Monitoring of Radiometric Sensitivity Changes of Space Sensors Using Deep Convective Clouds: Operational Application to PARASOL

Deep convective clouds have been tested to be used as stable reference for calibration purposes: the monitoring of the radiometric changes of space sensors in the spectral range from blue to short-wave infrared. After an appropriate selection, the clouds have been characterized for their brightness, spectral aspects, bidirectional signature, stability, and homogeneity. For this, radiative transfer computations using a discrete ordinate code, as well as remote sensing measurements from the PARASOL satellite, were analyzed. The first main result is a confirmation that the monthly mean reflectance over deep convective clouds is quite stable as suggested in other papers. Moreover, the excellent spectral properties of deep convective clouds are really convenient for a temporal monitoring if it can be assumed that a reference band is stable or well characterized with time. If the reference band is perfectly known, the accuracy of the temporal monitoring is about 0.2%. Experimental results are provided with PARASOL data for which the temporal drift is known with an accuracy better than 0.5% for the three years in orbit (accuracy which includes the uncertainty of the reference band).