Complex Layered Clouds Experiment (CLEX): Overview and recent results

Dr. Larry Carey,

Cooperative Institute for Research of the Atmosphere

Dept. of Atmospheric Sciences, Colorado State University

Mid-level, non-precipitating clouds are the forgotten clouds of the atmospheric science community. They are often overlooked in favor of modeling and observational studies of boundary layer stratus or cirrus clouds because of their known radiative forcing properties and potential climate effects. Altocumulus clouds suffer research neglect because they do not produce severe weather and rarely produce precipitation. As a result, there are only a handful of well-documented observational and modeling studies of these clouds.

Despite this lack of recent scientific attention, the microphysical, dynamical, and radiative properties of mid-level clouds are poorly understood. They typically reside at temperatures partially or wholly within the 0° to -30° C isotherms, often giving rise to a complex mixture of liquid and ice cloud particles. The precise spatial and temporal distribution of liquid and ice can have important implications for cloud processes and yet remain largely unknown. For example, details of ice crystal habit affect particle growth, density, terminal fall velocity, remote sensing of optical depth, and radiative forcing. Although the potential physical and dynamical forces responsible for the genesis, maintenance, and decay of these clouds are generally understood (e.g., subsidence/ascent, entrainment, sedimentation, radiative effects), there is no precise knowledge of their relative role. Because of this lack of detailed physical knowledge, the modeling and remote detection of mixed-phase, mid-level clouds is problematic. Forecasting skill for these clouds is poor.

This lack of forecasting and detection skill was clearly demonstrated during Operations Desert Shield and Desert Storm and again during the recent Balkan conflicts, when complex layered mid-level clouds routinely covered target areas, were poorly forecasted, and often hampered air missions. Mid-level clouds can also impact civilian aviation by restricting visibility and causing potential icing and turbulence hazards. Last but not least, these clouds are potentially important in the earth’s radiation budget. In order to make any progress on these issues, an improved understanding of their physical morphology and evolution is required.

The Complex Layered Cloud Experiment (CLEX) represents a multi-year investment by the DoD-funded Center for Geosciences at CSU-CIRA that was initiated to achieve this goal. I will present recent results by CIRA scientists from a field experiment involving in-situ microphysical measurements by the UND Citation II during Fall 1999 and Spring 2000. I will conclude by briefly outlining our future research plans, in particular our planned collaboration with the University of Wyoming King Air and Cloud Radar in the study of these clouds.