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Revision as of 15:20, 4 August 2010 by Bmy (Talk | contribs) (MERRA grid structure)

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The links below describe various aspects of the MERRA "reanalysis" met fields:

Also be sure to see our Overview of GMAO met data products page for some basic information about the different generations of GMAO met data used by GEOS-Chem.

--Bob Y. 09:16, 4 August 2010 (EDT)

MERRA grid structure

From the MERRA File Specification Document, page 2:

The analysis is performed at a horizontal resolution of 2/3 degrees longitude by 1/2 degree latitude and at 72 levels, extending to 0.01 hPa. Some products, such as the instantaneous analysis fields, are available on the native three-dimensional grid. Hourly two-dimensional diagnostic fields are also available at the native horizontal resolution. Other products are available on a coarser horizontal grid with resolution of 1.25 ×1.25 degrees or 1 × 1.25 degrees, the latter for use by the chemistry transport community. These may be on the model’s native vertical grid or at 42 pressure surfaces extending to 0.1 hPa. Surface data, near surface meteorology, selected upper air levels, and vertically integrated fluxes and budgets are produced at one-hour intervals, which will help the development of offline land and ocean models and data assimilation systems by resolving the diurnal cycle.

and on page 9:

Gridded products will use four different vertical configurations: Horizontal-only (can be vertical averages, single level, or surface values), pressure-level, model-level, or model-edge. Horizontal only data for a given variable appear as 3-dimensional fields (x, y, time), where multiple times can span multiple files, while pressure-level, model-level, or model-edge data appear as 4-dimensional fields (x, y, z, time). Pressure-level data will be output on the LMp=42 pressure levels. The GEOS-5 model layers used for MERRA output are on a terrain-following hybrid sigma-p coordinate. Model-level data will be output on the LM=72 layers. The model-edge products contain fields with LMe = LM + 1 levels representing the layer edges....

Note that the indexing for the GEOS-5 vertical coordinate system in the vertical is top to bottom, i.e., layer 1 is the top layer of the atmosphere, while layer LM is adjacent to the earth’s surface. The same is true for edge variables, with level 1 being the top of the model’s atmosphere (PTOP), and level LM+1 being the surface.

The table below lists the combination of the horizontal and vertical grids onto which the MERRA "raw" data products are placed:

Grid Name Horizontal
Used for Notes
single layer
Nx 0.5° x 0.666° 1 level Surface data fields
(e.g. PS, EVAP, HFLUX, etc.)
Identical to
GEOS-5 native horizontal grid
vertical centers
Nv 0.5° x 0.666° 72 hybrid levels Most 3D data fields
(e.g. U, V, T, etc.)
Identical to
GEOS-5 native horizontal grid +
GEOS-5 native vertical grid
vertical edges
Ne 0.5° x 0.666° 73 hybrid level edges Data fields defined on level edges
(e.g. PLE)
Identical to
GEOS-5 native horizontal grid +
GEOS-5 native vertical grid
Reduced FV,
single layer
Fx 1° x 1.25° 1 level Surface data fields
(e.g. T2M, U10M, V10M, etc.)
Identical to
GEOS-4 native horizontal grid
Reduced FV,
vertical centers
Fv 1° x 1.25° 72 hybrid levels Most 3D data fields
(e.g. U, V, T etc.)
Identical to
GEOS-4 native horizontal grid +
GEOS-5 native vertical grid
Reduced FV,
vertical edges
Fe 1° x 1.25° 73 hybrid level edges Data fields defined on level edges
(e.g. CMFMC)
Identical to
GEOS-4 native horizontal grid +
GEOS-5 native vertical grid
Pressure Cp 1.25° x 1.25° 42 pressure levels:

1000, 970, 950, 925, 900, 875,
850, 825, 800, 775, 750, 725,
700, 650, 600, 550, 500, 450,
400, 350, 300, 250, 200, 150,
100, 70, 50, 40, 30, 20,
10, 7, 5, 4, 3, 2, 1
0.7, 0.5, 0.4, 0.3, 0.1 hPa

Various 3D data fields
(e.g. DQIDTMST, etc.)
Cp is edged on (-180,-90)
unlike the "N" and "F" grids
which are centered on (-180,-90)

--Bob Y. 10:34, 4 August 2010 (EDT)

MERRA file naming convention

The following description of the naming convention used for MERRA "raw" data files is paraphrased from Section 5.1 of the MERRA File Specification Document, pp 10ff:

The standard full name for the assimilated GEOS-5 MERRA products will consist of five dot-delimited nodes:


The node fields, which vary from file to file, are defined as follows:

For mainstream MERRA data, the following runid's are used:
  1. MERRA100: Years from 1979-1993
  2. MERRA200: Years from 1993-2001
  3. MERRA300: Years from 2001-2010
The main GEOS-5 MERRA products will be from standard production runs (denoted by prod).
The GEOS-5 analysis and forecast system can run in different configurations. The mainline MERRA data product is assimilation data (denoted by assim). The assimilation uses a combination of atmospheric data analysis and model forecasting to generate a time-series of global atmospheric quantities.
Collection names are of the form freq_dims_group_HV, where the four attributes are:
  1. freq: time-independent (cnst), instantaneous (instF), or time-average (tavgF), where F indicates the frequency or averaging interval and can be any of the following:
    • 1 = Hourly
    • 3 = 3-Hourly
    • 6 = 6-Hourly
    • 0 = Not Applicable
  2. dims: Can be either:
    • 2d for collections with only 2-dimensional fields or
    • 3d for collections with a mix of 2- and 3-dimensional fields.
  3. group: A three-letter mnemonic for the type of fields in the collection.
  4. HV: Horizontal and Vertical grid.
    • H can be:
      • N: Native (2/3° x 1/2°) horizontal resolution
      • C: Reduced (1.25° x 1.25°) horizontal resolution
      • F: Reduced FV (1.25° x 1°) horizontal resolution
    • V can be:
      • x: horizontal-only data (surface, single level, etc.) ; dims must be 2D
      • p: pressure-level data (see Appendix D for levels) ; dims must be 3D
      • v: model layer centers (see Appendix D ) dims must be 3D
      • e: model layer edges (see Appendix D ) dims must be 3D
This node defines the date and time associated with the data in the file. It has the form yyyymmdd
  1. yyyy - year string (e.g. , 2002)
  2. mm - month string (e.g.., 09 for September)
  3. dd - day of the month string


This is an example of a MERRA filename from the production segment of the original version of the third (most recent) assimilation stream. The data are time-averaged, three-dimensional, temperature tendency products, at reduced horizontal resolution, interpolated to pressure levels. The file contains all data for 15 September 2002.