Calibration of the visible and near-infrared channels of NOAA-11 and -14 AVHRRs by using reflections from molecular atmosphere and stratus cloud

Abstract

A new method to determine the calibration coefficients for visible and near-infrared channels of Advanced Very High Resolution Radiometer (AVHRR) aboard NOAA satellite is presented and applied to NOAA-11 and -14 spacecrafts. The method uses the reflections from clear-sky ocean and stratus clouds. The clear-sky data analysis gives a minimum estimate of the slope coefficient (albedo per count) for a target month by using radiative transfer theory for molecular atmosphere. Cloudy-sky pixels were precisely excluded from that analysis by using multi-spectral data of AVHRR. Neighbouring pixels of cloud were also excluded to avoid three-dimensional radiative effects such as cloud shadow. On the other hand, the optical thickness (at a visible wavelength) of summer stratus clouds was retrieved from nominally calibrated reflectance of respective visible and near-infrared channels. This analysis was performed to adjust the balance between the two-channels' calibration coefficients because if the two channels were correctly calibrated, the cloud optical thickness retrieved from the two channels must be the same. Finally, the calibration coefficients were determined using iteration.