Abstract
Uniform radiometric calibration of satellite sensors is vital to the scientists and researchers around the world that use the satellite retrieved properties to study the Earth’s climate. In order to consistently calibrate sensors that have varying degrees of onboard calibration over time, an invariant target approach is needed. Fortunately, all satellite sensors are able to view deep convective clouds (DCC), which are bright, nearly-isotropic solar diffusers that can be easily identified via a simple infrared (IR) brightness temperature threshold. Also, most sensors have a window channel, which are historically well-calibrated using onboard black-bodies. If the DCC reflectivity is predictable in space and time, then the DCC can be used as an absolute reference for past, present, and future operational sensors. This study focuses on analyzing and optimizing the DCC identification criteria in order to derive the most consistent DCC reflectance fields over the tropics, while maintaining the greatest regional temporal stability. The DCC reflectances were characterized using the well-calibrated Aqua-MODIS 0.65 micron channel radiances. The DCC calibration technique revealed that the Aqua-MODIS 0.65µm radiances are temporally stable to within 0.5%/decade based on 9-years of data over the entire tropics. Replacing the DCC bidirectional reflectance distribution function (BRDF) model with a Lambertian BRDF model increased the Aqua-MODIS temporally stability to 0.7%, verifying that the DCC reflectance were nearly isotropic. It is known that the seasonal and diurnal distribution of DCC varies geographically over tropics. However, the inter-annual variability of the distribution is small and very predictable. The DCC reflectance was found to be dependent on the IR threshold criteria, indicating that a comparable IR temperature threshold is necessary to transfer the absolute visible calibration across visible sensors. It was also found that the lowest DCC reflectances were found over the tropical western pacific but the brightest DCC were not always found exclusively over land. These differences can easily be accounted for when using DCC as an invariant Earth target, allowing for a uniform and consistent calibration target for visible sensors.
Characterization of Deep Convective Clouds as Absolute Calibration Targets for Visible Sensors
Uniform radiometric calibration of satellite sensors is vital to the scientists and researchers around the world that use the satellite retrieved properties to study the Earth’s climate. In order to consistently calibrate sensors that have varying degrees of onboard calibration over time, an invariant target approach is needed. Fortunately, all satellite sensors are able to view deep convective clouds (DCC), which are bright, nearly-isotropic solar diffusers that can be easily identified via a simple infrared (IR) brightness temperature threshold. Also, most sensors have a window channel, which are historically well-calibrated using onboard black-bodies. If the DCC reflectivity is predictable in space and time, then the DCC can be used as an absolute reference for past, present, and future operational sensors. This study focuses on analyzing and optimizing the DCC identification criteria in order to derive the most consistent DCC reflectance fields over the tropics, while maintaining the greatest regional temporal stability. The DCC reflectances were characterized using the well-calibrated Aqua-MODIS 0.65 micron channel radiances. The DCC calibration technique revealed that the Aqua-MODIS 0.65µm radiances are temporally stable to within 0.5%/decade based on 9-years of data over the entire tropics. Replacing the DCC bidirectional reflectance distribution function (BRDF) model with a Lambertian BRDF model increased the Aqua-MODIS temporally stability to 0.7%, verifying that the DCC reflectance were nearly isotropic. It is known that the seasonal and diurnal distribution of DCC varies geographically over tropics. However, the inter-annual variability of the distribution is small and very predictable. The DCC reflectance was found to be dependent on the IR threshold criteria, indicating that a comparable IR temperature threshold is necessary to transfer the absolute visible calibration across visible sensors. It was also found that the lowest DCC reflectances were found over the tropical western pacific but the brightest DCC were not always found exclusively over land. These differences can easily be accounted for when using DCC as an invariant Earth target, allowing for a uniform and consistent calibration target for visible sensors.