soil moisture, which is free running during the assimilation cycle without control through direct or indirect observations.
Fig. 1 shows the difference in soil moisture in % in the layer from 3-9 cm between the 3D-Var scheme and the old OI-scheme from a parallel test which started in Dec. 2007.
The differences are valid for the day before the operational switch-over to the new system (i.e. 16. Sept. 2008). Users of the GME data for regional modelling will be affected from this sudden decrease in soil moisture in some areas in particular in the tropics - resulting e.g. in differences in forecasts for screen level temperatures and humidities and possibly also precipitation.
If you have any questions, please don't hesitate to contact Andreas Rhodin.