Calibration of the AVHRR shortwave channels .2. Application to NOAA II during early 1991

This article extends the calibration method of Mitchell, O'Brien, and Forgan (1992) to the calibration of the AVHRR on NOAA 11 using a data set obtained over the period January to May 1991. The method correlates AVHRR response against radiances computed from a radiative transfer model whose parameters are constrained by surface measurements. The data consist of cloud free AVHRR images over the ocean west of Tasmania and simultaneous sun photometer measurements of aerosol optical depth at Cape Grim, some 50 km from the ocean target. Ancilliary surface meteorological data are also employed. The best estimate of the mean gain in Channel 1 over the first half of 1991 is 1.75 +/- 0.09 counts per W m(-2) mu m(-1) sr(-1), in good agreement with several other determinations using a variety of techniques. However, differences among results from the various methods suggests that ultimate accuracy in the determination of sensor gain is limited to similar to +/- 5%. Spurious results are obtained when the aerosol depth measured at Cape Grim does not characterize the airmass over the ocean target. This situation can arise when winds are light and variable in direction during the observing period. Under these circumstances it is not valid to use the sun photometer aerosol optical depths to model the upwelling radiance over the ocean. In addition, uncertainty In wind speed over the target site introduces significant error into the method. The technique is found to be unreliable in Channel 2 due to problems connected with water vapour absorption.