Implementation of a 3D variational data assimilation system at the Canadian meteorological centre. Part II: The regional analysis

This paper describes the implementation of the 30 variational (3D-var) anal ysis in the Regional Data Assimilation System (RDAS) of the Canadian Meteor ological Centre. The RDAS, a 12-h data assimilation cycle, is nm twice dail y to provide analyses to the variable resolution Global El Environmental Mu lti-scale (GEM) model The same incremental 3D-var algorithm is used for bot h the regional and global data assimilation systems. In this algorithm, the innovations are calculated with respect to the full-resolution model while the analysis increments are calculated at a lower resolution on a Gaussian grid Background,and error correlations for the global and regional data assimila tion systems are examined. It is shown that the resolution of the analysis increments is determined to a great extent by the horizontal correlation le ngths. Although the horizontal resolution of the regional model is three ti mes higher than the global model, the correlation lengths for both models a re similar. Consequently the same horizontal resolution for the analysis in crements is used in both data assimilation systems. A pre-implementation evaluation showed that the RDAS maximises the coherenc e between the analysis and the forecast model This results in a higher-reso lution and more consistent analysis with respect to the regional model. The forecasts issued from the RDAS are generally improved especially the tempe rature and the geopotential above 300 hPa. The RDAS also reduces the precip itation spin-lip observed during the first 12 hours when initiated with an analysis from the global data assimilation system. Finally the impact of th e digital filter on the analysis from the RDAS is small, indicating that th e regional analysis is already well balanced.