| Popis: |
Near coincident data from satellite and aircraft overpasses of stratocumulus in the South Atlantic have been analyzed. The satellite data are from the Along Track Scanning Radiometer-2 (ATSR-2) onboard the European Remote Sensing satellite-2 (ERS- 2) and are measurements of reflectance at wavelengths of 0.87 micrometer and 1.6 micrometer. From these, retrieval algorithms estimate the cloud optical depth and effective drop size using well established principles. The aircraft data consist of reflectance measurements made using the Scanning Airborne Filter Radiometer (SAFIRE) from a transit made just above the cloud, and liquid water content (LWC) and effective drop size measurement made using an FSSP probe from a transit made approximately 100 m below cloud top. The aircraft flights were made as part of the South Atlantic Tropical Experiment-2 which took place off the Namibian coast in October 1995 and which was designed to coincide with ERS-2 overpasses where possible. One particular flight, A423, has been studied here as offering the best chance of intercomparison of the respective systems measurements. Two flight legs have been analyzed, one for validation of reflectance measurements and one for validation of cloud effective radius. Poor correlation in initial comparisons based on the geolocation of the two instruments was improved greatly by allowing for, and estimating, the advection of the cloud deck by the local wind. Other adjustments included compensating a small error in the ATSR geolocation and allowing for differences in the respective instrument view angles. Following these adjustments, good agreement is shown for the 11 micrometer brightness temperatures and for the 0.87 micrometer reflectances. Large biases in the 1.6 micrometer reflectances confirm calibration errors that were already suspected for both instruments. Using the same wind advection the cloud effective radius retrievals were compared for the previous flight leg. Agreement is shown to be within 0.5 micrometer for measurements within 5 minutes of the exact collocation time. This is a remarkable result considering the sensitivity of the ATSR retrievals to 1.6 micrometer calibration errors. |
