The characteristics of cloud fields associated with midlatitude circulation systems in a general circulation model

Cloud fields and concurrent dynamical structures associated with extratropical cyclones in a general circulation model are investigated with emphasis on the cool season (October-March). An ensemble approach is used for constructing the respective anomaly patterns in the storm track region of the northwestern Atlantic. The simulated composite patterns are in many respects similar to those derived from satellite cloud data and operational weather analyses. The results also agree reasonably well with traditional conceptual frameworks for the organization of clouds in the vicinity of warm and cold frontal zones. The vertical structure and wave characteristics of various dynamical, thermal, and hydrological fields reveal the typical patterns found in observed baroclinic wave disturbances such as a westward tilt with height of pressure anomalies and an eastward displacement of the uppertropospheric clouds relative to the center of largest upward motion at 500 hPa. However, this displacement is not as distinct as in the observations. Similar to indications from surface weather reports, the largest precipitation anomalies are simulated westward of the reference site defined by the largest anomaly in cloud water path and cloud optical thickness, respectively. The changes in cloud cover, cloud water, humidity, and temperature during the passage of a synoptic-scale weather disturbance have a distinct impact on the energy budget with a pronounced seasonality in all components except for the longwave radiation. Due to changes in the fluxes of sensible heat, latent heat, and shortwave and longwave radiation, the atmospheric column in most parts of the cyclonic systems is cooled in winter and heated in summer. On the other hand, extratropical cyclones cause anomalous surface heating in winter and cooling in summer. The total effect on the earth-atmosphere system, as identified by the changes in the top-of-atmosphere radiation balance, is a warming during winter and a cooling during summer.