MERRA implementation details

NOTE: Page under construction!

On this page we list the necessary modifications to GEOS-Chem for MERRA compatibility.

Overview

The GMAO MERRA data product is a 30-year reanalysis done with the GEOS-5.2.0 assimilation system. Therefore, within GEOS-Chem we can (for the most part) treat MERRA in the same way as we do for the GEOS-5 meteorology.

However, in order to reduce the volume of data produced (and for other technical concerns), GMAO chose to reduce the vertical and/or horizontal resolution of some important MERRA data fields. Also, some MERRA data fields have different units than in the GEOS-5 product. Because of this, some additional special handling may be required within GEOS-Chem.

Key similarities

The following aspects of MERRA are identical to the GEOS-5 operational data product.

1. The MERRA high-resolution grid (0.5° x 0.666°) is identical to that of GEOS-5.
2. The MERRA vertical grid (72 hybrid levels) is identical to that of GEOS-5.

Key differences

The following aspects of MERRA differ from the GEOS-5 operational data product:

1. The MERRA temporal resolution differs from GEOS-5 as follows:
   • Surface data: hourly averages (MERRA) vs. 3-hour averages (GEOS-5)
   • 3-D data: 3-hour averages (MERRA) vs. 6-hour averages (GEOS-5)
2. Differences in individual MERRA met fields:
   • Most of the releveant MERRA 3-D data for GEOS-Chem are only available at 1° x 1.25° horizontal resolution.
   • OMEGA, PV, RH are 6-hr instantaneous data in MERRA but are 6-hr averaged data in GEOS-5
   • SNOMAS (snow mass) has units of kg/m2 in MERRA but mm in GEOS-5
   • Many of the precipitation fields (e.g. DQIDTMST, DQVDTMST, DQLDTMST, etc.) are saved on a 1.25° x 1.25° x 42 pressure levels instead of 72 hybrid levels. P -> hybrid interpolation is necessary.
   • The MERRA fields FRLAKE, FRLAND, FRLANDIC, FROCEAN, PHIS are time-invariant.