Utilizing Increments in Model Development

MERRA data has complete budgets for water and energy, including the incremental analysis updates (IAU) that constrain the model forecast to the analyzed observations. The increments can be interpreted instantaneously (at the six hour analysis) as a representation of the forecast error, or for longer terms as the mean model bias. The magnitude of the IAU terms are not trivial, and should be accounted for, certainly in budget studies, but can also be useful in understanding the representation of weather and climate phenomena in reanalyses.

As an example of utilizing the increments to evaluate the background model, Mapes and Bacmeister (2012) have evaluated MERRA's tropical climate and convection, relating significant IAU values to weaknesses in the representation of physical processes. They suggest diagnostics and potential areas for model development.

Extrapolation to P > Ps

As many have found, MERRA pressure level data does not provide values for pressure surfaces when they are greater than the surface pressure (e.g. high topography).  Other reanalyses extrapolate the data using the surface meteorology and assumed lapse rates. This data may be useful in some cases such as zonal averaging, stream functions and thickness calculations.

A recent post at reanalysis.org provides user developed codes to fill these undefined grid points. This should be useful as one could adapt the codes to the filling method applied in other reanalyses to better match their extrapolated data.

See Extrapolation of MERRA Reanalyses to obtain continuous fields for more information.

Reanalysis Conference Report

The Report of the 4th International Conference on Reanalyses (May 7-11, 2012, Silver Spring MD) is presently available from the World Climate Research Programme.