On the Nature of Optically-Thin Low Clouds over Land and Ocean
Leahy, Louise V.
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University of Washington Abstract On the Nature of Optically-Thin Low Clouds over Land and Ocean Louise V. Leahy Chair of the Supervisory Committee: Professor Robert Wood Atmospheric Sciences This work describes macrophysical properties of optically-thin low clouds over non-polar land and oceans (60S-60N) measured using two years of full-resolution nighttime data from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). Optically-thin clouds are herein defined as the subset of low clouds that do not fully attenuate the lidar signal. Over both land and marine domains, approximately half of the low clouds detected at full resolution are found to be optically thin. Regionally, the fraction of marine low clouds that are optically thin (<italic>f</italic><sub>thin,cld</sub>) exhibits a strong inverse relationship with the mean low-cloud cover (<italic>f</italic><sub>cld</sub>), with maxima in the tropical trades ((<italic>f</italic><sub>thin,cld</sub>)> 0.8) and minima in regions of persistent marine stratocumulus ((<italic>f</italic><sub>thin,cld</sub>) < 0.3). Such a relationship is not evident over land at the high end of the <italic>f</italic><sub>cld</sub> range. An <italic>f</italic><sub>thin,cld</sub> maximum at the low end of the land <italic>f</italic><sub>cld</sub> range, as seen in the marine data, is observed, but the tight relationship with <italic>f</italic><sub>cld</sub> is not. Results indicate increased occurrences of horizontally extensive optically-thin low cloud over land, relative to ocean regions. On average, both land and ocean data indicate that low clouds are optically thicker during daytime, however the opposite behavior is observed for a land cumulus region analyzed. Domain-mean fcld is largest during daytime (nighttime) for land (ocean) regions. Over the marine and land domains the cloud length distribution of low clouds is described by a power law fit with exponents β = 2.03±0.06, and 2.21±0.13, respectively (± 95% confidence interval), over the size range 0.335 km to ~550 km. Geographical variations in the marine cloud length distribution explains three-quarters of the variance in marine <italic>f</italic><sub>thin,cld</sub>, which implies that optically-thin low cloud properties are strongly connected with the properties of all low clouds. Comparison of CALIOP observed trade wind cumulus lengths with airborne High Spectral Resolution Lidar (HSRL), which has a higher spatial resolution than CALIOP, reveals that clouds with sizes smaller than are resolvable by CALIOP contribute approximately half of the marine low clouds in the region sampled. We discuss the implications of this for future spaceborne lidar remote sensing. A bounded cascade model is used to show that the observed marine optically-thin low cloud behavior is consistent with a power law scaling of cloud optical depth, and that most marine optically-thin trade wind cumulus clouds only partially fill the CALIOP footprint. This result highlights the difficulty in accurate detection of cloud cover, especially in regions of broken cloud.
- Atmospheric sciences