Comparing different cloud schemes of a single column model by using mesoscale forcing and nudging technique

Different cloud schemes are compared using the single column model (SCM) ve rsion of the general circulation model of the Canadian Centre for Climate M odelling and Analysis. Emphasis is placed on the differences between a stat istical cloud scheme and an explicit one, two approaches commonly used in G CMs. The micro-physical processes are identical in both schemes so that the differences can be attributed to cloud formation and dissipation only. Two case studies are chosen, one for a day during the European Cloud and Radia tion Experiment (EUCREX) and one for a day during the North Atlantic Region al Experiment (NARE). During the EUCREX case study the SCM is forced by adv ection from the mesoscale model GESIMA (Geesthacht Simulation Model of the Atmosphere). The comparison of ice water content as a function of height sh ows that the SCM cannot reproduce the observed nearly linear decrease with height as well as GESIMA does above 8.5 km. If temperature, specific humidi ty: and cloud ice advection are used to force the SCM, the explicit scheme simulates a coherent thick cirrus cloud, which is in better agreement with observations than the separate cloud layers simulated with the statistical scheme. Sensitivity studies show that cloud ice advection is crucial for th e formation of the cirrus deck in this case study, but omitting specific hu midity advection improves the agreement with observations. During the NARE case study four sequential vertical profiles are available so that wind, te mperature, and moisture of the SCM can be nudged toward their observed valu es. The observed lifting of the boundary layer cloud with time is captured best by the statistical scheme when adjusted toward observations with a rel axation timescale of one hour or less.