Impact of an improved shortwave radiation scheme in the MAECHAM5 General Circulation Model

Abstract

Abstract. In order to improve the representation of ozone absorption in the stratosphere of the MAECHAM5 general circulation model, the spectral resolution of the shortwave radiation parameterization used in the model has been increased from 4 to 6 bands. Two 20-years simulations with the general circulation model have been performed, one with the standard and the other with the newly introduced parameterization respectively, to evaluate the temperature and dynamical changes arising from the two different representations of the shortwave radiative transfer. In the simulation with the increased spectral resolution in the radiation parameterization, a significant warming of almost the entire model domain is reported. At the summer stratopause the temperature increase is about 6 K and alleviates the cold bias present in the model when the standard radiation scheme is used. These general circulation model results are consistent both with previous validation of the radiation scheme and with the offline clear-sky comparison performed in the current work with a discrete ordinate 4 stream scattering line by line radiative transfer model. The offline validation shows a substantial reduction of the daily averaged shortwave heating rate bias (1-2 K/day cooling) that occurs for the standard radiation parameterization in the upper stratosphere, present under a range of atmospheric conditions. Therefore, the 6 band shortwave radiation parameterization is considered to be better suited for the representation of the ozone absorption in the stratosphere than the 4 band parameterization. Concerning the dynamical response in the general circulation model, it is found that the reported warming at the summer stratopause induces stronger zonal mean zonal winds in the middle atmosphere. These stronger zonal mean zonal winds thereafter appear to produce a dynamical feedback that results inCorrespondence to: C. Cagnazzo (cagnazzo@bo.ingv.it) a dynamical warming (cooling) of the polar winter (summer) mesosphere, caused by an increased downward (upward) circulation in the winter (summer) hemisphere. In addition, the comparison of the two simulations performed with the general circulation model shows that the increase in the spectral resolution of the shortwave radiation and the associated changes in the cloud optical properties result in a warming (0.5-1 K) and moistening (3%-12%) of the upper tropical troposphere. By comparing these modeled differences with previous works, it appears that the reported changes in the solar radiation scheme contribute to improve the model mean temperature also in the troposphere.

The global aerosol-climate model ECHAM-HAM, version 2: sensitivity to improvements in process representations

“…The shortwave scheme has 4 or 6 bands depending on model version. The 6-band version adapted from the ECMWF model by Cagnazzo et al (2007) has the 250-690 nm interval subdivided to better consider the absorption by water vapor at 440-690 nm. In addition, an ultra-violet band is added to consider the absorption by ozone, resulting in a total of 3 bands in the ultra-violet and visible range (185-250 nm, 250-440 nm and 440-690 nm).…”

“…(Roeckner et al, 2003(Roeckner et al, , 2006a Stratiform cloud microphysics scheme by Lohmann and Roeckner (1996); Solar radiation scheme with four bands (Fouquart and Bonnel, 1980). Two-moment stratiform cloud microphysics scheme by Lohmann et al (2007); Solar radiation scheme extended to six bands (Cagnazzo et al, 2007) Sulfuric acid gas Sources and sinks include transport, chemical production, condensation, and aerosol nucleation.…”

“…The refractive index for black carbon has been updated to reduce the negative bias of absorption aerosol optical depth . The four-band shortwave radiative transfer scheme in the atmospheric model has been extended with two more bands (Cagnazzo et al, 2007). A newer aerosol nucleation parameterization has been adopted, and additional mechanisms included (Kazil et al, 2010).…”

Intercomparison of shortwave radiative transfer schemes in global aerosol modeling: results from the AeroCom Radiative Transfer Experiment

“…In a two-stream approximation, scattering and absorption by molecules and aerosols are taken into account. Since the update by Cagnazzo et al (2007), six bands are used, with intervals between 0.185 µm, 0.25 µm, 0.44 µm, 0.69 µm, 1.19 µm, 2.38 µm and 4.0 µm. The range 0.185-0.69 µm is considered the visible range, the range 0.69-4.0 µm, the near-infrared.…”

Influences of in-cloud aerosol scavenging parameterizations on aerosol concentrations and wet deposition in ECHAM5-HAM

“…Convective clouds, and transport are based on the mass-flux scheme of Tiedtke (1989) with modifications following Nordeng (1994). The solar radiation scheme has 6 spectral bands (Cagnazzo et al, 2007) and the infrared has 16 spectral bands (Mlawer et al, 1997;Morcrette et al, 1998).…”