Photolysis mechanism

This page describes some of the updates to the FAST-J photolysis mechanism, as is currently implemented in GEOS-Chem.

Input files for FAST-J

The following input files are required for the FAST-J photolysis mechanism:

ratj.d

This file is where you specify each of the FAST-J photolysis species. Each species is mapped to a corresponding entry of the GEOS-Chem chemical mechanism.

jv_atms.dat

This file specifies the reference O3 climatology for FAST-J. NOTE: GEOS-Chem will overwrite this reference climatology with TOMS/SBUV data for those months and locations where such data exists.

jv_spec.dat

This file is where the various quantum yields and aerosol cross-sections are specified.

O1D reaction updated to JPL 2006

As of GEOS-Chem v8-01-02, the rate constants in the "FAST-J" jv_atms.dat file have been updated by Lin Zhang.

These were the old values:

O3_1d  180 9.000E-01 9.000E-01 3.824E-01 8.092E-02 7.650E-02       0.0       0.0
O3_1d  260 9.000E-01 9.000E-01 4.531E-01 1.438E-01 7.654E-02       0.0       0.0
O3_1d  300 9.000E-01 9.000E-01 5.273E-01 2.395E-01 7.659E-02       0.0       0.0

which are now replaced by the new values from JPL 2006:

O3_1d  180 9.000E-01 9.000E-01 3.824E-01 8.092E-02 7.650E-02       0.0       0.0
O3_1d  260 9.000E-01 9.000E-01 4.531E-01 1.438E-01 7.654E-02       0.0       0.0
O3_1d  300 9.000E-01 9.000E-01 5.273E-01 2.395E-01 7.659E-02       0.0       0.0

For more information, please contact Lin Zhang (lzh@io.as.harvard.edu).

--Bob Y. 11:16, 23 May 2008 (EDT)

Cloud overlap options in FAST-J

GEOS-Chem now has 3 cloud overlap options in the FAST-J photolysis mechanism:

1. Linear cloud overlap assumption:

Grid Box Optical depth = In-cloud optical depth * Cloud fraction

This

1. Approximate random overlap assumption:

           !===========================================================
           ! CLOUD OVERLAP : LINEAR ASSUMPTION 
           ! Directly use OPTDEPTH = TAUCLD * CLDTOT
           ! 
           ! NOTE: Use this option if you want to compare to results
           !       from GEOS-Chem v7-04-12 and prior versions.
           !===========================================================
           IF ( OVERLAP == 1 ) then

              ! Call FAST-J routines to compute J-values
              CALL PHOTOJ( NLON,  NLAT, YLAT,    DAY_OF_YR,  
    &                      MONTH, DAY,  CSZA,    TEMP,    
    &                      SFCA,  OPTD, OPTDUST, OPTAER )

           !===========================================================
           ! CLOUD OVERLAP : APPROXIMATE RANDOM OVERLAP
           ! Use OPTDEPTH = TAUCLD * CLDTOT**1.5
           !===========================================================
           ELSE IF ( OVERLAP == 2 ) THEN

              ! Column cloud fraction (not less than zero)
              CLDF1D = CLDF(1:LLPAR,NLON,NLAT)
              WHERE ( CLDF1D < 0d0 ) CLDF1D = 0d0
              
              ! Adjust optical depth
              OPTD = OPTD * SQRT( CLDF1D )

              ! Call FAST-J routines to compute J-values
              CALL PHOTOJ( NLON,  NLAT, YLAT,    DAY_OF_YR,  
    &                      MONTH, DAY,  CSZA,    TEMP,  
    &                      SFCA,  OPTD, OPTDUST, OPTAER )

           !===========================================================
           ! CLOUD OVERLAP : MAXIMUM RANDOM OVERLAP
           !
           ! The Maximum-Random Overlap (MRAN) scheme assumes that 
           ! clouds in adjacent layers are maximally overlapped to 
           ! form a cloud block and that blocks of clouds separated by 
           ! clear layers are randomly overlapped.  A vertical profile 
           ! of fractional cloudiness is converted into a series of 
           ! column configurations with corresponding fractions 
           ! (see Liu et al., JGR 2006; hyl,3/3/04). 
           !
           ! For more details about cloud overlap assumptions and 
           ! their effect on photolysis frequencies and key oxidants 
           ! in the troposphere, refer to the following articles:
           ! 
           ! (1) Liu, H., et al., Radiative effect of clouds on 
           !      tropospheric chemistry in a global three-dimensional 
           !      chemical transport model, J. Geophys. Res., vol.111, 
           !      D20303, doi:10.1029/2005JD006403, 2006.
           ! (2) Tie, X., et al., Effect of clouds on photolysis and 
           !      oxidants in the troposphere, J. Geophys. Res., 
           !      108(D20), 4642, doi:10.1029/2003JD003659, 2003.
           ! (3) Feng, Y., et al., Effects of cloud overlap in 
           !      photochemical models, J. Geophys. Res., 109, 
           !      D04310, doi:10.1029/2003JD004040, 2004.
           ! (4) Stubenrauch, C.J., et al., Implementation of subgrid 
           !      cloud vertical structure inside a GCM and its effect 
           !      on the radiation budget, J. Clim., 10, 273-287, 1997.
           !-----------------------------------------------------------
           ! MMRAN needs IN-CLOUD optical depth (ODNEW) as input 
           ! Use cloud fraction, instead of OPTD, to form cloud blocks
           ! (hyl,06/19/04)
           !===========================================================