Wednesday, 17 January 2007
Cloud top temperatures of cirrus clouds retrieved from radiances in the NPOESS/VIIRS 8.55-Ám and 12.0-Ám Bandpasses
217D (Henry B. Gonzalez Convention Center)
This poster describes a new approach developed for the National Polar-orbiting Operational Environmental Satellite System (NPOESS) to retrieve pixel-level, cirrus cloud top temperatures (CTT) from radiances observed in the 8.55 Ám and 12.0 Ám bandpasses. The CTT is retrieved based on the numerical solution of two nonlinear algebraic equations derived from the theory of radiative transfer in these two bandpasses. The retrievals are further refined by using cloud optical thickness and effective particle size determined from parameterized correlations of the cloud emissivity in the 3.7 and 10.7 Ám bandpasses. This new approach has been demonstrated using MODIS as a proxy to VIIRS data. A number of proxy scenes have been analyzed covering a wide-range of geophysical conditions, including single and multi-layered cirrus cloud situations along with diverse backgrounds and seasons. For single-layer clouds, the new approach compares favorably with the MODIS 5-km resolution cloud products; mean CTT for both methods are close, while the standard deviation for the new approach is smaller. However, in multi-layered cloud situations, mean CTT's for the new approach appear to be slightly colder than CTT's from MODIS cloud products, which are acknowledged to be too warm. Furthermore, partly because the VIIRS approach is applied at the pixel level, cloud top temperatures do not increase toward cloud edges as are commonly seen in the MODIS products. Based on these encouraging results, this new approach to retrieve cloud top temperature and optical properties has been incorporated into the ground-based data processing segment for the NPOESS system. Pre-launch testing of this new VIIRS algorithm is continuing and the performance of this algorithm will be established with synthetic data to be generated with a radiative transfer model.