Motivation For An Avhrr Albedo Algorithm

The Commonwealth Scientific and Industrial Research Organisation (CSIRO) is Australia's primary public organisation for scientific research. The Atmospheric Research division of CSIRO has developed a general circulation model and regional climate models, and these models require as input the surface broadband albedo. The albedo fields used currently are those of the Simple Biosphere (SiB) model developed by Dorman and Sellers (1989), in which a single albedo value is assigned to each land cover class. The albedo for each class has a fixed seasonal cycle and no interannual variation, whereas large regions of Australia experience significant inter-annual rainfall variations that could be expected to produce significant inter-annual variations in the land cover and hence in the albedo. Also, while the SiB albedo of the desert class is assigned the value appropriate for African deserts, there is evidence that a different value would be more appropriate for Australian deserts, which cover much of the Australian continent. A multiyear time series of albedo maps derived from satellite observations would provide more realistic estimates of the mean and range of variation of albedo. The time series of albedo maps would be of even greater value if it was analysed in conjunction with datasets of those parameters that influence albedo, such as soil moisture and vegetation properties. This would help to refine model parameterisations of the dependence of albedo on those parameters.

The CSIRO has some twenty divisions, which conduct research for the benefit of Australia in diverse fields including the ocean, forestry, agriculture, inland water resources, biodiversity and mining exploration. Many divisions have a small complement of specialists in satellite-based earth observation. In 1996 the CSIRO Earth Observation Centre (EOC) was formed to coordinate remote sensing research across CSIRO. One of the EOC's tasks has been to standardise CSIRO's processing of AVHRR data. To this end, the EOC has established and funded teams to develop CSIRO best practice algorithms for navigation, calibration, and atmospheric and angular corrections, either by comparing methods already in use, or by conducting research where no method has been developed. Furthermore, these algorithms are being implemented in a new efficient and flexible software package called the Common AVHRR Processing System (CAPS), which is based on the Tcl/Tk environment. The completion of this effort is expected to see all Australian providers and processors of AVHRR data using the same software and the same algorithms.

The EOC has also embarked on taking the historical data from several AVHRR reception sites around Australia and merging them into a single archive with a uniform format. A feature of this effort is that acquisitions of a single pass from several stations will be stitched into a superpass, resulting in a reduction of data volume and allowing the correction of reception errors in cases of overlap. The application of the best practice algorithms to the stitched archive will enable the generation of uniformly processed, higher level, continental scale AVHRR datasets, extending back fifteen years for some parts of the country.

The availability of the stitched archive and good processing algorithms and software makes the generation of a multiyear time series of continental albedo maps from AVHRR feasible. Two steps in the processing need development: the allowance for changes in surface reflectance due to the large variations in the direction from which AVHRR views any particular site; and the conversion from albedo in the narrow AVHRR spectral bands to albedo in the solar broadband. In the next section an approach to deriving narrowband albedo and an angular correction from AVHRR is described. Narrowband to broadband conversion for AVHRR will not be touched on here but is discussed, for example, in Li and Leighton (1992) and references therein.

0 0

Post a comment