Coupling GEOS-Chem with RRTMG

From Geos-chem
Revision as of 22:37, 30 October 2013 by David Ridley (Talk | contribs) (Overview)

Jump to: navigation, search

On this page we provide information about the coupling of GEOS-Chem with the RRTMG radiative transfer model (by AER, Inc.).


The GEOS-Chem model with online radiative transfer calculations (referred to as GCRT) was developed to allow GEOS-Chem users to produce gas and aerosol direct radiative effect (DRE) output for both the longwave and shortwave. This alternative to offline coupling allows better temporal resolution in the RT calculations and provides a consist platform for GEOS-Chem users with the widely used radiative transfer package RRTMG.

Most of the added code is 'transparent', therefore this version of the GEOS-Chem model can still be run with the radiation code switched off. The optical properties are calculated at multiple wavelengths so that the user is no longer restricted to 550nm as default, so there are associated changes regardless of whether the radiative code is invoked. However, these cause negligible slow down (the default model is actually slightly faster). Compiling with RRTMG=yes requires approximately double the amount of RAM (~15Gb for a 2x2.5 simulation) and takes between 40% and 100% longer depending on the settings used.

This wiki explains the key changes and requirements. If you want to dig deeper into the changes and assumptions made, or are looking for a quick-start guide to running the model, download the GCRT User Guide

Authors and collaborators

Questions regarding GCRT can be directed at David (e-mail linked above).

--David R. 18:00, 08 October 2013 (EST)

GCRT User Groups

User Group Personnel Projects
Atmospheric Chemistry, Massachusetts Institute of Technology Colette Heald
David Ridley
Xuan Wang
  • GCRT Publication
  • Inter-annual variability and trends in African dust
  • Modeling black and brown carbon
Add yours here

Key Changes

1. A new menu for Radiation exists in input.geos (see below). This also includes a wavelength selection for optical depth output that is independent of whether RRTMG is switched on (up to three optical depths can be output in the standard OD-MAP-$ diagnostic, but only the first is used for diagnostics other than the standard bpch output e.g. ND51).

2. A new code folder, GeosRad/, is required for the RRTMG code. The module "rrtmg_rad_transfer_mod.F" is the driver code (within GeosCore/) that interfaces with RRTMG.

3. The optics look-up tables are updated, containing multiple wavelengths and separated into files for each species (soot.dat, so4.dat, org.dat, dust.dat, ssa.dat, ssc.dat) that are stored in the run directory. To prevent discrepancies jv_spec.dat no longer contains optical properties for aerosol, Fast-J uses aerosol optics from from the same speciated look-up tables. The new files mean that jv_spec_aod.dat is redundant and no longer used.

4. Several new input files are required. Surface albedo and emissivity climatologies have been generated and must be stored in modis_surf_201210/ within the root data directory (e.g. GEOS_2x2.5/). Climatologies of gases (CH4, N2O, CFC-11, CFC-12, CFC-22 and CCl4) must be stored within the root data directory.

5. A new set of diagnostics (ND71) are available providing the change in radiative flux (DRE) for gases and aerosol, LW/SW, top of atmosphere (TOA) and surface, and clear-sky and all-sky conditions. These also include AOD, SSA and asymmetry parameter for each aerosol species at the requested wavelengths.

Running GCRT

As with the latest version of GEOS-Chem, NetCDF libraries must be installed (see Installing libraries for GEOS-Chem). The code should be compiled using the compile switch RRTMG, i.e. "make RRTMG=yes". A fresh compilation after a "make realclean" will take 15-20 mins (subsequent compiles are much faster).

Once the code is compiled the input.geos file will need configuring:

AOD Wavelength (nm)     : 550 440 532 
Turn on RRTMG?          : T
Calculate LW fluxes?    : T
Calculate SW fluxes?    : T
Clear-sky flux?         : T
All-sky flux?           : T
Radiation Timestep [min]: 180
Species fluxes          : 0 0 1 0 0 0 0 0 0 1

The species for which DRE output is currently available are listed with two-letter identifiers. In the example the DRE is calculated for sulfate (SU) and all particulate matter (PM) - see the table below for all identifier descriptions. The radiation time step defaults to 180 mins but can be set for any multiple of the chemistry time step. The baseline (BA) flux is always output by default and is the flux (W/m^2) with all aerosol and species included.

In this set up all diagnostics from ND48 to ND51b produce optical depth output at 550nm, but the main diagnostic output contains three copies of the optical depths (and SSA and asymmetry parameters if requested). The output variables have the wavelength appended to the variable name e.g. OPBC550.

Abbreviation Species
Black carbon
Organic aerosol
Sea salt
Mineral dust
All particulate matter
Baseline flux
  • note that the radiative impacts of gases (ozone and methane) are relatively untested at this stage and should be interpreted with caution.

Finally, the ND71 diagnostic must be set to 1 if RRTMG is switched on.

Once all the auxiliary files are in place (new optics data files, surface albedo climatologies, trace gas profile climatologies) and the input.geos set up, the model can be run in the usual way.

For more information please consult the documentation [link]

Example Output

Below, example output fluxes and direct radiative effect (DRE) from GCRT are displayed for March 2013. The top row show the total flux with all aerosol and gases included, subsequent rows show the DRE for all particulate matter, black carbon, and sulfate. The left column shows top of atmosphere (TOA) and the right column shows surface DRE. All results are for all-sky conditions (i.e. including clouds). The identifier for each figure is |F|SW|PM|T|A| (F - denotes a flux output, SW/LW - shortwave or longwave, PM - species identifier, T/S - TOA or surface, A/C - All-sky or clear sky)

Example DRE figures.jpg