An initial analysis of the pixel-level uncertainties in global MODIS cloud optical thickness and effective particle size retrievals

摘要

Operational Moderate Resolution Imaging Spectroradiometer (MODIS) retrievals of cloud optical thickness and effective particle radius employ well-known solar reflectance techniques using pre-calculated reflectance look-up tables. We develop a methodology for evaluating the quantitative uncertainty in simultaneous retrievals of cloud optical thickness and particle size for this type of algorithm and present example results. The technique uses retrieval sensitivity calculations derived from the reflectance look-up tables, coupled with estimates for the effect of various error terms on the uncertainty in inferring the reflectance at cloud-top. The error terms include the effects of the measurements, surface spectral albedos, and atmospheric corrections on both water and ice cloud retrievals. Results will deal exclusively with pixel-level uncertainties associated with plane-parallel clouds; real-world radiative departures from a plane-parallel model are an additional consideration. While we demonstrate the uncertainty technique with operational 1 km MODIS retrievals from the NASA Earth Observing System (EOS) Terra and Aqua satellite platforms, the technique is generally applicable to any reflectance-based satellite- or air-borne sensor retrieval using similar spectral channels.