The new capabilities of global storm-resolving models to resolve individual clouds allow for a more physical perspective on the tropical high-cloud radiative effect and how it might change with warming. In this study, we develop a conceptual model of the high-cloud radiative effect as a function of cloud thickness measured by ice water path. We use atmospheric profiles from a global ICON simulation with 5 km horizontal grid spacing to calculate the radiation offline with the ARTS line-by-line radiative transfer model. The conceptual model of the high-cloud radiative effect reveals that it is sufficient to approximate high clouds as a single layer characterised by an albedo, emissivity and temperature, which vary with ice water path. The increase of the short-wave high-cloud radiative effect with ice water path is solely explained by the high-cloud albedo. The increase of the long-wave high-cloud radiative effect with ice water path is governed by an increase of emissivity for ice water path below 10-1 kg m-2, and by a decrease of high-cloud temperature with increasing ice water path above this threshold. The total high-cloud radiative effect from the ARTS simulations for the chosen day of this ICON model run is 2.59 W m-2, which is closely matched by our conceptual model with 2.56 W m-2. Because the high-cloud radiative effect depends on the assumed radiative alternative, assumptions on low clouds make a substantial difference. The conceptual model predicts that doubling the fraction of low clouds causes a doubling of the high-cloud radiative effect.

Changes in the concentration of greenhouse gases within the atmosphere lead to changes in radiative fluxes throughout the atmosphere. The value of this change, called the instantaneous radiative forcing, varies across climate models, due partly to differences in the distribution of clouds, humidity, and temperature across models, and partly due to errors introduced by approximate treatments of radiative transfer. This paper describes an experiment within the Radiative Forcing Model Intercomparision Project that uses benchmark calculations made with line-by-line models to identify parameterization error in the representation of absorption and emission by greenhouse gases. The clear-sky instantaneous forcing by greenhouse gases to which the world has been subject is computed using a set of 100 profiles, selected from a re-analysis of present-day conditions, that represent the global annual mean forcing with sampling errors of less than 0.01 \si{\watt\per\square\meter}. Six contributing line-by-line models agree in their estimate of this forcing to within 0.025 \si{\watt\per\square\meter} while even recently-developed parameterizations have typical errors four or more times larger, suggesting both that the samples reveal true differences among line-by-line models and that parameterization error will be readily resolved. Agreement among line-by-line models is better in the longwave than in the shortwave where differing treatments of the water vapor vapor continuum affect estimates of forcing by carbon dioxide and methane. The impacts of clouds on instantaneous radiative forcing are roughly estimated, as are adjustments due to stratospheric temperature change. Adjustments are large only for ozone and for carbon dioxide, for which stratospheric cooling introduces modest non-linearity.

The article describes a new practical model for the infrared absorption of chlorofluorocarbons and other gases with dense spectra, based on HITRAN absorption cross-sections. The model is very simple, consisting of frequency-dependent polynomial coefficients describing the pressure and temperature dependence of absorption. Currently it is implemented for the halocarbon species required by the Radiative Forcing Model Intercomparison Project (RFMIP). This approach offers practical advantages compared to previously available options, and is traceable, since the polynomial coefficients follow directly from the laboratory spectra. The model is applied to the problem of computing instantaneous clear-sky halocarbon radiative efficiencies and present day radiative forcing. Results are in reasonable agreement with earlier assessments that were carried out with the less explicit Pinnock method, and thus broadly validate that method. Overall, halocarbons are responsible for a substantial share of the present-day forcing, 0.573 Watt/squaremeter (instantaneous clear-sky at the TOA), corresponding to approximately 20% of the total anthropogenic forcing, or 44% compared to anthropogenic CO2 forcing alone.

We quantify the temperature-dependence of the clear-sky climate sensitivity in a one-dimensional radiative-convective equilibrium model. The atmosphere is adjusted to fixed surface temperatures between 280 and 320 K while preserving other boundary conditions in particular the relative humidity and the CO2 concentration. We show that an out-of-bounds usage of the radiation scheme RRTMG can lead to an erroneous decrease of the feedback parameter and an associated “bump” in climate sensitivity as found in other modelling studies. Using a line-by-line radiative transfer model, we find an almost constant longwave radiative feedback at surface temperatures above 300 K. However, the line-by-line simulations also show a slight decrease in climate sensitivity when surface temperatures exceed 310 K. This decrease is caused by water-vapor masking the radiative forcing at the flanks of the CO2 absorption band, which reduces the total radiative forcing by about 18%.

We quantify the temperature-dependence of clear-sky radiative feedbacks in a tropical radiative-convective equilibrium model. The longwave radiative fluxes are computed using a line-by-line radiative transfer model to ensure accuracy in very warm and moist climates. The one-dimensional model is tuned to surface temperatures between 285 and 313 K by modifying a surface enthalpy sink, which does not directly interfere with radiative fluxes in the atmosphere. The total climate feedback increases from -1.7 to -0.8 Wm^-2K^-1 for surface temperatures up to 305 K due to a strengthening of the water-vapor feedback. The temperature-dependence maximizes at surface temperatures around 297 K, which is close to the present-day tropical mean temperature. At surface temperatures above 305 K, the atmosphere becomes fully opaque and the radiative feedback is almost constant. This near-constancy is in agreement with a theoretical model of the water-vapor feedback presented by Ingram (2010), but in disagreement with other modeling studies.