Sensitivity of cloud and radiation parameterizations to changes in vertical resolution

The importance of vertical resolution to the parameterization of cloud-radi ation processes in climate models is examined. Using a one-dimensional sing le-column model containing a typical suite of physical parameterizations, t he authors test 12 different vertical resolutions, ranging from 16 to 60 la yers. The model products are evaluated against observational data taken dur ing three intensive observation periods from the Atmospheric Radiation Meas urement Program. The simulated values of cloud-radiation variables display a marked sensitivity to changes in vertical resolution. This sensitivity is apparent in all the model variables examined. The cloud fraction varies ty pically by approximately 10% over the range of resolutions tested, a substa ntial amount when compared to the typical observed values of about 50%. The outgoing longwave radiation typically changes by approximately 10-20 W m(- 2) as resolution is varied, which is of the order of 5%-10% of the observed value. The downwelling shortwave radiation change is somewhat smaller but is still significant. Furthermore, the model results have not converged eve n at a resolution of 60 layers, and there are systematic differences betwee n model results and observations.