The GEWEX Cloud System Study (GCSS, GEWEX is the Global Energy and Water Cycle Experiment) is a community activity aiming to promote development of improved cloud parameterizations for application in the large-scale atmospheric models used for climate research and numerical weather prediction. GCSS makes use of cloud-resolving models (CRMs); that is, "process" models with sufficient spatial and temporal resolution to represent individual cloud elements but spanning a wide range of space and time scales to enable statistical analysis of simulated cloud systems.

The GCSS Working Group on cirrus clouds has conducted an intercomparison of CRM simulations of idealized cirrus cases. These represent cloud formation in typical mid-latitude ("warm") and sub-tropical ("cold") atmospheres. Key aims of these experiments were (i) to determine what level of microphysical complexity is necessary to correctly represent the processes occurring in these clouds, (ii) to examine the interactions between microphysical processes, radiative heating and turbulence, and (iii) to examine the impact of key physical properties of the cirrus ice crystals, in particular their fallspeed, on the evolution of the simulated clouds. This paper examines the simulation results in further detail.

The onset of cloud is related to the type of ice nucleation process operating in the different models. A number of models use both bulk water and explicit size-resolved microphysics schemes with parametrized ice nucleus activity according to the expression given by Meyers et al. (1992), which is a function of the ice saturation ratio. These models produce cloud at the start of the simulation. By contrast, models that describe the homogeneous freezing of haze droplets require periods of between 40 and 90 minutes before initial cloud formation. This represents the period required for the imposed forcing (a fixed cooling rate to represent large-scale ascent) to achieve the critical saturation ratio.

The highest ice water path (IWP) is maintained, as expected, by a model that has zero fallspeed for cloud ice. In all other simulations, ice fallout depletes the IWP and evaporation of the falling ice allows cloudbase gradually to descend. Due to the imposed cooling, the cloudbase is expected to lower as air below the initial cloud layer is brought to saturation. Additional lowering of the cloudbase below this level represents depletion of the IWP by precipitating ice and evaporation. For "warm" simulations, several models have a cloudbase that is close to that expected from their residual IWP. Other models allow greater penetration of precipitating large ice particles into unsaturated air, and hence produce lower than expected cloudbases.

Simulations were also done using fixed fallspeeds of 0.2 and 0.6 ms^-1, values within the range of those generated by the models. These produce some convergence in the character of the simulated cloud fields. With the lower value, models tend to have an ice water content (IWC) profile peaked near the cloud top. This generates stronger cloud-top radiative cooling and more intense turbulence in the cloud layer. With the higher fallspeed, IWC peaks toward cloudbase and radiative heating is less able to generate turbulence.