Cloud and Ocean Remote Sensing Around Africa (CORSA) from the NOAA AVHRR Radiometer (GAC), 1981-1992, from SAI/JRC

General information about NOAA-AVHRR can be queried by interested
users in the category 'Sensor' and 'Source'. Some basic information is
given hereafter.

The Advanced Very High Resolution Radiometer (AVHRR) onboard NOAA 6
and TIROS-N measured in four spectral bands, while the NOAA 7,
9 and 11 are measured in 5 bands. The primary objective of the
AVHRR instrument is to provide cloud top and sea surface temperatures
through passively measured visible, near infra-red and infra-red
spectral radiation bands. Nevertheless these data are widely used for
terrestrial applications, such as land cover mapping and vegetation
monitoring.

The available data set provides a comprehensive time series of Sea
Surface Temperature (SST) and different cloud parameters for the ocean
surrounding the African continent derived from daytime NOAA GAC
data. The total number of satellite passes is approximately 12000. The
time period covered by the data set is from August 1981 to December
1992 with the intention to extend up to present (1995) as data will be
purchased. Geographical coverage is from 45 S to 45 N and from 55 E to
30 W.

Initially, emphasis has been put on SST for studying coastal upwelling
processes in the Northwest African and Benguela upwelling systems in
continuation of the SAI/JRC on-going activity on coastal upwelling
research, Nykjaer and Van Camp (1994). In parallel, different studies
are carried out for establishing algorithms for cloud optical
properties and their validation. The elaboration of this data set is
carried out in the frame of the Cloud and Ocean Remote Sensing around
Africa project (CORSA).

The five channels of the AVHRR are calibrated to 'top of atmosphere'
reflectances for the channels 1 and 2 and brightness temperatures for
channels 3, 4 and 5 following the recommendations of Kidwell (1991).
Clouds over the ocean are identified using a modified Saunders and
Kriebel (1988) approach. For cloud free areas SST is calculated using
a classical 'split-window' algorithm from Castagne et al. (1986).

After SST the individual images are resampled into weekly and monthly
composites maintaining the original 4 km resolution. Validation of the
SST fields are done by comparing to lower resolution data sets such as
Comprehensive Ocean- Atmosphere Data Set (COADS), National
Meteorological Center SST Data (IGOSS) and Global Ocean Surface
Temperature Atlas (GOSTA).

To accomodate the need for the reprocessing the data as algorithms
evolve, all raw data have been written to an on-line optical file
server system. In the processing, intermediate products are not stored
but only the weekly and monthly composites are retained. This reduces
the disk storage requirements for the user without sacrificing
computational speed. Weekly and monthly composites of SST for one
month, ca. 100 satellite passes, are generated in approximately 5
hours on a SUN Sparc 10 workstation.

The weekly and monthly composites are stored on the on-line optical
server and made available through collaborative agreements with the
Joint Research Centre.

Example SST time series can be found on the CEO World Wide Web home
page: "http://www.ceo.org/".