Vertically resolved minimal-set k-distribution for thermal infrared absorption: an application to the atmosphere of Venus

TL;DR

This paper introduces a vertically resolved minimal-set k-distribution method tailored for Venus's atmosphere, enabling efficient and accurate thermal infrared absorption modeling suitable for climate simulations.

Contribution

It develops a new FKDM k-distribution technique that avoids the correlated k assumption, providing height-dependent k-functions for Venus's atmosphere with fewer terms and high accuracy.

Findings

Achieves <1.2 K/day cooling rate accuracy below 90 km

Flux calculations within 2% error

Uses fewer k-terms than correlated-k methods

Abstract

The FKDM $k$-distribution technique is applied to parameterize absorption of thermal radiation in the lower and middle atmosphere of Venus, targeting modeling scenarios where the cost of full radiative transfer calculations necessitates efficient parameterizations (e.g. climate modeling). Line-by-line reference modeling based on a Monte Carlo method for radiative transfer is built into the $k$-distribution terms construction process, explicitly controlling accuracy. From 16 bands across $10$--$6000~\mathrm{cm^{-1}}$, the method produces 32 $k$-terms, band-averaged Planck function values and per-band spectral points for computing Venus cloud optical properties. The FKDM $k$-distribution technique does not require the inter-level correlation assumption common for the correlated $k$-distribution method. We supply height-dependent $k(z)$ functions tabulated on the same vertical grid as the input temperature-pressure profile, designed for direct use in radiative transfer solvers and avoiding additional remapping of the pre-tabulated $k$-data. Our implementation of the technique yielded acceptable accuracy below 90~km ($<1.2~\mathrm{K\,day^{-1}}$ for cooling rates; $<2\%$ for fluxes), while requiring substantially fewer $k$-terms than recent implementations of the correlated-$k$ method. A Fortran driver that generates $k(z)$ functions for an arbitrary Venus atmospheric profile is provided in a public repository.