A Scheme for Continuous Data Assimilation

Abstract

Major revisions to the Geophysical Fluid Dynamics Laboratory's (GFDL) continuous data-assimilation system have been implemented and tested. Shortcomings noted during the original processing of data from FGGE [First GARP (Global Atmospheric Research Program) Global Experiment) served as the basis for thew improvements. This new system has been used to reanalyze the two FGGE special observing periods. The main focus here will be on assessing the changes to the assimilation system using comparisons of rerun test results with results from the original FGGE processing. The key new features in the current system include: a reduction in the assimilation cycle from 12 to 6 h; the use of a 6-h forecast first guess for the OI (optimum-interpolation analysis) as opposed to the previous use of persistence as a first guess; an extension of the OI search range from 250 to 500 km with an increase in the maximum number of observations used per analysis point from 8 to 12; the introduction of incremental linear normal-mode initialization, eliminating the periodic nonlinear normal-mode initialization; and an increase in the horizontal resolution of the assimilating model from 30 waves to 42 waves, rhomboidally truncated. Tests of the new system show a significant reduction in the level of noise, improved consistency between mass and momentum analyses, and a better fit of the analyses to observations. In addition, the new system has demonstrated a greater ability to resolve rapidly moving and deepening transient features, with an indication of less rejection of surface pressure data. In addition to the quantities archived during the original FGGE data processing, components of diabatic heating from the assimilating model have also been archived. They should be used with caution to the extent that they reflect model bias and spinup in addition to real features of the general circulation.