Abstract
Abstract. A fast pseudo-monochromatic radiative transfer package using a singular value decomposition (SVD) compressed atmospheric optical depth database has been developed, primarily for simulating radiances from hyperspectral sounding instruments (resolution ≥0.1 cm−1). The package has been tested extensively for clear-sky radiative transfer cases, using field campaign data and satellite instrument data. The current database uses HITRAN 2016 line parameters and is primed for use in the spectral region spanning 605 to 2830 cm−1. Optical depths for other spectral regions (15–605 and 2830–45 000 cm−1) can also be generated for use by kCARTA. The clear-sky radiative transfer model computes the background thermal radiation quickly and accurately using a layer-varying diffusivity angle at each spectral point; it takes less than 30 s (on a 2.8 GHz core using four threads) to complete a radiance calculation spanning the infrared. The code can also compute non-local thermodynamic equilibrium effects for the 4 µm CO2 region, as well as analytic temperature, gas and surface Jacobians. The package also includes flux and heating rate calculations and an interface to an infrared scattering model.
Highlights
Recent years have seen the launch and routine operation of new-generation infrared sounders on board Earth-orbiting satellites, for the purposes of providing measurements for data assimilation into numerical weather prediction (NWP)centers and for monitoring atmospheric composition
We have described the details of a very fast and accurate pseudo-monochromatic code, optimized for the thermal infrared spectral region used by operational weather sounders for thermodynamic retrievals
It is much faster than line-byline codes, and the accuracy of its spectroscopic database has been extensively compared to GENLN2 and more recently to Line-by-Line Radiative Transfer Model (LBLRTM)
Summary
Recent years have seen the launch and routine operation of new-generation infrared sounders on board Earth-orbiting satellites, for the purposes of providing measurements for data assimilation into numerical weather prediction (NWP). Using true MNLBL codes to produce optical depths for training the fast models is computationally intensive, as accurate line shapes needed to be computed for millions of spectral points, each at about 100 layers spanning a 0–80 km atmosphere, for about 40–50 gases; this has to be done for 50 or more profiles. The acceleration of this part of the process, needed to develop a fast RTA for the AIRS sounder, was the motivating factor behind the development of the work presented here. The paper discusses in detail some of the internal machinery of kCARTA, such as a background thermal computation developed for kCARTA, flux computations, and scattering packages
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