A Tool for Generating Fast k‐Distribution Gas‐Optics Models for Weather and Climate Applications. (1st October 2022)

• Record Type: Journal Article
• Authors: Hogan, Robin J.; Matricardi, Marco
• Is Part Of: Journal of advances in modeling earth systems. Volume 14:Number 10(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-10-01
• Subjects: radiation parameterization -- radiative forcing -- tcorrelated k‐distribution method -- climate modeling; Geological modeling -- Periodicals; Climatology -- Periodicals; Geochemical modeling -- Periodicals; 551.5011
• DOI: 10.1029/2022MS003033
• Languages: English
• ISSNs: 1942-2466

Abstract

Abstract: One of the most important components of an atmospheric radiation scheme is its treatment of gas optical properties, which determines not only the accuracy of its radiative forcing calculations fundamental to climate prediction, but also its computational cost. This paper describes a free software tool "ecCKD" for generating fast gas‐optics models by optimally dividing the spectrum into pseudo‐monochromatic spectral intervals (known as k ‐terms) according to a user‐specified error tolerance and the range of greenhouse‐gas concentrations that needs to be simulated. The models generated use the correlated k ‐distribution method in user‐specified bands, but can also generate accurate "full‐spectrum correlated‐ k " models that operate on the entire longwave or near‐infrared (NIR) parts of the spectrum. In the NIR, the large spectral variation in cloud absorption is represented by partitioning the parts of the spectrum where gases are optically thin into 2–6 sub‐bands, while allowing k ‐terms for the optically thicker parts of the spectrum (where clouds and surface reflectance are less important) to span the entire NIR spectrum. Candidate models using only 16 and 32 k ‐terms in each of the shortwave and longwave are evaluated against line‐by‐line calculations on clear and cloudy profiles. The 32‐term models are able to accurately capture the radiative forcing of varying greenhouse gases including CO2 concentrations spanning a factor of 12, and heating rates at pressures down to 1 Pa. Plain Language Summary: A crucial component of atmospheric computer models used to make climate projections and weather forecasts is the "gas optics scheme, " which represents the interaction of sunlight and infrared radiation with greenhouse gases. This paper describes a free software tool "ecCKD" that uses a number of novel techniques to generate new gas optics schemes that are computationally faster than most existing schemes while still being very accurate. For example, the schemes are able to simulate variations in carbon dioxide concentration spanning a factor of 12 and methane concentration spanning a factor of 10. Users of ecCKD can generate schemes that are optimized for specific applications, such as short‐term weather forecasting or simulating past climates. A special focus has been placed on the near‐infrared part of the solar spectrum to ensure that the schemes work well when computing the interactions of sunlight simultaneously with gases and clouds, important to ensure that the impact of clouds on weather and climate is well simulated. Key Points: We describe a free software tool for generating accurate gas optics models for radiation schemes and test with line‐by‐line calculations Extra efficiency is achieved via the use of the full‐spectrum correlated‐ k method in the thermal‐ and near‐infrared (NIR) The spectral properties of clouds are treated accurately via the use of sub‐bands in the NIR and several other techniques