Category Archives: MERRA

MERRA data

“The Modern Era Retrospective-analysis for Research and Applications (MERRA) products are generated using Version 5.2.0 of the GEOS-5 DAS with the model and analysis each at 1/2×2/3 degrees resolution. Three-dimensional analyses are generated every 6 hours, and 3- dimensional diagnostics, describing the radiative and physical properties of the atmosphere, are 3-hourly. The product suite includes analyses on the native vertical grid as well on pressure surfaces. Two-dimensional data, including surface, fluxes, and vertical integrals, are produced hourly. The product suite includes monthly and monthly diurnal files. The MERRA production is being conducted in 3 separate streams, 1979 – 1989; 1989 – 1998; 1998 – present. Data are being uploaded to the MDISC after undergoing quality assurance in the GMAO.”

A summary of MERRA can be found in the [brouchure](\ Brochure.pdf), and the best detailed information about MERRA is found in the Readme file. Data can be accessed from Goddard Earth Sciences Data Centre.

“Hourly, three-hourly, and six hourly collections consists of daily files. For collections of monthly or seasonal means, each month or season is in a separate file.”

One attraction of MERRA is the hourly resolution of some variables. Only ‘2D’ variables are available at hourly resolution, but these ‘2D’ variables are available at different heights.

Underlying file names

File names consist of five dot-delimited nodes, runid.runtype.config.collection.timestamp. Where:

Node Description
run_id MERRASVv where S=stream number and Vv=Version number
runtype prod=standard product,
ovlp=overlapping product,
spnp=spin up product,
rosb=reduced observing system product,
cers=CERES observing system product
config assim = assimilation. Uses a combination of atmospheric data analysis and model forecasting to generate a time-series of global atmospheric quantities.
simul=simulation. Uses a free-running atmospheric model with some prescribed external forcing, such as sea-surface temperatures.
frcst=forecasts. Uses a free-running atmospheric model initialized from an analyzed state.
collection All MERRA data are organized into file collections that contain fields with common characteristics. Collection names are of the form freq_dims_group_HV, where
freq can be cnst=time-independent, instF=instantaneous, tavgF=time-average and F indicates the frequency or averaging interval and can be 1 = Hourly, 3 = 3-Hourly, 6 = 6-Hourly, M = Monthly mean, U = Monthly-Diurnal mean, 0 = Not Applicable.
dims=2D or 3D,
group lowercase (cryptic) nemomic
HV, where H (horizontal) can be N=native (2/3×1/2), C=reduced (1.25×1.25), F=reduced (1.25×1), and V (vertical) can be: x=horizontal only, p=pressure levels, v=model level centres, e=model level edges
timestamp yyyymmdd

There are two collection naming conventions in operation, described part-by-part in the table below:

Short Name Standard name
C: Configuration, where C is one of:
A = Assimilation
F = Forecast; 
S = Simulation;
ffff: Frequency Type, where ffff is one of:
inst = Instantaneous; 
tavg = Time average; 
const = Time independent
T: Time Description, where T is one of:
I = Instantaneous; 
T = Time Averaged with
C = Time Independent
N: Frequency, where N is one of:
1 = 1-hourly; 
3 = 3-hourly; 
6 = 6-hourly; 
M = Monthly Mean; 
U = Monthly Diurnal Mean; 
0 = Not Applicable;
H: Horizontal Resolution, where H is one of:
N = Native (2/3 x 1/2 deg);
F = Reduced Resolution Version of Model Grid (1.25 x 1 deg);
C = Reduced Resolution (1.25 x 1.25 deg)
ggg: Group, where ggg is one of:
ana = Direct analysis products;
asm = Assimilated state variables;
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;
slv = Single level;
int = Vertical integrals;
chm = Chemistry forcing
V: Vertical Location, where V is one of:
X = Two-dimensional;
P = Pressure;
V = Model Layer Center;
E = Model Layer Edge
H: Horizontal Resolution, where H is one of:
N = Native (2/3 x 1/2 deg);
F = Reduced Resolution Version of Model Grid (1.25 x 1 deg);
C = Reduced Resolution (1.25 x 1.25 deg)
GGG: Group, where GGG is one of:
ANA = Direct analysis products;
ASM = Assimilated state variables;
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;
OCN = Ocean quantities;
MST = Moist processes;
CLD = Clouds;
RAD = Radiation;
TRB = Turbulence;
SLV = Single level;
INT = Vertical integrals;
CHM = Chemistry forcing
v: Vertical Location, where v is one of:
x = Two-dimensional;
p = Pressure;
v = Model Layer Center;
e = Model Layer Edge

Useful collections

Collection Description
2D IAU Diagnostic, Single Level Meteorology, Time Average 1-hourly, on 2/3×1/2 grid
2D Surface Fluxes, Single Level Meteorology, Time Average 1-hourly, on 2/3×1/2 grid
3-Dimensional Instantaneous 6-hourly, on pressure levels, at native resolution
3D Meteorology Instantaneous 6-hourly analyzed fields on model layer center on 2/3×1/2 grid
3D analyzed state, meteorology, instantaneous (monthly), on pressure levels, at native resolution
MERRA 3D analyzed state, meteorology, instantaneous (diurnal), on pressure levels, at native resolution

A particularly useful dataset (for me) is MAT1NXSLV. Using OpenDAP (more later) to examine the files shows this structure:
<type 'netCDF4.Dataset'>
root group (NETCDF3_CLASSIC file format):
missing_value: 1e+15
Conventions: CF-1.0
title: MERRA reanalysis. GEOS-5.2.0
history: File written by CFIO
institution: Global Modeling and Assimilation Office, NASA Goddard Space Flight Center, Greenbelt, MD 20771
source: Global Modeling and Assimilation Office. GEOSops_5_2_0
comment: GEOS-5.2.0
dimensions: TIME, XDim, YDim
variables: SLP, PS, U850, U500, U250, V850, V500, V250, T850, T500, T250, Q850, Q500, Q250, H1000, H850, H500, H250, OMEGA500, U10M, U2M, U50M, V10M, V2M, V50M, T10M, T2M, QV10M, QV2M, TS,     DISPH, TROPPV, TROPPT, TROPPB, TROPT, TROPQ, CLDPRS, CLDTMP, XDim, YDim, TIME, XDim_EOS, YDim_EOS, Time

Where the XDim=540 (i.e. 2/3 degree), YDim=361 (i.e. 1/2 degree), and the long names of the variables are:

Variable name long name
SLP Sea level pressure
PS Time averaged surface pressure
U850 Eastward wind at 850 hPa
U500 Eastward wind at 500 hPa
U250 Eastward wind at 250 hPa
V850 Northward wind at 850 hPa
V500 Northward wind at 500 hPa
V250 Northward wind at 250 hPa
T850 Temperature at 850 hPa
T500 Temperature at 500 hPa
T250 Temperature at 250 hPa
Q850 Specific humidity at 850 hPa
Q500 Specific humidity at 500 hPa
Q250 Specific humidity at 250 hPa
H1000 Height at 1000 hPa
H850 Height at 850 hPa
H500 Height at 500 hPa
H250 Height at 250 hPa
OMEGA500 Vertical pressure velocity at 500 hPa
U10M Eastward wind at 10 m above displacement height
U2M Eastward wind at 2 m above the displacement height
U50M Eastward wind at 50 m above surface
V10M Northward wind at 50 m above the displacement height
V2M Northward wind at 2 m above the displacement height
V50M Northward wind at 50 m above
T10M Temperature at 10 m above the displacement height
T2M Temperature at 2 m above the displacement height
QV10M Specific humidity at 10 m above the displacement height
QV2M Specific humidity at 2 m above the displacement height
TS Surface skin temperature
DISPH Displacement height
TROPPV PV based tropopause pressure
TROPPT T based tropopause pressure
TROPPB Blended tropopause pressure
TROPT Tropopause temperature
TROPQ Tropopause specific humidity
CLDPRS Cloud-top pressure
CLDTMP Cloud-top temperature
XDim longitude
YDim latitude
TIME time
Time Time

Getting the data

There are at least three different ways of getting the data:

MIRADOR allows http or ftp access to the full global hdf4 files

Simple Subset Wizard Web front-end to an OpenDAP server. Use the web interface to select the collection (using the short name, e.g. MAI6NPANAY), and define a bounding box. The web interface then constructs an OpenDAP request e.g.[0:3][0:41][226:337][188:344],U[0:3][0:41][226:337][188:344],H[0:3][0:41][226:337][188:344],V[0:3][0:41][226:337][188:344],O3[0:3][0:41][226:337][188:344],SLP[0:3][226:337][188:344],QV[0:3][0:41][226:337][188:344],PS[0:3][226:337][188:344],XDim[188:344],TIME[0:3],Height,YDim[226:337]

Use OpenDAP directly. There are some instructions here about using OpenDAP via GRADS to download MERRA data. Information gleaned from that:

  • There are three OpenDAP servers:
    • Meteorological fields for Chemical Transport Modeling
    • Two-dimensional fields
    • Three-dimensional fields

On the servers, the files are grouped by year and month, e.g.

Then you need to know the long name of the underlying hdf file, e.g.

Note that the stream number comes into play here, i.e the first digit after the final ‘MERRA’:

  • Stream 1: 1979 – 1989;
  • Stream 2: 1989 – 1998;
  • Stream 3: 1998 – present.

If you have OpenDAP-enabled NetCDF Operators can use ncks:

ncks -v <variables> -d XDim,<xmin><xmax> -d YDim,<ymin><ymax><year>/<month>/MERRA<stream_number><version>.prod.assim.tavg1_2d_slv_Nx.<year><month><day>.hdf  <out>.nc

Concrete example:

ncks -v SLP,U850,U500,V850,V500,T850,T500,T250,Q850,Q500,H1000,H850,H500,OMEGA500,U10M,U2M,U50M,V10M,V2M,V50M,T10M,T2M,QV10M,QV2M,TS,DISPH,XDim,YDim,TIME,XDim_EOS,YDim_EOS,Time -d XDim,225,315 -d YDim,262,322 $HOME/data/reanalysis/MERRA/