Introduction
Most radiation schemes in weather and climate models use the "correlated k-distribution" (CKD) method to treat gas absorption, which approximates the integration over hundreds of thousands of spectral lines by N pseudo-monochromatic radiative transfer calculations, where N is in the range tens to hundreds. In principle, there is a trade-off to make: larger N means more accuracy and a wider range of atmospheric conditions can be simulated, but at greater computational cost. Unfortunately most CKD schemes are a black box: there is no way for the user to adjust N according to their application. The purpose of CKDMIP is to address these issues, and specifically:
To use benchmark line-by-line calculations to evaluate the accuracy of existing CKD models for applications spanning short-range weather forecasting to climate modelling, and to explore how accuracy varies with number of g-points in individual CKD schemes.
To understand how different choices in way that CKD models are generated affects their accuracy for the same number of g-points.
To provide freely available datasets and software to facilitate the development of new gas-optics models, with the ultimate aim of producing a community tool to allow users to generate their own gas-optics models targeted at specific applications.
Documentation
More information on the design of the experimental protocol is here:
- Hogan and Matricardi: draft GMD protocol paper
- Technical Guide: describes the CKDMIP datasets and software, and what should be produced by the CKDMIP participants
Software
- ckdmip-0.5.tar.gz: Software package for performing longwave radiative transfer calculations both on line-by-line absorption spectra and CKD files produced by CKDMIP participants
Datasets
- ftp://dissemination.ecmwf.int: The location of the absorption spectra and associated CKDMIP datasets (username ckdmip, password on request from Robin Hogan, r.j.hogan@ecmwf.int)
Table 3 of Hogan and Matricardi defines four datasets consisting of a number of gas-concentration profiles:
- Evaluation-1 - 50 realistic profiles for training and evaluation: ckdmip_evaluation1_concentrations.nc
- Evaluation-2 - 50 further realistic profiles for independent evaluation (not yet available)
- MMM - profiles of median, minimum and maximum temperature, H2O and O3: ckdmip_mmm_concentrations.nc
- Idealized - idealistic profiles of temperature, pressure and humidity: ckdmip_idealized_concentrations.nc
As described in the technical guide, participants are requested to provide files containing the output from their CKD model in the form of the optical depth and Planck function for each g-point of their model. Here is an example of this file, and the fluxes that the CKDMIP software package will produce from it:
- ecrad-rrtmg_evaluation1_optical-depth_present.nc: Gas optical depth in the 140 g-points of the RRTMG gas-optics model in its implementation in the ecRad radiation scheme
- ecrad-rrtmg_evaluation1_lw_fluxes_present.nc: Broadband and spectral upwelling and downwelling fluxes computed from the RRTMG gas optical depths
- ckdmip_evaluation1_lw_fluxes_present.h5: The corresponding reference line-by-line fluxes
