Characteristics of Optically-Thin Coastal Florida Cumuli from Surface-Based Lidar and Radiometer Measurements

The optical depths and sizes of optically-thin passive shallow cumuli representative of the trade-wind regime are characterized primarily by a micropulse lidar. A zenith-viewing narrow-field-of-view spectral radiometer provides a secondary assessment of the cloud optical depth only. The focus is on clouds that do not attenuate the lidar signal, meaning an explained variance of the molecular scattering fit exceeding 0.5. The measurements from a coastal south Florida location span a ten-week winter time period and are mostly of clouds located near the lifting condensation level. In the lidar data, the clouds are distinguished from aerosol by an instrument-specific particle backscatter coefficient threshold established through visual comparison to zenith-viewing time-lapse camera images. Sun photometers provide a continuous independent aerosol optical depth estimate applied within the lidar retrieval of cloud optical depth. The two independently-derived cloud optical depths correspond reasonably well. Micropulse lidar measurements indicate that clouds with optical depths less than one constitute 12% of the sampled dataset. The frequency of clouds increases exponentially with decreasing optical depth consistent with a cloud fraction that is fundamentally a function of the cloud optical depth threshold. The lidar-derived mode of the horizontal extent of such optically-thin cloudy regions is 110 m. To our knowledge, this study is the first to apply micropulse lidar and zenith radiometer data towards characterizing these ubiquitous, low, shallow, broken clouds, too small for robust characterization from space but whose existence is consistent with other recent surface-based and in-situ observations of optically-thin cloud.