Pressure levels greater than surface pressure

Some questions have come in regarding differences between MERRA and other reanalysis at pressure 1000mb and 850mb pressure levels. It is very important to note that MERRA does not extrapolate pressure level data vertically greater than the surface pressure. The result is that there is undefined data points in much of the 1000mb fields. This will affect the representativeness of both time and area averages of MERRA data compared with reanalyses or other data that extrapolates gridpoints to pressure levels greater than the surface pressure. The GMAO provides a summary of the impact that this has on averaging. In addition, the FAQ will be updated to call out this difference with other data sets. The main GMAO www page also has other pages with useful practical information about the data and assimilation system.