Sensitivity of a regional atmospheric model to a sea state-dependent roughness and the need for ensemble calculations

The sensitivity of an atmospheric high-resolution limited area model to a s ea state-dependent roughness is examined. Two sets of Monte Carlo experimen ts are compared. In the first set the sea state was explicitly accounted fo r in the computation of the sea surface roughness. In the second set the ro ughness was parameterized by the standard Charnock relation. On climatic ti mescales of months and longer, the differences between the two sets are sma ll. On the daily timescale large deviations between individual realizations of the two ensembles in the order of several hectopascals are occasionally found suggesting a considerable impact of the sea state-dependent roughnes s on the atmospheric circulation. It is shown, however, that the comparison of individual realizations, a frequently used approach in regional sensiti vity studies, can be misleading. It is found here that the largest differen ces between the two ensembles occurred simultaneously with high inherent mo del variability. In these situations an eventually existing impact of the s ea state-dependent roughness on the atmospheric circulation could therefore not be discriminated from the background variability and the null hypothes is that both ensembles stem from the same population could not be rejected at given risk. At times at which the internal model variability was small a statistically significant impact of the sea state-dependent roughness on t he atmospheric circulation was found. However, the impact was small and it is concluded that compared with the sea state-dependent parameterization us ed in this study the Charnock relation represents a reasonable parameteriza tion in regional atmospheric climate models.