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.