Coupling GEOS-Chem with RRTMG

Overview

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. This alternative to offline coupling allows better temporal resolution in the RT calculations and provides a consist platform for GEOS-Chem users with the reliable radiative transfer package RRTMG.

Most of the added code is 'transparent', therefore GEOS-Chem model can still be run with the radiation code switched off. The optical properties are calculated at multiple wavelengths so the user is no longer restricted to 550nm as default, or indeed a single wavelength, so there are associated changes regardless of whether the radiative code is invoked. However, these cause negligible slow down (the model is actually slightly faster).

This wiki explains the key changes, why they were chosen, and links to the documentation for getting started running the model and digging deeper into the changes and the auxiliary files required.

Key Changes

1. A new menu for Radiation exists in input.geos. This also includes a wavelength selection for optical depth output (up to three optical depths, but only the first is used for diagnostics other than the standard bpch output).

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. 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:

---

%%% RADIATION MENU %%%  :

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

[O3,ME,SU,NI,AM,BC,OA,SS,DU,PM]

---

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). The radiation timestep defaults to 180 minutes but can be set for any multiple of the chemistry time step.

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.

[Under Construction]

Authors and collaborators

• Colette Heald (Civil and Environmental Engineering, MIT) -- Principal Investigator
• [sbarrett@mit.edu Steven Barrett] (Aeronautics and Astronautics, MIT)
• David Ridley (Civil and Environmental Engineering, MIT)
• Karen Cady-Pereira (AER)
• Matthew Alvarado (AER)

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	David Ridley Colette Heald	GCRT Publication Xuan's work Dust work
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