The Community Radiative Transfer code (CouRT)

An atmospheric radiative transfer package for consideration in CCSM4

Principal Designers: William Collins (NCAR) and Andrew Conley (NCAR)

Rationale for a new atmospheric radiative transfer (RT) code:

The current radiative transfer methods employed in the Community Atmosphere Model (CAM3) are described in CAM technical note, Collins et al (2005), and references therein. These longwave parameterization was developed for CCM1 (Williamson et al, 1987), and the shortwave parameterization was developed for CCM2 (Hack et al, 1993). These codes have been used for a wide variety of scientific applications with the atmospheric model and the Community Climate System Model (CCSM3). Since the original development, the codes have been extended to include the effects of trace gases, aerosols, improved understanding of the spectroscopy of H₂O, and more general assumptions for cloud geometrical overlap.

However, it has become clear that the CCSM project should adopt new codes based upon

• Recent theoretical developments in radiative transfer;
• Modern spectroscopic data for all the radiative species; and
• Current software engineering practices.

Over the period 2005-2009, the CouRT designers will develop a new atmospheric radiative transfer code for consideration in the CAM4 and CCSM4. The ultimate decisions regarding which formulation of radiative transfer is adopted for these models is at the discretion of the CCSM Scientific Steering Committee (SSC) based upon recommendations from the Atmospheric Model Working Group (AMWG). Our objective is to have a new code ready for consideration by the SSC and AMWG in time for the release of CCSM4 (tentatively in 2009) for use in simulations for the IPCC 5^th Assessment Report (AR5).

The new code is designed to provide several advantages over the current implementation, including:

• Greater accuracy with respect to benchmark calculations;
• Greater flexibility to add or substract radiatively active gaseous and condensed species;
• Greater flexibility to handle arbitrary cloud geometry and cloud inhomogeneity;
• Simpler implementation to facilitate a wider range of applications;
• Traceability of the descriptions of radiatively active species to basic data and theory;
• Flexibility to include more diagnostics, for example TOA radiances and radiative forcing; and
• Flexibility to run the RTE solver easily as part of CAM, as part of a diagnostic computation, and as part of a single-column-model.

The basic components in the RTE solver are:

1. A solver for the radiative transfer equations;
2. Optical characterization for gaseous species;
3. Optical characterization for condensed species; and
4. An interface to the cloud parameterizations to implement the Independent Column Approximation (ICA).

These are described in greater detail in the software design and numerical requirements.

Major Design Elements:

Science requirements:

• Boundary conditions and interaction with surface models
• Atmospheric constituents input to the code
• Radiative fields output from the code
• Interactive outputs (e.g., flux divergences)
• Diagnostic outputs
• Accuracy requirements for the outputs
• Implementation phases for each of the mandatory and optional outputs

Software design and numerical requirements:

• Methods for solution of the RT equations
• Description of the portable RTE solver package
• Description of the offline computations
• Data flow in the offline computations and RTE solver
• Data dependencies of the RTE solver
• Interface and communication prescriptions
• Requirements for unit and physics testing
• Requirements for numerical and physical accuracy

Implementation and Review plan:

• Phases and tasks of the project
• Initial timeline for each phase
• Initial design review
• Periodic software review
• Science review

References:

1. Literature search for 1991-2005 for correlated-k methods and other aspects of radiative transfer relevant to our parameterization work.