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Quantitative Assessment of the Sampling Properties of a Spaceborne Lidar (ATLID)

Jörg AckermannMeteorologisches Institut der Universität Muünchen, Munich, Germany

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Abstract

The European Space Agency plans to install the backscatter lidar system ATLID (atmospheric lidar) on a polar-orbiting platform at the beginning of the next century. This kind of active remote sensing will provide highly accurate information about cloud-top height, which, in addition to collocated passive sounder's measurements of brightness temperature, might improve retrieved vertical temperature profiles and serve as a supplementation of present cloud climatologies. Due to technical constraints, ATLID will not provide spatially continuous information about cloud-top height. The representativeness of the lidar measurements for the whole cloud field constitutes the sampling problem and is investigated in two steps: first, a scan mode for ATLID is developed, which on the assumption that the cloud field is a two-dimensional random variable gives an equal pixel spacing along and across the flight track of the orbiter. Second, the simulated lidar measurements given by the elaborated scan mode are contributed to a spatially continuous cloud field represented by Advanced Very High Resolution Radiometer images. From the dispersed lidar measurements with a footprint diameter of about 1 km the cloud field is restored by a spatial interpolation scheme and compared with the original cloud field by a linear regression analysis. It turns out that the sampling error and hence the benefits of ATLID strongly depend on the meteorological situation: if the required vertical accuracy of the lidar measurement is about 250 m corresponding approximately to half of the vertical resolution of present retrieval schemes, the probability for a meaningful ATLID information is between 40% and 70%. Since an imager cannot provide a useful brightness temperature in case of multilayered or broken clouds within one imager pixel, the synergism of ATLID with a passive instrument also depends on the homogeneity of cloud-top height within the range of 1 km. To cheek this small-scale variability of cloud tops data from the European Lidar Airborne Campaign 1990 are evaluated. Results show that for optically thick clouds the variability exceeds in 3% to 38% of all considered cases a threshold of 250 m. Additionally, power-spectrum analyses confirm the result of the sampling analyses.

Abstract

The European Space Agency plans to install the backscatter lidar system ATLID (atmospheric lidar) on a polar-orbiting platform at the beginning of the next century. This kind of active remote sensing will provide highly accurate information about cloud-top height, which, in addition to collocated passive sounder's measurements of brightness temperature, might improve retrieved vertical temperature profiles and serve as a supplementation of present cloud climatologies. Due to technical constraints, ATLID will not provide spatially continuous information about cloud-top height. The representativeness of the lidar measurements for the whole cloud field constitutes the sampling problem and is investigated in two steps: first, a scan mode for ATLID is developed, which on the assumption that the cloud field is a two-dimensional random variable gives an equal pixel spacing along and across the flight track of the orbiter. Second, the simulated lidar measurements given by the elaborated scan mode are contributed to a spatially continuous cloud field represented by Advanced Very High Resolution Radiometer images. From the dispersed lidar measurements with a footprint diameter of about 1 km the cloud field is restored by a spatial interpolation scheme and compared with the original cloud field by a linear regression analysis. It turns out that the sampling error and hence the benefits of ATLID strongly depend on the meteorological situation: if the required vertical accuracy of the lidar measurement is about 250 m corresponding approximately to half of the vertical resolution of present retrieval schemes, the probability for a meaningful ATLID information is between 40% and 70%. Since an imager cannot provide a useful brightness temperature in case of multilayered or broken clouds within one imager pixel, the synergism of ATLID with a passive instrument also depends on the homogeneity of cloud-top height within the range of 1 km. To cheek this small-scale variability of cloud tops data from the European Lidar Airborne Campaign 1990 are evaluated. Results show that for optically thick clouds the variability exceeds in 3% to 38% of all considered cases a threshold of 250 m. Additionally, power-spectrum analyses confirm the result of the sampling analyses.

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