Difference between revisions of "MERRA"

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(Subsetting MERRA data)
(Subsetting MERRA data)
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--[[User:Bmy|Bob Y.]] 14:20, 15 December 2010 (EST)
 
--[[User:Bmy|Bob Y.]] 14:20, 15 December 2010 (EST)
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== Reading MERRA raw data files with IDL ==
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You can read any of the MERRA data files directly into IDL.  The [http://acmg.seas.harvard.edu/gamap/ GAMAP visualization package] contains a routine callee <tt>EOS_GETGR</tt> that you can use to read the MERRA data in its native in HDF4-EOS format.
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'''Step 1:''' Download the data file that you need.  The MERRA data are available via a simple FTP file transfer.  However, we recommend using the GNU wget utility, which will simplifly the download process.  For example, to download a data file that contains the MERRA FRSEAICE data field, use this command:
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wget –nd “ftp://goldsmr2.sci.gsfc.nasa.gov/data/s4pa/MERRA/MAT1NXFLX.5.2.0/2008/04/MERRA300.prod.assim.tavg1_2d_flx_Nx.20080412.hdf
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'''Step 2:''' Once you have downloaded a MERRA data file, you can read it into IDL with this sequence (or include this into your own IDL program):
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IDL> FID = EOS_GD_OPEN( 'MERRA300.prod.assim.tavg1_2d_flx_Nx.20080412.hdf', /READ )
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IDL> DATA = EOS_GETGR( fId, 'FRSEAICE', GRIDNAME='EOSGRID' )
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IDL> help, data, /stru
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DATA            FLOAT    = Array[540, 361, 24]
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IDL> STATUS = EOS_GD_CLOSE( FID )
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etc.
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For more information, please see:
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# [http://disc.sci.gsfc.nasa.gov/daac-bin/DataHoldings.pl MERRA data holdings web page].  (This web page lists the file names and the FTP data paths.)
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# [[List of MERRA met fields]]
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--[[User:Bmy|Bob Y.]] 17:11, 7 March 2011 (EST)

Revision as of 22:11, 7 March 2011

This page describes some basic information about the GMAO MERRA data product.

Overview

The GMAO MERRA data product is a 30-year reanalysis done with the GEOS-5.2.0 assimilation system. This was done to supplement the GEOS-5 operational data product, which only has data from 2004-present.

Please visit the links below to learn more about the MERRA data:

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. 14:41, 16 August 2010 (EDT)

MERRA grid structure

This section describes the horizontal and vertical grids used by the MERRA data products.

Input grids

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
Resolution
Vertical
Resolution
Used for Notes
Native,
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
Native,
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
Native,
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)


The longitudes and latitudes of the Native (Nx, Nv, Ne) grids are constructed as follows:

Lon_Edge(I)   = ( -180° - DI/2 ) + ( DI * I ),  I = 0, 540
Lat_Edge(J)   = ( -90°  - DJ/2 ) + ( DJ * J ),  J = 0, 361

Lon_Center(I) = -180° + ( DI * ( I - 1 ) ),     I = 1, 540
Lat_Center(J) =  -90° + ( DJ * ( J - 1 ) ),     J = 1, 361

where DI = 2/3° and DJ = 1/2°.

The longitudes and latitudes of the Reduced FV (Fx, Fv, Fe) grids are constructed as follows:

Lon_Edge(I)   = ( -180° - DI/2 ) + ( DI * I ),  I = 0, 288
Lat_Edge(J)   = ( -90°  - DJ/2 ) + ( DJ * J ),  J = 0, 181

Lon_Center(I) = -180° + ( DI * ( I - 1 ) ),     I = 1, 288
Lat_Center(J) =  -90° + ( DJ * ( J - 1 ) ),     J = 1, 181

where DI = 1.25° and DJ = 1°.

The longitudes and latitudes of the Pressure (Cp) grid are constructed as follows:

Lon_Edge(I)   = -180° + ( DI * I ),             I = 0, 288
Lat_Edge(J)   =  -90° + ( DJ * J ),             J = 0, 144

Lon_Center(I) = -180° + ( DI * ( I - 1/2° ) ),  I = 1, 288
Lat_Center(J) =  -90° + ( DJ * ( J - 1/2° ) ),  J = 1, 144

where DI = 1.25° and DJ = 1.25°.

--Bob Y. 10:19, 6 August 2010 (EDT)

Output grids

The grid structure of MERRA precludes the construction of the 0.5° x 0.667° nested grids, as for certain individual data fields the finest resolution available is 1.25° x 1.25°.

At present, we foresee the use of MERRA only at the following resolutions:

  1. GEOS-Chem 2° x 2.5° grid
  2. GEOS-Chem 4° x 5° grid

As of August 2010, Harvard will only regrid MERRA data to the 4° x 5° grid (due to both time and disk space constraints). The 2° x 2.5° data can be produced later if there is a strong research need. Bob Yantosca will make the MERRA regridding code available to GEOS-Chem users who would like to process the MERRA data for their own applications.

--Bob Y. 10:19, 6 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 and 5.2 of the MERRA File Specification Document, pp 10ff:

Standard names

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

       runid.runtype.config.collection.timestamp

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

runid
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
runtype
The main GEOS-5 MERRA products will be from standard production runs (denoted by prod).
config
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
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
timestamp
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


EXAMPLE:

       MERRA300.prod.assim.tavg3_3d_tdt_Cp.20020915.hdf

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.

ESDT names

To accommodate certain software requirements, all MERRA files are associated with a nine character ESDT. The ESDT is a short (and rather cryptic) handle for users to access sets of files. In MERRA the ESDT will be used to identify collections and will consist of a compressed version of the collection name of the form:

       MCTFHVGGG

where

  1. C: Configuration
    • A = Assimilation
  2. T: Time Description:
    • I = Instantaneous
    • T = Time-averaged
    • C = Time-independent
  3. F: Frequency
    • 1 = Hourly
    • 3 = 3-Hourly
    • 6 = 6-Hourly
    • 0 = Not Applicable
  4. H: Horizontal Resolution
    • N = Native.
    • F = Reduced resolution version of model grid
    • C = Reduced resolution
  5. V: Vertical Location
    • X = Two-dimensional
    • P = Pressure
    • V = model layer center
    • E = model layer edge
  6. GGG: Group
    • ANA = direct analysis products
    • ASM = assimilated state variables (from the IAU corrector, see Appendix A)
    • TDT = tendencies of temperature
    • UDT = tendencies of eastward and northward wind components
    • QDT = tendencies of specific humidity
    • ODT = tendencies of ozone
    • LND = land surface variables
    • FLX = surface turbulent fluxes and related quantities
    • MST = moist processes
    • CLD = clouds
    • RAD = radiation
    • TRB = turbulence

The ESDT name is also used in the various FTP file paths to the MERRA data directories at GSFC.

--Bob Y. 12:51, 4 August 2010 (EDT)

MERRA data file collections

In this section we list the MERRA "raw" data files that are used to create the data archive for GEOS-Chem. This is a subset of all of the available MERRA data. Information in the table is taken from Section 6 of the MERRA File Specification Document, page 15ff:

File collection name ESDT Name
(aka "short name")
Description Times Size (GB/day)
const_2d_asm_Nx MAC0FXCHM Constant fields Time-invariant 0.0002
tavg1_2d_flx_Nx MAT1NXFLX Surface turbulent fluxes and related quantities 1-hr time averages 0.267
tavg1_2d_lnd_Nx MAT1NXLND Land related surface quantities 1-hr time averages 0.146
tavg1_2d_rad_Nx MAT1NXRAD Surface and TOA radiative fluxes 1-hr time averages 0.189
tavg1_2d_slv_Nx MAT1NXSLV Single-level atmospheric state variables 1-hr time averages 0.285
tavg3_2d_chm_Fx MAT3FXCHM Chemistry related 2-D Single-level 3-hr time averages 0.020
tavg3_3d_chm_Fv MAT3FVCHM Chemistry related 3-D at model layer centers 3-hr time averages 0.329
tavg3_3d_chm_Fe MAT3FECHM Chemistry related 3-D at model layer edges 3-hr time averages 0.166
tavg3_3d_mst_Cp MAT3CPMST Upper-air diagnostics from moist processes 3-hr time averages 0.056
tavg3_3d_qdt_Cp MAT3CPQDT Upper-air humidity tendencies by process 3-hr time averages 0.166
inst3_3d_asm_Cp MAI3CPASM Basic assimilated fields from IAU corrector 3-hr instantaneous 0.231
inst6_3d_ana_Nv MAI6NVANA Analyzed fields on model layers 6-hr instantaneous 0.452
TOTAL       2.307


As described in Section 3, the full file name for mainstream MERRA data products is of the form:

MERRAXXX.prod.assim.CCCCCCCCCCCCCCC.YYYYMMDD.hdf

where

  1. XXX is the runid (e.g. "100", "200", or "300")
  2. CCCCCCCCCCCCCCC is the collection name (i.e. one of the entries in the first column of the above table)
  3. YYYYMMDD is the year/month/day date

--Bob Y. 12:56, 4 August 2010 (EDT)

MERRA time archiving

Raw data

The MERRA raw data are archived/averaged at the following times:

  1. const: Time-invariant data
  2. inst3: 3-hr instantaneous data
    • Times: 00:00, 03:00, 06:00, 09:00, 12:00, 15:00, 18:00, 21:00 GMT
  3. inst6: 6-hr instantaneous data
    • Times: 00:00, 06:00, 12:00, 18:00 GMT
  4. tavg1: 1-hour time-averaged data
    • Center times: 00:30, 01:30, 02:30 ... 23:30 GMT
  5. tavg3: 3-hour time-averaged data
    • Center times: 01:30, 04:30, 07:30, 10:30, 13:30, 16:30, 19:30, 22:30 GMT

Regridded data

The regridded MERRA data for GEOS-Chem are archived at the following times:

  1. CN (time-invariant data)
    • Filenames 20000101.cn.2x25 and/or 20000101.cn.4x5
  2. A1 (1-hr time-averaged data)
    • Filenames YYYYMMDD.a1.2x25 and/or YYYYMMDD.a1.4x5
  3. A3 (3-hr time-averaged data)
    • Filenames YYYYMMDD.a3.2x25 and/or YYYYMMDD.a3.4x5
  4. I6 (6-hr instantaneous data)
    • Filenames YYYYMMDD.i6.2x25 and/or YYYYMMDD.i6.4x5


The CN files contain time-invariant data.

The I6 files contain instantaneous (i.e. snapshot) data at 00:00, 06:00, 12:00, 18:00 GMT.

The A1 and A3 files use the following bins for time-averaging:

GEOS5 MERRA times.png

Please also see our List of MERRA met fields wiki page for detailed information about which fields are stored in each of these file types.

--Bob Y. 16:09, 11 August 2010 (EDT)

Implementation into GEOS-Chem

Please see our MERRA implementation details wiki page for more information about the specifics of how GEOS-Chem is being modified to use the MERRA met product.

--Bob Y. 10:42, 16 August 2010 (EDT)

Create plots from MERRA data online

For those of you who want to see monthly mean plots of MERRA data, you can do that online via the GIOVANNI utility at NASA. GIOVANNI is a Web-based application developed by the GES-DISC that provides a simple and intuitive way to visualize, analyze, and access vast amounts of Earth science remote sensing data without having to download the data.

Please see the following web links:

  1. GIOVANNI home
  2. Create plots of MERRA monthly-mean 2-dimensional fields
  3. Create plots of MERRA monthly-mean 3-dimensional fields

Subsetting MERRA data

You have the option of asking NASA's GES-DISC (i.e. the department that handles data download requests) for MERRA data files that have been subsetted, or "cut down to size". This may help you to avoid having to download and archive the large global MERRA data files.

Please see the MERRA data subset web interface for more information.

--Bob Y. 14:20, 15 December 2010 (EST)

Reading MERRA raw data files with IDL

You can read any of the MERRA data files directly into IDL. The GAMAP visualization package contains a routine callee EOS_GETGR that you can use to read the MERRA data in its native in HDF4-EOS format.

Step 1: Download the data file that you need. The MERRA data are available via a simple FTP file transfer. However, we recommend using the GNU wget utility, which will simplifly the download process. For example, to download a data file that contains the MERRA FRSEAICE data field, use this command:

wget –nd “ftp://goldsmr2.sci.gsfc.nasa.gov/data/s4pa/MERRA/MAT1NXFLX.5.2.0/2008/04/MERRA300.prod.assim.tavg1_2d_flx_Nx.20080412.hdf

Step 2: Once you have downloaded a MERRA data file, you can read it into IDL with this sequence (or include this into your own IDL program):

IDL> FID = EOS_GD_OPEN( 'MERRA300.prod.assim.tavg1_2d_flx_Nx.20080412.hdf', /READ )
     
IDL> DATA = EOS_GETGR( fId, 'FRSEAICE', GRIDNAME='EOSGRID' )

IDL> help, data, /stru
DATA            FLOAT     = Array[540, 361, 24]

IDL> STATUS = EOS_GD_CLOSE( FID )

etc.

For more information, please see:

  1. MERRA data holdings web page. (This web page lists the file names and the FTP data paths.)
  2. List of MERRA met fields

--Bob Y. 17:11, 7 March 2011 (EST)