This page evaluates the ecRad implementation of the RRTMG gas optics scheme operational in the ECMWF model. It is indistinguishable from version 3.9 of RRTMG available from AER. RRTMG is 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. This page should be compared to the RRTMGP evaluation, where RRTMGP is an updated version of RRTMG.
Longwave
The longwave gas optics scheme uses a total of 140 g-points (k terms) in 16 bands. The evaluation has been performed using radiative transfer with four zenith angles in each hemisphere (8 streams). The following plots evaluate fluxes and heating rates for the four CKDMIP greenhouse-gas scenarios "Present", "Preindustrial", "Glacial Maximum" and "Future" (click on individual plots to expand). The shaded regions in the central three panels of each plot encompass 95% of the data. Generally the downwelling fluxes are unbiased, the upwelling TOA fluxes are underestimated by around 0.4 W m-2, and there are some systematic "wiggles" in the heating-rate profile errors.
The following plot evaluates fluxes and heating rates for the broadband (leftmost column of panels) and the 13 CKDMIP longwave bands (other panels). This provides more information about the source of the broadband errors. The black dashed and red solid lines correspond to the average of the 50 profiles for the "present-day" scenario, while the shaded regions encompass 95% of the error.
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 RRTMG represents the radiative forcing of all gases very well except for low values of methane, the radiative effect of which is underestimated.
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. RRTMG correctly represents the fact that carbon dioxide radiative forcing is very weakly affected by the concentrations of the other two gases. However, it over-predicts the sensitivity of methane and nitrous oxide radiative forcing to the concentrations of the other gases.
Shortwave
The shortwave RRTMG gas optics scheme uses a total of 112 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 blue lines show the "raw" results from RRTMG, but since it is known that the solar spectrum is somewhat outdated in RRTMG, the fluxes in each of the 13 "narrow" CKDMIP bands have been scaled to match the more up-to-date solar spectrum used in CKDMIP - these results are in red. It can be seen that this correction substantially improves the agreement with the reference calculations.
To understand the broadband errors further, the following figure evaluates fluxes and irradiances in each of the 13 CKDMIP shortwave bands in the case that the solar spectrum is not corrected. It can be seen that excess incoming shortwave in the 29000-38000 cm-1 band is largely responsible for the excess stratospheric heating, although there are errors in other bands too.
The following provides the same band-wise evaluation but after correcting the incoming shortwave in each band to match the values used in the reference calculations, where the substantial improvement in the important 29000-38000 cm-1 band is apparent. There are also shortcomings in other bands.
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. In this case we see that RRTMG tends to underestimate the radiative forcing from changes to carbon dioxide and methane, and does not represent the shortwave effect of nitrous oxide at all.
