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What is CIRC?

CIRC is in many respects the successor to the seminal ICRCCM (Intercomparison of Radiation Codes in Climate Models) effort that spanned the late 80's - early 00's. CIRC distinguishes itself from ICRCCM by its emphasis on using observations to build its catalog of cases. It is intended as an evolving and regularly updated reference source for GCM-type radiative transfer (RT) code evaluation, and similar to ICRCCM, its goal is to contribute to the improvement of solar and thermal RT parameterizations. CIRC has received support by DOE's Atmospheric Radiation Measurement (ARM) program and is a project of GEWEX's GDAP and GASS panels as well as a working group within IAMAS's International Radiation Commission (IRC). More information on the rationale behind CIRC can be found here. The invitation letter that launched Phase I on June 4, 2008 is available in this page

Instrument clusters near Barrow, Alaska, gather data useful in refining global climate models. (photo by Mark Ivey)

Register as a CIRC participant

While anybody can download the input files needed for the radiative transfer runs and the reference output results, we urge users of this website to register as "CIRC participants". Registered CIRC participants will enjoy benefits such as: 

  • Updates via e-mail about improvements, additions, and corrections to the reference dataset and the accompanying documentation.
  • An opportunity to have their results compared to those of other participants.
  • Invitation to workshops on CIRC. 
  • Invitation to coauthor scientific papers on CIRC. 

  Please register as a CIRC participant by sending your name, affiliation and e-mail address to Lazaros Oreopoulos

What we provide and what we request

The CIRC Phase I cases, with one exception, are based on ARM BBHRP cases satisfying preset criteria that make them appropriate for the purposes of the intercomparison. The main criterion was:

  • satisfactory radiative closure at the surface and TOA (for both the solar and thermal part of the spectrum),

while for the cloudy cases additional criteria were:

  • overcast conditions
  • the presence of only one water phase (liquid)
  • cloud homogeneity (as indicated by small variability in the observed surface irradiances),

  and for the clear sky cases additional criteria were:

  • a wide range of precipitable water loadings
  • a significant range of aerosol loadings
  • a significant range of solar geometries

We provide all the input typically needed by a GCM-type radiative transfer algorithm to calculate profiles of radiative fluxes and heating rates, namely profiles of atmospheric pressure, temperature, gas concentrations, aerosol single scattering properties, cloud fraction/water path/effective particle size, and surface albedo. A comprehensive list of these quantities and details about how they were specified or derived can be found here, while the input data themselves can be downloaded from here. The reference output consists of surface and TOA fluxes resolved at 1 cm-1 resolution and broadband longwave (LW) flux and heating rate profiles. The longwave results were obtained with the line-by-line radiation code LBLRTM, while the shortwave (SW) results were obtained with the doubling-adding code CHARTS (Code for High-Resolution Accelerated Radiative Transfer with Scattering) which uses LBLRTM gaseous absorption optical depths.  CHARTS output is currently limited to radiative fluxes at the boundaries of the atmospheric column (TOA and surface), but fluxes at additional atmospheric levels may be provided in the future.  The output requested from CIRC participants consists of broadband SW and LW flux and heating rate profiles.

Phase I Cases

CIRC Phase I consists of seven cases, five cloud-free, and two with overcast liquid clouds. The cloudless cases come from BBHRP Southern Great Plains (SGP) cases (three), and from one BBHRP Northern Slope of Alaska (NSA) case which spawns two experiments, one with nominal and one with doubled carbon dioxide.  The cloudy cases come from a BBHRP SGP case and from a Pt. Reyes, CA ARM Mobile Facility (AMF) deployment case . In early 2010, runs for new simplified "subcases" were requested in order to aid the interpretation of Phase I submissions. The detailed list of cases with description and links to download the corresponding input and output is provided here.


Radars at the ARM Southern Great Plains site.
Microwave Radiometer with Stratus (Image courtesy of the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) user facility.)

4xCO2 cases

In late 2014 Robert Pincus led an effort to evaluate approximate RT codes with respect to forcing performance under 4xCO2 conditions. The CIRC Phase I cloud-free pristine (no-aerosol) subcases with spectrally flat surface albedo (for SW) were used. The effort provided the opportunity to evaluate updated versions of the RT codes that participated in CIRC Phase I, as well as codes that had not yet participated in CIRC both under nominal as well as quadrupled CO2 concentrations. The most important results of the exercise are described in this paper. The article contains information on how to download input and output data relevant to the exercise.


Contact information

For more information on CIRC please contact Lazaros Oreopoulos. For questions on BBHRP, LBLRTM and CHARTS contact Eli Mlawer.