Observational Constraints on Cumulus Parameterizations

Abstract

Observational tests of cumulus parameterizations are difficult. Since cumulus convection is generally embedded in a larger-scale flow pattern in which many feedbacks are acting, it is hard to tell what is causing what to happen. For those aspects of convection tightly embedded in feedback loops, the change in convective behavior caused by environmental changes needs to be correctly represented in parameterizations. However, it is precisely these changes that are difficult to observe, since negative large-scale feedback often minimizes the range of naturally occurring variations. For instance, suppose the convective mass flux were very sensitive to environmental instability, such that a slight increase in instability resulted in a large increase in convective mass flux. This in turn would generate large amounts of convective heating, which in turn would quickly stabilize the environment, thus reducing the vertical mass flux. In normal circumstances, large deviations from neutral stability would rarely occur, and observations of convective behavior in highly unstable conditions would be difficult to obtain. However, it is this abnormal behavior that needs to be studied in order to understand the feedback mechanisms that prevent it from occurring more frequently.