This page evaluates the RRTMGP gas optics scheme (v1801204), designed for weather and climate applications. The comparisons below use the 50 profiles of the "Evaluation-1" CKDMIP dataset. The reference calculations were performed using LBLRTM to generate the high resolution absorption spectra and the CKDMIP software to perform the radiative transfer. See also the results for the later version 1.5 of RRTMGP (including full and reduced spectral resolution), as well as earlier RRTMG gas optics scheme, and also those from a Neural Network approximation to RRTMGP.
Longwave
The longwave gas optics scheme uses a total of 256 g-points (k terms) in 16 bands. The evaluation has been performed using radiative transfer with four zenith angles in each hemisphere (8-stream). The following plots evaluate fluxes and heating rates for the four CKDMIP greenhouse-gas scenarios "Present", "Preindustrial", "Glacial Maximum" and "Future". The comparison shares some of the features of the ecRad-RRTMG results, but are generally improved with less wiggles in the heating rate errors.
To examine the performance in more detail, the following plot evaluates fluxes and heating rates in each of the 13 CKDMIP longwave bands. The performance is noticeably better than in the equivalent plot for the ecRad-RRTMG simulations. Note that the apparent errors in the first two bands are simply because the RRTMGP band boundary is 250 cm-1 internally rather than 350 cm-1.
The following plot compares the instantaneous radiative forcing (change to net flux) at top-of-atmosphere and the surface, from perturbing the concentrations of individual well-mixed greenhouse gases from their present-day values, found by averaging over the 50 profiles of the Evaluation-1 dataset. For the minimum and maximum concentrations, the change to mean atmospheric heating rate is also evaluated. In this case we see that RRTMGP represents the radiative forcing of all gases well, and is much better than RRTMG at representing low concentrations of methane.
The following plot evaluates the representation of the overlap of the longwave absorption by carbon dioxide, methane and nitrous oxide. In each case, the x-axis shows the top-of-atmosphere radiative forcing from perturbing a gas to either its climatic minimum or maximum value, using the ranges stated by Hogan and Matricardi (2020). These radiative forcings are computed keeping the concentrations of all other well-mixed gases at their present-day values, except for the gas on the y-axis which is perturbed to its own climatic minimum or maximum values. The performance is improved somewhat from RRTMG.
Shortwave
The shortwave RRTMGP gas optics scheme uses a total of 224 g-points (k terms) in 14 bands. The following plots evaluate fluxes and heating rates for the four CKDMIP greenhouse-gas scenarios "Present", "Preindustrial", "Glacial Maximum" and "Future". Five solar zenith angles have been used with a fixed surface albedo of 0.15, the approximate global-mean value. The red line in the central column of panels quantifies the bias averaging over all five solar zenith angles, so should be considered as a daytime average (divide by two to get an approximate diurnal average). The shaded regions in these panels encompass 95% of the data. The solar spectrum in RRTMGP is based on recent measurements of the sun, consistent with those used in the reference calculations used in CKDMIP, so no correction needs to be performed as in the analysis of the earlier RRTMG model. The upwelling fluxes are much more accurate in terms of both bias and RMSE than the RRTMG results (after correcting the a posteriori correction of the RRTMG solar spectrum).
The following figure evaluates fluxes and irradiances in each of the 13 CKDMIP shortwave bands, indicating good performance in all bands; note that discrepancy in the first two bands is largely due to the fact that RRTMGP uses 2680 cm-1 as the boundary between these two bands, not 2600 cm-1 used in the CKDMIP reference calculations.
The following plot compares the instantaneous radiative forcing (change to net flux) at top-of-atmosphere and the surface, from perturbing the concentrations of individual well-mixed greenhouse gases from their present-day values. It has been found by averaging over the 50 profiles of the Evaluation-1 dataset, and averaging over the five solar zenith angles; therefore these forcings correspond to daytime only. The CFCs have a tiny shortwave effect so have been excluded. For the minimum and maximum concentrations, the change to mean atmospheric heating rate is also evaluated. We see that the radiative forcing from changes to carbon dioxide and methane is excellent, fixing the underestimates found in RRTMG. The nitrous oxide forcing is underestimated, but is still better than in RRTMG which ignores this gas in the shortwave.
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