Comparing Different Cloud Schemes of a Single Column Model by Using Mesoscale Forcing and Nudging Technique

Abstract

Different cloud schemes are compared using the single column model (SCM) version of the general circulation model of the Canadian Centre for Climate Modelling and Analysis. Emphasis is placed on the differences between a statistical cloud scheme and an explicit one, two approaches commonly used in GCMs. The microphysical 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 Radiation Experiment (EUCREX) and one for a day during the North Atlantic Regional Experiment (NARE). During the EUCREX case study the SCM is forced by advection from the mesoscale model GESIMA (Geesthacht Simulation Model of the Atmosphere). The comparison of ice water content as a function of height shows that the SCM cannot reproduce the observed nearly linear decrease with height as well as GESIMA does above 8.5 km. If temperature, specific humidity, 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 the formation of the cirrus deck in this case study, but omitting specific humidity advection improves the agreement with observations. During the NARE case study four sequential vertical profiles are available so that wind, temperature, and moisture of the SCM can be nudged toward their observed values. The observed lifting of the boundary layer cloud with time is captured best by the statistical scheme when adjusted toward observations with a relaxation timescale of one hour or less.