MERRA implementation details
NOTE: Page under construction!
On this page we list the necessary modifications to GEOS-Chem for MERRA compatibility.
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.
The following aspects of MERRA are identical to the GEOS-5 operational data product.
- The MERRA high-resolution grid (0.5° x 0.666°) is identical to that of GEOS-5.
- The MERRA vertical grid (72 hybrid levels) is identical to that of GEOS-5.
The following aspects of MERRA differ from the GEOS-5 operational data product:
- 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)
- 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.
Required coding changes in GEOS-Chem
As noted in the section above, the similarities between GEOS-5 and MERRA (especially of the vertical grids) means that in most cases we can just treat MERRA in the same way as for GEOS-5. In many areas of the code, the coding changes are straightforward and simply involve an extension of C-preprocessor statements such as:
#if defined( GEOS_5 )
#if defined( GEOS_5 ) || defined( MERRA )
In other areas of GEOS-Chem the changes are more involved. Below we provide a description of the areas of GEOS-Chem that were modified for MERRA:
The following modifications for MERRA were made in the Headers/ subdirectory:
- Add a new C-preprocessor switch named "MERRA".
- Added size parameters for MERRA 4 x 5 and MERRA 2 x 2.5 grids.
The following modifications for MERRA were made in the GeosUtil/ subdirectory:
- GET_MODELNAME: Add "MERRA_47L" and "MERRA" modelnames to the #if block
- GET_NAME_EXT: Return grid name "geos5" for MERRA fields
- GET_PRESSURE: Compute PEDGE for MERRA in the same way as for GEOS-5
- INIT_PRESSURE: Initialize Ap and Bp for MERRA in the same way as for GEOS-5
- Collapse vertical levels in the stratosphere for MERRA in the same way as for GEOS-5