Extrapolation of MERRA Reanalyses to obtain continuous fields

 

Purpose: To perform basic extrapolations resulting in continuous fields of  NASA MERRA reanalyses for selected model levels where pressure values are greater than the surface pressure (i.e., filling in model grid points "below ground" with defined values). This could be useful where a continuous global field is needed, such as in modeling applications or spherical harmonic analyses. Code samples are included, below.  Geopotential heights are extrapolated by applying the hydrostatic equation and equation of state, as are temperature fields using adiabatic lapse rate approximations (moist or dry). 

Background reference from the MERRA FAQs (https://gmao.gsfc.nasa.gov/research/merra/faq.php)

Q. 6 "Why are there such large discrepancies at 1000MB and 850MB between MERRA and other Reanalyses?"

A. "The GEOS5 data assimilation system used to produce MERRA does not (or did not at the time of production) extrapolate data to pressure levels greater than the surface pressure. These grid points are marked by undefined values. The result is that area averages that include these points will not be representative compared to other data sets without additional screening. Time averages, such as monthly means, may also have substantial differences at the edges of topography. The lowest model level data and surface data are available so that users can produce their own extrapolation. A page discussing this issue is available. See http://gmao.gsfc.nasa.gov/research/merra/pressure_surface.php.   "

Below, note regions corresponding to high model topography where contours end due to undefined values in a sample MERRA Reanalysis plot (for example, portions of Rocky Mountains, Andes Mountains, Tibetan Plateau, Antarctica):

 

With extrapolation, the values are "filled in", as this figure shows (below):

 

Here is another example with Geopotential height:

 

And, after extrapolation:

 

Below are sample codes and scripts that perform extrapolation on 5 variables from a MERRA NetCDF file.

Software needed: GrADS, Fortran compiler, Unix/Linux shell environment.

Need a MERRA NetCDF or HDF file along with the following:

1.Shell executable extrap.csh:   Runs programs, creates output.  Calls grads scripts and fortran executable

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2. Two GrADS control files are needed:

check_filled_9999.ctl (views output w/-9999 set as undefined):

 

'reinit'

'sdfopen MERRA300.prod.assim.inst3_3d_asm_Cp.20100110.SUB_1.nc'

'set x 1 288'

'set y 1 144'

'set gxout fwrite'

'set fwrite tot_merra_filled_9999.prs'

t=1

while (t <= 8)

 'set t 't

say 'Analysis time is 't

*height

  z=1

  while (z<=17)

    'set  z 'z

    'd const(maskout(h,h),-9999.,-u)'

    z=z+1

  endwhile

*

*Temperature

  z=1

  while (z<=17)

    'set  z 'z

    'd const(maskout(t,t),-9999.,-u)'

    z=z+1

  endwhile

*

*u-wind

  z=1

  while (z<=17)

    'set  z 'z

    'd const(maskout(u,u+1000),-9999.,-u)'

    z=z+1

  endwhile

*

*v-wind

  z=1

  while (z<=17)

    'set  z 'z

    'd const(maskout(v,v+1000),-9999.,-u)'

    z=z+1

  endwhile

*




*RH

  z=1

  while (z<=17)

    'set  z 'z

    'd const(maskout(rh,rh),-9999.,-u)'

    z=z+1

  endwhile

*

 t = t + 1

endwhile

'disable fwrite'

'quit'

 

 

 

 4.  Fortran file extrap_merra.f:

 Code to perform extrapolation, called in extrap.csh.




 

 

 

 

 

Contact: Lee Byerle