Abstract
A number of changes were made to the National Meteorological Center global data assimilation system (GDAS) during 1982 and 1983. The most significant of these changes was the replacement of the quasi-univariate, sigma-coordinate optimum interpolation analysis procedure by a version that performs a fully multivariate analysis on mandatory pressure surfaces. The new analysis technique is described in some detail. The impact of this and other changes made during 1982 and 1983 on the performance of the GDAS is evaluated.
