Application of different 3d radiative flux parameterizations in the global atmospheric circulation model ECHAM 5

Non cloud-resolving atmospheric circulation models often account for 3d effects in solar radiative transfer by means of artificially scaling the cloud optical thickness in the internal radiative transfer model (e.g. Cahalan et at. 1994). Based on intensive 3d radiative transfer calculations for a large number of 3d cloud scenarious it was shown that domain averaged cloud properties like total water content, cloud cover, or cloud base height are strongly correlated with the domain averaged shortwave radiative fluxes at the top and at the surface of the model domain (Schewski and Macke, 2003). From these correlations nonlinear parameterizations of transmitted, absorbed, and reflected broadband solar radiative fluxes have been obtained. The present study indirectly applies these parameterizations to the atmospheric general circulation model ECHAM 5 in the following way: Radiative fluxes for uncorrected cloud properties have been compared to the fluxes from the parameterization by Schewski and Macke (2003) for each model grid box and selected time steps. New correction factors have been derived to match the transmitted and the reflected solar irradiation. Mean diurnal cycles of the correction factors for different climatological regimes as well as global distributions of monthly mean correction factors have ben determined and will be discussed in detail. The current ECHAM5 model applies a liquid water path dependent correction factor (Roeckner et al. 2003) whereas the parameterization by Schewski and Macke (2003) accounts for solar elevation, and up to three most dominant domain averaged cloud properties. Differences in the newly derived correction factors to those from the standard parameterization will be presented.

Cahalan, R. F., Ridgway, W., Wiscombe, Warren J., Bell, Thomas L., Snider, Jack B., The Albedo of Fractal Stratocumulus Clouds, Journal of the Atmospheric Sciences, 1994 51: 2434-2455

Roeckner, E., Bäuml, G., Bonaventura, L., Brokopf, R., Esch, M., Girogetta, M., Hagemann, S., Kirchner, I., Kornblueh, L., Manzini, E., Rhodin, A., Schlese, U., Schulzweida, U., und Tompkins, A., 2003. The atmospheric general circulation model ECHAM 5, Part I, Report 213, Max-Planck-Institut für Meteorologie, Bundesstr. 55, 20146 Hamburg, Germany.

Schewski, M. and Macke, A., Correlation between domain averaged cloud properties, and solar radiative fluxes for three-dimensional inhomogeneous mixed phase clouds, Meteorologische Zeitschrift", 2003:12, 293-299