Formation of fair-weather cumuli

CUMULI

Fair-weather cumuli are among the most common phenomena in the boundary layer. Exploring why and how these clouds form under different conditions can provide insight into the interaction between the atmosphere and the underlying surface. In a study addressing the formation of fairweather cumuli, we find that moisture entrainment at the top of the boundary layer and fluxes at the surface play important roles in the formation of these clouds. The effect of these processes on cloud formation can be determined by a combination of the surface Bowen ratio (between sensible and latent heat flux) and the ratio between the entrainment moisture flux and the surface moisture flux.

The stratification above the inversion and large-scale subsidence are the other keys to cloud formation. Weaker stability provides a greater potential for cloud formation. Generally, subsidence tends to reduce the chances of cloud formation, but the intensity of this reduction may vary depending on boundary layer conditions. For a moist boundary layer with a small surface Bowen ratio, subsidence may be a minor factor affecting cloud formation even though it can reduce the boundary layer depth substantially. However, subsidence may be a major obstacle to cloud formation when the surface Bowen ratio increases.

Over land the cloud formation process is complicated not only by changes of surface and ambient meteorological conditions, but also by other factors such as terrain, land use, and variations in source and abundance of cloud condensation nuclei. Thus, for continental fair-weather cumuli, cloud formation could be as important as other cloud-related issues.

This study systematically applies mixed-layer theory and uses observations from the Atmospheric Radiation Measurement (ARM) program. The analyses shed new light on the mixed-layer model and provide some useful avenues for future research on cloud formation processes. They also provide the framework for effectively using datasets like those collected by the ARM program to comprehensively study the factors that affect cloud formation and fractional cloudiness.-PING ZHU (MPO/RSMAS, UNIVERSITY OF MIAMI) AND BRUCE A. ALBRECHT. "A Theoretical and Observational Analysis on the Formation of FairWeather Cumuli, "appearing in the 15 June Journal of the Atmospheric Sciences.