Liquid Water Path and Plane-Parallel Albedo Bias during ASTEX

Abstract

Inhomogeneous distributions of liquid water like those observed in real clouds generally reflect less solar radiation than idealized uniform distributions assumed in plane-parallel theory. Here the authors determine cloud reflectivity and the associated plane-parallel albedo bias from distributions of liquid water path derived from 28 days of microwave radiometer measurements obtained on Porto Santo Island in the Madeiras during June 1992 as part of the Atlantic Stratocumulus Transition Experiment (ASTEX). The distributions are determined for each hour of the day, both for composites of the full act of 28 days and for a subset of 8 days having a high fraction of relatively thick cloud. Both sets are compared with results obtained from California stratocumulus during FM [First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment]. In FIRE the albedo bin was dominated by variability of the cloud optical depth, as measured by a fractal parameter, 0≤ f 0 ≤ 1, while the ASTEX results are more complex. Mean cloud fraction above a 10 g m−1 threshold is about 50% in the 28-day set, compared to 76% in the 8-day subset and 82% in FIRE. Cloud fraction is sensitive to the threshold for the 28 ASTEX days, probably due to a large fraction of thin cloud below the threshold, but this is not the case for the 8-day subset or for FIRE. Clear fractions during ASTEX are generally of shorter duration than those in FIRE, as are those in the 8-day subset. The diurnal mean fractal parameter is about 0.6 in ASTEX compared to 0.5 in FIRE, while the 8-day subset has nearly the same mean but a wider range. The diurnal cycle in cloud albedo mid and albedo bias is computed from the cloud parameters for both sets, assuming zero clear-sky albedo. The total absolute albedo bias rises to values above 0.3 at sunrise and sunset, but since there is little incident energy at that time, the reflected flux is more affected by the midday bias. The total albedo bias has a 10OO LST maximum of about 0.3, largely due to a cloud fraction contribution of 0.2, absent in FIRE because in that case cloud frontier remains near 100% until after 1000 LST. The albedo bias has a second maximum of about 0.2 at noon, again mainly from cloud fraction and then drops to a minimum of about 0.1 at 1400 LST, when cloud fraction and fractal structure contribute about equally. Finally, a third maximum due to cloud fraction occurs at 1600 LST. In the, 8-day subset the 1000 LST maximum becomes dominated by the frontal structure, since the cloud fraction remains near 100% until 1000 LST, as in FIRE. The noon maximum receives roughly equal contributions, while the 1400 LST minimum bias is mainly due to fractal structure. Finally, the 1600 LST maximum and the evening limb bias are similar to those of the full 28-day set. These results show lids cloud fractal and radiative properties can vary considerably from one site and time to another mid at different times within the same site, as meterological conditions change.