CALIBRATION OF GOES-VISSR, VISIBLE-BAND SATELLITE DATA AND ITS APPLICATION TO THE ANALYSIS OF A DUST STORM AT OWENS LAKE, CALIFORNIA

As part of a joint Russian/American dust-storm experiment, GOES-VISSR (Geostationary Operational Environmental Satellite, Visible-Infrared S pin-Scan Radiometer), data from a visible-band satellite image of a la rge dust storm emanating from Owens Lake, California were acquired on March 10 and 11, 1993. The satellite data were calibrated to targets o f known ground reflectance factors and processed with radiative transf er techniques to yield aerosol (dust) optical depth at those stages of the dust storm when concurrent ground-based measurements of optical d epth were made. Calibration of the satellite data is crucial for compa ring surficial changes in remotely sensed data acquired over a period of time from the same area and for determining accurate concentrations of atmospheric aerosols using radiative transfer techniques. The cali bration procedure forces the distribution of visible-band, DN (digital number) values, acquired on July 1, 1992, at 1731 GMT from the GOES-V ISSR sensor over a large test area, to match the distribution of visib le-band, DN values concurrently acquired from a Landsat MSS (Multispec tral Scanner) sensor over the same test area; the Landsat MSS DN value s were directly associated with reflectance factors measured from grou nd targets. The calibrated GOES-VISSR data for July 1, 1992, were then used to calibrate other GOES-VISSR data acquired on March 10 and 11, 1993, during the dust storm, Uncertainties in location of ground targe ts, bi-directional reflectance and atmospheric attenuation contribute an error of approximately +/-0.02 in the satellite-inferred ground ref lectance factors. On March 11 at 1031 PST the satellite-received radia nces during the peak of the storm were 3 times larger than predicted b y our radiative transfer model for a pure clay dust plume of infinite optical depth. This result supported ground-based measurements that th e plume at that time was composed primarily of large salt grains, prob ably sodium sulfate, which could not be properly characterized in our radiative transfer model. Further, the satellite data showed that the salt fell out of the plume within 35 km from the source. Finer-grained , clay dust was observed to extend beyond the salt-laden plume and was the major component of the dust plume after 1131 PST, when erosion of the salt crust on Owens Lake ceased. By 1331 and 1401 PST satellite-i nferred, optical depths compared favorably with measurements concurren tly acquired at the ground. Uncertainties in bi-directional reflectanc e, atmospheric attenuation, and locating ground points in the satellit e data manifest errors between the inferred and measured optical depth s in the range of 20 to 50%; these errors would be much greater withou t the calibration of the GOES-VISSR data. Changes in satellite-inferre d reflectance factors over the lake bed during the course of the storm showed that 76 km(2) of the surface was disrupted during the March 11 storm, suggesting as much as 76 x 10(3) m(3) of crustal material were displaced for each millimeter of several estimated to have been moved during the storm; an unknown fraction of the displaced material was s uspended. The satellite data also showed dust fallout on mountain snow fields. Whereas fallout may have removed most of the salt, satellite d ata acquired at 1631 PST, when the plume had a large brightness contra st with the ground, showed that it covered over 2500 km(2) and contain ed at least 1.6 x 10(9) g of sediment. For such a small source area, t he dust represents a substantial contribution to the regional and glob al load of aerosols.