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. This is the default in GEOS-Chem versions [GEOS-Chem_versions_under_development#v8-01-01|v8-01-01]] and prior. The option is:

Grid Box Optical depth = In-cloud optical depth * Cloud fraction.  This is the default

2. Approximate random overlap assumption: This option will be introduced into the standard code in GEOS-Chem v8-01-02 (benchmark run #1).

Grid Box Optical Depth = In-Cloud Optical Depth * ( Cloud Fraction )^1.5 

3. Maximum random overlap assumption: This option is much more computationally intensive, and therefore is not used as the default option. However, if you wish to use this option, then manually edit the fast_j.f source code file such that OVERLAP = 3.

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., 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.