Microphysical observations of warm cumulus clouds in Cearل, Brazil

Microphysical properties of shallow, warm cumulus clouds, such as droplet concentration, shape of the spectra, etc., may vary due to several factors, from the large-scale environment to microphysical processes on very small scales. Microphysical characteristics of clouds present a significant variability due to different CCN sources. For instance, it is well known that there are crucial differences between maritime and continental clouds regarding their microstructure. In this paper, we analyze microphysical data obtained inside shallow cumuli with an instrumented aircraft in Ceará State, Northeast Brazil, during a field campaign carried out during the first half of 1994. A brief description of the field campaign is presented and a cloud classification is established. Significant differences regarding droplet concentration and spectrum shape were observed among four different cloud regimes: maritime, coastal, continental and “urban” clouds. Different functions were examined (exponential, gamma, lognormal and Weibull) in order to determine how appropriate are bulk parameterizations of droplet spectra in the representation of the microphysical properties of shallow cumulus clouds. The exponential distribution was shown to be unsuitable for most of the observed spectra. The gamma and lognormal distributions were better with the Weibull distribution providing the best fit. However, a significant variability of the width (or shape) parameter was verified for the three distributions, regarding different cloud regimes (maritime, coastal, continental and urban), from cloud to cloud, and in association with different regions of a cloud. Such variability imposes important limitations to bulk-microphysical modeling using distributions with prescribed width/shape parameters. A brief discussion is presented on how physical processes in a cloud can alter the shape of the droplet spectra, focusing on idealized distributions.