An Investigation of the Failed Retrievals in the MODIS Cloud Product: Frequency and Potential Causes

Abstract

This study investigates the frequency of occurrence of the failed optical thickness and effective radius retrievals and their causes for Moderate Resolution Imaging Spectroradiometer (MODIS) onboard Aqua satellite together with Cloud Profiling Radar (CPR) observations on CloudSat satellite. In satellite-based passive remote sensing, the cloud optical thickness (τ) and droplet effective radius (re), which has a strong influence on cloud radiative effects, are derived simultaneously from reflectance measurements at two wavelengths, the so called bi-spectral method. Since biases in the τ and re of marine water clouds can lead errors in estimates of global radiative forcing, it is critical to identify the sources and magnitudes of errors in the retrieval. The look-up tables (LUTs) used in the bi-spectral method have upper and lower limits to τ and re. These limitations might be problematic for clouds having very large or very small radius droplets, and may be even worse for optically very thin clouds. Alternatively, retrievals near or outside the LUTs boundary might be indicative of retrieval artifacts.