Effect of vertical resolution on cloudy-sky radiation calculations: Tests with two schemes

The effect of vertical resolution on cloudy-sky radiation calculations is i nvestigated through idealized, single-column experiments and extensive test s with a high vertical resolution (100 layers) general circulation model (G CM)-generated data set. As examples of GCM-type radiation codes, the Europe an Centre for Medium-Range Weather Forecasts (ECMWF) and Deutscher Wetterdi enst (DWD) schemes are considered. The basic assumption in the tests is tha t vertical discretization distorts the profiles of temperature, absorbing g ases, liquid water, and ice only by removing the subgrid-scale details. On the whole, cloudy-sky radiation calculations appear significantly more sens itive to vertical resolution than clear-sky calculations, owing to the foll owing reasons: (1) Both in the longwave and in the shortwave, a critical fa ctor is how the assumed subgrid-scale horizontal distribution of cloud wate r changes with vertical resolution. This issue also depends on the cloud ov erlap assumptions, random overlap tending to lead to increasing cloud cover and cloud forcing with improving resolution, (2) Application of maximum-ra ndom overlap to effective cloud fractions in the ECMWF longwave scheme lead s, in particular, to severe underestimation of cloud forcing at the top of the atmosphere at high resolution. (3) The assumption (in the ECMWF scheme) that cloud layers emit upward (downward) at the exact layer top (bottom) t emperature tends to lead to overestimated longwave cloud forcing at coarse resolution; the same also occurs if (4) cloud top (bottom) heights are over estimated (underestimated) at coarse resolution. (5) Cloud optics parameter izations may be dependent on vertical resolution (this affects to some exte nt, the shortwave results of both schemes considered). On the positive side , both the longwave and shortwave results of the DWD scheme depend little o n vertical resolution if vertical discretization does riot distort the clou d properties.