Incremental Analysis Update

GEOS5 uses an Incremental Analysis Update (IAU) to constrain the atmospheric numerical model by observations. The following figure shows the schematic of the procedure. Starting at 09Z, a 6 hour forecast is run, and forecast data from 09Z, 12Z and 15Z are used to create the analysis (blue diamond). From the analysis and the forecast, a tendency is calculated. This tendency is applied to another model forecast cycle in the prognostic equations (green arrow and light blue box). This is called the corrector segment, so that the tendencies are nudging the model forecast in the direction of the observations at every time step.

MERRA will have two primary products. First, the analyses will consist of the model state variables written instantaneously after the analysis every 6 hours (00Z, 06Z, 12Z and 18Z). There will be model level (72 eta levels) and pressure interpolated (42 pressure levels) for each analysis time. Second, the diagnostic fields are written from the model corrector segment. These include 1 hourly average 2 dimensional (1/2 deg latitude by 2/3 degree longitude) surface, single level (e.g. H500), radiation, land specific and vertically integrated fields. In addition, 3 hourly average 3 dimensional coarse resolution 1.25 deg x 1.25 degree) atmospheric diagnostics are produced from the corrector segment. These include all the tendencies for the state variables, as well as fluxes and budget terms.



One advantage of IAU is that it allows the corrector segment data to be written. This data is exposed to the observational forcing spread out in time, rather than a large change in the initial conditions. The spin up spin down problems in the forecast, associated with initializing a forecast system with an analysis data, are much smaller. Essentially, this allows the production of 1 hourly precipitation and other physics fields. The figure below shows a global average precipitation time series (data is written at every model time step, ~30 min) using the synoptic analysis as initial conditions, IAU and a pure model forecast. Reinitializing the forecasts with the analysis causes jumps in the time series. The free running model tries to have a global precipitation rate of ~3 mm/day. The analysis tried to reduce that, but after the initial time the forecast starts to drift back to it's preferred climate state. The IAU provides forcing at every time step, constraining the system with the observations.