Uncertainties in Defining Cloud Fraction in Stratocumulus Clouds -- A Study Using Aircraft Measurements

Low level marine stratocumulus clouds play an important role in global radiation budget and regulate the air-sea interaction and thermodynamic balance in the cloud-topped boundary layer. Predicting the evolution of the stratocumulus cloud is hence crucial to a full understanding of the Earth's climate system, which calls for adequate representation of the physical processes on various scales in these clouds. Statistical cloud parameterization is one approach that have been used by many researchers in the past years, in which Large Eddy Simulations (LES) played an essential role in generating/modifying the various probability distribution functions and the corresponding moments. In the effort of evaluating the various cloud parameterizations, a key question is how one should obtain the corresponding variables from observations. One such important variable in statistical cloud parameterization is cloud fraction.

This study uses observations from two major field projects on stratocumulus-topped boundary layers to identify issues in defining cloud fraction of stratocumulus clouds. One data source is from the Atlantic Stratocumulus Transition Experiment (ASTEX) conducted over the subtropical Atlantic Ocean in 1992; the other is the Second Dynamics and Chemistry of Marine Stratocumulus experiment (DYCOMS-II) made off the coast of southern California in 2001. The ASTEX experiment represents cases with deeper boundary layers and much variable cloud fraction, while the boundary layers from DYCOMS-II have much larger cloud fraction. Several methods are used and compared in deriving cloud fraction involving both level legs and slant-path soundings with measurements of radiometric temperature as well as cloud liquid water. The sources and magnitude of uncertainties will be examined to consider sampling representativeness and calculation methods.