The major distinction is that during a given reanalysis, no changes are permitted to the assimilation algorithms. This is done to eliminate false climatic signals that sometimes result from intermittent algorithm changes. Approximately once every 4 years the DAO reanalyzes the past 20 years of data using the latest data assimilation techniques. That is, it gathers observations in 6 hour blocks and uses them to correct the GCM's estimates for that same time period. Figure 1 shows a typical cycle.
Processes are rectangles and data files are ovals.
Vertical interpolation steps between sigma and pressure levels are shown in the small rectangles. First Guess Each cycle begins with a restart file from the previous cycle. For example, if the first guess valid time is 06 UTC, then observations from 03 to 09 UTC are used. An objective analysis system compares these observations with the first guess and produces a set of gridded corrections. These corrections are called analysis increments.