The GEWEX Cloud System Study (GCSS, GEWEX is the Global Energy and Water Cycle Experiment) is an international community activity aiming to promote development of improved cloud parameterizations for application in the large-scale general circulation models (GCMs) used for climate research and for numerical weather prediction. GCSS makes use of cloud-system models (CSMs); 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 CSM simulations of idealized cirrus cases. These represent cloud formation in conditions typical of "warm" cirrus (midlatitude with cloud top at about -47°C) and "cold" cirrus (sub-tropical with cloud top at about -66°C). Key objectives of the project 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 describes the basic experiments and the overall results. It also examines the simulation results in further detail with regard to (ii). A companion paper addresses (iii) in more detail (Brown et al).

The 16 participating models include 3-dimensional large eddy simulation (LES) models, 2-dimensional cloud resolving models (CRMs), and single column model (SCM) versions GCMs. The model microphysical components are similarly varied, ranging from single-moment bulk (relative humidity) schemes to fully size-resolved (bin) treatments where ice crystal growth is explicitly calculated. Radiative processes are included in the physics package of each model.

Cloud formation occurs in an ice supersaturated layer, about 1 km in depth, with an ice pseudoadiabatic thermal stratification (neutral) in the upper half under nighttime conditions (only infrared radiative processes). Additional forcing via an imposed diabatic cooling representing a 3 cm s-1 uplift over a 4-hour time span followed by a 2-hour dissipation stage with no cooling. Variations of these baseline cases include simulations with no-radiation or stable-thermal-stratification in the cloud generating region. Significant inter-model differences were found. Results indicate the great importance of the ice crystal fallout in determining the overall cloud characteristics (Brown et al).

Model behavior was found to be quite variable and sometimes surprising. For example, some models generate vigorous turbulence while others are quiescent for a given test case. The shape of the vertical distribution of ice mass can also vary appreciably from model to model. Generalization of results based on simple criteria, such as bin versus bulk microphysics, proved more complex than had been anticipated. The reason for this complexity is largely based on the importance of interactions among radiation, microphysics and turbulence in cirrus. The ice water fallspeed plays a significant role in modulating these interactions. Examples will be given to illustrate model behavior in these respects.