Analysis of Solar Transmission Data from SABLE 89

This study analyzes solar transmissometer data that was taken during the South Atlantic Backscatter Lidar Experiment SABLE. The instrument operated on six days between 25 June and 7 July 1989. The instrument was located on Ascension Island, and it measured solar transmission at 532 nm versus time and transmission across the solar spectrum. Time series of solar transmission at 532 nm are used to study the properties of cirrus and boundary layer stratocumulus that passed in front of the sun while the instrument was working. For 25 June 1989, a qualitative assessment shows the transmissions through cirrus to be between 0.4 and 0.8, and the cirrus thicknesses are estimated to be between 1.0 and 2.2 km. These estimates are consistent with cirrus observations and from aircraft. The analysis also focuses on the properties of the stratocumulus cloud edges and thin spots. Stratocumulus are shown to have very similar edge characteristics from day-to-day there is a uniform distribution of cloud transmission values between about 0.1 and 0.7, and the percentages increase significantly as cloud transmissions increase from 0.7 to 0.9. The number of occurrences of stratocumulus thin spots and edges are then compared against the total number of stratocumulus occurrences. The comparisons clearly show that the stratocumulus over Ascension Island often are not optically thick and they frequently transmit partial direct solar radiation.