A Color Ratio Method for Simultaneous Retrieval of Aerosol and Cloud Optical Thickness of Above-Cloud Absorbing Aerosols From Passive Sensors: Application to MODIS Measurements

The presence of absorbing aerosols above cloud decks reduces the amount of upwelling ultraviolet (UV), visible (VIS), and shortwave infrared radiation reaching the top of atmosphere. This effect is often referred to as “cloud darkening,” which can be seen by eye in images and quantitatively in the spectral reflectance measurements made by passive sensors such as the Moderate Resolution Imaging Spectroradiometer (MODIS) in the regions where light-absorbing carbonaceous and dust aerosols overlay low-level clouds. Radiative transfer simulations support the observational evidence and further reveal that the reduction in the spectral reflectance and color ratio between a pair of wavelengths is a function of both aerosol and cloud optical thickness (AOT and COT). For a prescribed set of aerosol and cloud properties and their vertical profiles, thus, the measured reflectance can be associated with a pair of AOT and COT. Based on these results, a retrieval technique has been developed, which is named as the “color ratio method,” which utilizes the measurements at a shorter (470 nm) and a longer (860 nm) wavelength for the simultaneous derivation of AOT and COT. The retrieval technique has been applied to the MODIS 1-km reflectance measurements for the two distinct above-cloud smoke and dust aerosols events. This study is an extension of the previously developed near-UV method to the VIS spectral region. However, it constitutes the first attempt to use non-UV wavelengths to retrieve above-cloud AOT by a passive nonpolarized sensor. An uncertainty analysis has been presented, which estimates the expected error associated with these retrievals.