View source for MERRA implementation details

'''''NOTE: Information subject to change!'''''

On this page we list the necessary modifications to GEOS-Chem for MERRA compatibility.

== Overview ==

The [[MERRA|GMAO MERRA data product]] is a 30-year reanalysis done with the [http://acmg.seas.harvard.edu/geos/wiki_docs/geos5/GEOS-5.2.0-File_Specification.pdf 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|GEOS-5 meteorology]].

However, in order to reduce the volume of data produced (and for other technical concerns), GMAO chose to reduce the [[MERRA#MERRA grid structure|vertical and/or horizontal resolution]] of some important [[List of MERRA met fields|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.

=== Key similarities to GEOS-5 === 

The following aspects of MERRA are identical to the [[GEOS-5]] operational data product.

# The MERRA high-resolution grid ([http://acmg.seas.harvard.edu/geos/doc/man/appendix_2.html#A2.6 0.5&deg; x 0.666&deg;]) is identical to that of GEOS-5. 
# The MERRA vertical grid ([http://acmg.seas.harvard.edu/geos/doc/man/appendix_3.html#A3.5 72 hybrid levels]) is identical to that of GEOS-5.
# By and large, the same data fields that were in the GEOS-5 product are also contained in MERRA.

=== Key differences from GEOS-5 ===

The following aspects of MERRA differ from the GEOS-5 operational data product:

# The [[MERRA#MERRA time archiving|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 [[List of MERRA met fields|individual MERRA met fields]]:
#* Most of the relevant MERRA 3-D data for GEOS-Chem are only available at [http://acmg.seas.harvard.edu/geos/doc/man/appendix_2.html#A2.4 1&deg; x 1.25&deg; horizontal resolution].
#* OMEGA, PV, RH are 6-hr instantaneous data in MERRA but are 6-hr averaged data in GEOS-5
#* [[List of MERRA met fields#SNOMAS|SNOMAS (snow mass)]] in MERRA was missing coverage over Antarctica and Greenland (this has been corrected for).
#* Many of the precipitation fields (e.g. DQIDTMST, DQVDTMST, DQLDTMST, etc.) are saved on a 1.25&deg; x 1.25&deg; 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 )

to 
 
 #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:

=== Headers directory ===

The following modifications for MERRA were made in the <tt>Headers/</tt> directory:

{| border=1 cellspacing=0 cellpadding=5 
|- bgcolor="#CCCCCC"
!File
!Modifications made
|-
|define.h      
| 
*Add a new C-preprocessor switch named "MERRA". 
|-
|CMN_SIZE      
|
*Added size parameters for MERRA 4 x 5 and MERRA 2 x 2.5 grids.
|}

=== GeosUtil directory ===

The following modifications for MERRA were made in the <tt>GeosUtil/</tt> directory:

{| border=1 cellspacing=0 cellpadding=5 
|- bgcolor="#CCCCCC"
!File
!Modifications made
|-
|bpch2_mod.f     
| 
*In routine GET_MODELNAME: Add "MERRA_47L" and "MERRA" modelnames to the #if block
*In routine GET_NAME_EXT: Return grid name "geos5" for MERRA fields
|-
|directory_mod.f
|
*Added MERRA_DIR variable for MERRA met fields
|-
|file_mod.f
|
*Add IU_A1 parameter for logical unit # of MERRA 1hr time avg'd data
*In routine CLOSE_FILES: Now close the IU_A1 logical unit #
|-
|pressure_mod.f
|
*In routine GET_PRESSURE: Compute PEDGE for MERRA in the same way as for GEOS-5
*In routine INIT_PRESSURE: Initialize Ap and Bp for MERRA in the same way as for GEOS-5
|-
|time_mod.f
|
*Add variable CT_A1
*Add routines SET_CT_A1 and GET_CT_A1
*Add routine ITS_TIME_FOR_A1
*Update comments in various places
|-
|transfer_mod.f
|
*Change instances of <tt>#if defined( GEOS_5 )</tt>&nbsp;&nbsp;&nbsp;to&nbsp;&nbsp;&nbsp;<tt>#if defined( GEOS_5 ) || defined( MERRA )</tt>
|}

=== GeosCore directory ===

The following modifications for MERRA were made in the <tt>GeosCore/</tt> directory:

{| border=1 cellspacing=0 cellpadding=5 
|- bgcolor="#CCCCCC"
!File
!Modifications made
|-
|acetone_mod.f
|
*In routine OCEAN_SOURCE_ACET: Compute the ocean source of acetone for MERRA in the same way as for GEOS-5.<br>'''''(NOTE: We may need to eventually rescale this.)'''''
|-
|convection_mod.f
|
*In routine DO_CONVECTION: Call the same convection routine (NFCLDMX) for MERRA as is done for GEOS-5
|-
|dao_mod.f
|
*Add the following allocatable arrays for MERRA: DQRCU, DQRLSAN, FRSEAICE, FRSNO, PFICU, PFILSAN, PFLCU, PFLLSAN, PREANV, PRELSC, RH1, RH2, REEVAPCN, REEVAPLS, SEAICE{00..90}
*In routine COPY_I6_FIELDS: Rewrote #if block for clarity; Now copy MERRA RH fields
*In routine INIT_DAO: Initialize MERRA met field arrays
*In routine IS_ICE: Changed #if block line to <tt>#elif defined( GEOS_4 ) || defined( GEOS_5 ) || defined( MERRA )</tt>
*In routine IS_LAND: Changed #if block line to <tt>#elif defined( GEOS_4 ) || defined( GEOS_5 ) || defined( MERRA )</tt>
*In routine IS_NEAR: Rewrote logic of #if block for clarity
*In routine IS_WATER: Changed #if block line to <tt>#elif defined( GEOS_4 ) || defined( GEOS_5 ) || defined( MERRA )</tt>
*In routine INTERP: Rewrite #if block logic for clarity
*In routine CLEANUP_DAO: Deallocate MERRA met field arrays
|-
|depo_mercury_mod.f
|
*Change instances of <tt>#if defined( GEOS_5 )</tt>&nbsp;&nbsp;&nbsp;to&nbsp;&nbsp;&nbsp;<tt>#if defined( GEOS_5 ) || defined( MERRA )</tt>
|-
|diag3.f
|
*Modify ND66 diagnostic to divide by proper # of increments (i.e. every 3 hours)
*Modify ND67 diagnostic to divide by proper # of increments (i.e. every hour)
|-
|dust_mod.f
|
*Change instances of <tt>#if defined( GEOS_5 )</tt>&nbsp;&nbsp;&nbsp;to&nbsp;&nbsp;&nbsp;<tt>#if defined( GEOS_5 ) || defined( MERRA )</tt>
*Convert SNOMAS from mm H2O to m H2O in SRC_DUST_DEAD
|-
|dust_dead_mod.f
|
*Assign parameter FLX_MSS_FDG_FCT = 4.9d-4 for both MERRA and GEOS-5.<br>'''''(NOTE: We may need to eventually rescale this.)'''''
|-
|fast_j.f
|
*Do cloud overlapping for MERRA the same way as for GEOS-5
|-
|gamap_mod.f
|
*In routine INIT_TRACERINFO: 
*#Define units of CMFMC field w/ same units as GEOS-5
*#For ND28 diagnostic, only print out SOA tracers to "tracerinfo.dat" if LSOA=T
|-
|global_ch4_mod.f
|
*Change instances of <tt>#if defined( GEOS_5 )</tt>&nbsp;&nbsp;&nbsp;to&nbsp;&nbsp;&nbsp;<tt>#if defined( GEOS_5 ) || defined( MERRA )</tt>
|-
|global_hno3_mod.f
|
*Change instances of <tt>#if defined( GEOS_5 )</tt>&nbsp;&nbsp;&nbsp;to&nbsp;&nbsp;&nbsp;<tt>#if defined( GEOS_5 ) || defined( MERRA )</tt>
|-
|global_o3_mod.f
|
*Rewrote logic of #if block, now treat MERRA in the same way as GEOS-5
|-
|initialize.f
|
*Now also reset the counter for A1 timesteps by calling routine&nbsp;&nbsp;<tt>SET_CT_A1( RESET=.TRUE. )</tt>&nbsp;&nbsp; from time_mod.f
|-
|input_mod.f
|
*In routine READ_SIMULATION_MENU: Read value for MERRA_DIR from <tt>input.geos</tt> file
*In routine READ_EMISSIONS_MENU: Turn off LMEGANMONO when LMEGAN is turned off
|-
|land_mercury_mod.f
|
*Change instances of <tt>#if defined( GEOS_5 )</tt>&nbsp;&nbsp;&nbsp;to&nbsp;&nbsp;&nbsp;<tt>#if defined( GEOS_5 ) || defined( MERRA )</tt>
|-
|lightning_nox_mod.f
|
*In routine LIGHTNING: Eliminate the shallow-cloud inhibititon trap for MERRA, as we already do for GEOS-5
*In routine GET_OTD_LIS_SCALE: Compute <tt>ANN_AVG_FLASHRATE</tt> for MERRA in the same way as for GEOS-5.<br>'''''(NOTE: We may have to rescale this in the future.)'''''
|-
|Makefile
|
*Added dependencies for merra_cn_mod.f
*Added dependencies for merra_a1_mod.f
*Added dependencies for merra_a3_mod.f
*Added dependencies for merra_i6_mod.f
|-
|main.f
|
*Rewrote #if block logic where met fields are read in from disk
*Read RH from disk, in the same way as for GEOS-5.<br>'''''(NOTE: MERRA RH is a 6-hr instantaneous field; GEOS-5 RH is 6-hr average field.)'''''
*Now calls routines from <tt>merra_cn_mod.f</tt>, <tt>merra_a1_mod.f</tt>, <tt>merra_a3_mod.f</tt>, <tt>merra_i6_mod.f</tt> to read in met data
*Now compute lightning emissions every 3 hours (immediately after reading in the A3 fields).
|-
|megan_mod.f
|
*At top of module:
*#Move the <tt>#include "CMN_SIZE"</tt> declaration to top of module
*#For MERRA only: define DAY_DIM parameter = 24 for hourly temperature data
*#For all other met data: define DAY_DIM parameter = 8 for 3-hourly temperature data
*In routine INIT_MEGAN:
*#For MERRA only: call routines OPEN_MERRA_A1_FIELDS and READ_MERRA_A1_FIELDS every hour
*#For all other met data: call routines OPEN_A3_FIELDS and READ_A3_FIELDS every 3 hours
*#Now only read NUM_DAYS of initial sfc temp data instead of 15 days
|-
|mercury_mod.f
|
*Change instances of <tt>#if defined( GEOS_5 )</tt>&nbsp;&nbsp;&nbsp;to&nbsp;&nbsp;&nbsp;<tt>#if defined( GEOS_5 ) || defined( MERRA )</tt>
|-
|merra_a1_mod.f
|
*New module.  Reads in the MERRA 1-hour time-averaged (aka "A1") fields from disk.
|-
|merra_a3_mod.f
|
*New module.  Reads in the MERRA 3-hour time-averaged (aka "A3") fields from disk.
|-
|merra_cn_mod.f
|
*New module.  Reads in the MERRA constant (aka "CN") fields from disk.
|-
|merra_i6_mod.f
|
*New module.  Reads in the MERRA 6-hour instantaneous (aka "I6") fields from disk.
|-
|nei2005_anthro_mod.f
|
*Change instances of <tt>#if defined( GEOS_5 )</tt>&nbsp;&nbsp;&nbsp;to&nbsp;&nbsp;&nbsp;<tt>#if defined( GEOS_5 ) || defined( MERRA )</tt>
|-
|schem.f
|
*Change instances of <tt>#if defined( GEOS_5 )</tt>&nbsp;&nbsp;&nbsp;to&nbsp;&nbsp;&nbsp;<tt>#if defined( GEOS_5 ) || defined( MERRA )</tt>
|-
|sulfate_mod.f
|
*Change instances of <tt>#if defined( GEOS_5 )</tt>&nbsp;&nbsp;&nbsp;to&nbsp;&nbsp;&nbsp;<tt>#if defined( GEOS_5 ) || defined( MERRA )</tt>
|-
|tagged_co_mod.f
|
*Change instances of <tt>#if defined( GEOS_5 )</tt>&nbsp;&nbsp;&nbsp;to&nbsp;&nbsp;&nbsp;<tt>#if defined( GEOS_5 ) || defined( MERRA )</tt>
|-
|upbdflx_mod.f
|
*Change instances of <tt>#if defined( GEOS_5 )</tt>&nbsp;&nbsp;&nbsp;to&nbsp;&nbsp;&nbsp;<tt>#if defined( GEOS_5 ) || defined( MERRA )</tt>
|-
|vdiff_mod.f90
|
*Change instances of <tt>#if defined( GEOS_5 )</tt>&nbsp;&nbsp;&nbsp;to&nbsp;&nbsp;&nbsp;<tt>#if defined( GEOS_5 ) || defined( MERRA )</tt>
|-
|wetscav_mod.f
|
*Change instances of <tt>#if defined( GEOS_5 )</tt>&nbsp;&nbsp;&nbsp;to&nbsp;&nbsp;&nbsp;<tt>#if defined( GEOS_5 ) || defined( MERRA )</tt>
|}

=== GeosTomas directory ===

The following modifications for MERRA were made in the <tt>GeosTomas/</tt> directory:

{| border=1 cellspacing=0 cellpadding=5 
|- bgcolor="#CCCCCC"
!File
!Modifications made
|-
|Makefile
|
*Added dependencies for merra_cn_mod.f
*Added dependencies for merra_a1_mod.f
*Added dependencies for merra_a3_mod.f
*Added dependencies for merra_i6_mod.f
|-
|diag3.f
|
*Modify ND66 diagnostic to divide by proper # of increments (i.e. every 3 hours)
*Modify ND67 diagnostic to divide by proper # of increments (i.e. every hour)
|-
|dust_mod.f
|
*Change instances of <tt>#if defined( GEOS_5 )</tt>&nbsp;&nbsp;&nbsp;to&nbsp;&nbsp;&nbsp;<tt>#if defined( GEOS_5 ) || defined( MERRA )</tt>
|-
|gamap_mod.f
|
*In routine INIT_TRACERINFO: Define units of CMFMC field w/ same units as GEOS-5
|-
|input_mod.f
|
*In routine READ_SIMULATION_MENU: Read value for MERRA_DIR from <tt>input.geos</tt> file
|-
|sulfate_mod.f
|
*Change instances of <tt>#if defined( GEOS_5 )</tt>&nbsp;&nbsp;&nbsp;to&nbsp;&nbsp;&nbsp;<tt>#if defined( GEOS_5 ) || defined( MERRA )</tt>
|-
|wetscav_mod.f
|
*Change instances of <tt>#if defined( GEOS_5 )</tt>&nbsp;&nbsp;&nbsp;to&nbsp;&nbsp;&nbsp;<tt>#if defined( GEOS_5 ) || defined( MERRA )</tt>
|}

--[[User:Bmy|Bob Y.]] 12:20, 26 August 2010 (EDT)

== Resolved issues ==

=== Fractional sea ice ===

In order to preserve some information about sea ice leads during the regridding process, we have binned the fractional sea ice coverage into deciles (0-10%, 10-20% ... 90-100% coverage) and saved those as separate fields in the MERRA data archive for GEOS-Chem.  Please see [[List of MERRA met fields#Fractional sea ice|this wiki post]] for more information.

=== SNOMAS ===

The SNOMAS (snow depth) field in MERRA differs from that in GEOS-5.  It appears that in GEOS-5 a default snow mass value was applied to grid boxes that had > 90% land ice coverage.  This was not done in MERRA.  Bob Yantosca [[List of MERRA met fields#SNOMAS|has implemented an algorithm]] to adjust the MERRA SNOMAS field to be backwards-compatible with GEOS-5.

== Outstanding issues ==

=== Lightning ===

For the time being, GEOS-Chem/MERRA will read in the same the OTD/LIS redistribution factors as for GEOS-5.  MERRA is based on GEOS-5.2.0, so this should be OK for a first-cut.

However, for proper treatment, the OTD/LIS redistribution factors for MERRA will be rescaled for the entire OTD/LIS observation period.  This cannot be done until we have processed and regridded several years of MERRA met data. 

=== Wet deposition and scavenging ===

We are currently looking into how some of the MERRA precipitation fields can be used to improve the existing [[Wet deposition|GEOS-Chem wet scavenging algorithm]].

=== Acetone ocean source ===

As a first cut, the ocean source of acetone for MERRA (computed in routine <tt>OCEAN_SOURCE_ACET</tt> in <tt>acetone_mod.f</tt>) is computed in the same way as for GEOS-5.  The following #if block was based on the GEOS-5 code:

 #elif defined( MERRA )
 
       !%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
       !%%% NOTE: Treat MERRA in the same way as GEOS-5, although    %%%
       !%%% we may want to rescale this in the future (bmy, 8/13/10) %%%
       !%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
 #if defined( GRID4x5 )
       REAL*8, PARAMETER :: SCALE_FACTOR = 0.9551d0   ! MERRA 4x5
 #elif defined( GRID2x25 )
       REAL*8, PARAMETER :: SCALE_FACTOR = 0.2212d0   ! MERRA 2x25
 #endif

At some point we may need to rescale this.  However this cannot be done until we have sufficient MERRA met fields processed and regridded.

--[[User:Bmy|Bob Y.]] 15:03, 16 August 2010 (EDT)