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

In order to improve the representation of the shortwave radiative transfer in the
MAECHAM5 general circulation model, the spectral resolution of the shortwave radiation
parameterization used in the model has been increased and extended in the
UV-B and UV-C bands. The upgraded shortwave parameterization is first validated
offline with a 4 stream discrete-ordinate line-by-line model. Thereafter, two 20-years
simulations with the MAECHAM5 middle atmosphere general circulation model are
performed to evaluate the temperature changes and the dynamical feedbacks arising
from the newly introduced parameterization. The offline clear-sky comparison of
the standard and upgraded parameterizations with the discrete ordinate model shows
considerable improvement for the upgraded parameterization in terms of shortwave
fluxes and heating rates. In the simulation with the upgraded ratiation parameterization,
we report a significant warming of almost the entire atmosphere, largest at 1 hPa
at the stratopause, and stronger zonal mean zonal winds in the middle atmosphere.
The warming at the summer stratopause alleviates the cold bias present in the model
when the standard radiation scheme is used. The stronger zonal mean zonal winds
induce a dynamical feedback that results in a dynamical warming (cooling) of the polar
winter (summer) mesosphere, caused by an increased downward (upward)circulation
in the winter (summer) hemisphere. In the troposphere, the changes in the spectral
resolution and the associated changes in the cloud optical parameters introduce a relatively
small warming and, consistenly, a moisteneing. The warming occurs mostly
in the upper troposphere and can contribute to a possible improvement of the model
temperature climatology.