MODIS Retrievals of Cloud Effective Radius in Marine Stratocumulus Exhibit No Significant Bias

Abstract

Satellite retrievals of cloud effective radius re are frequently validated using aircraft in situ measurements. Past intercomparisons have found a significant bias toward larger remotely sensed re. Explanations for this bias have focused on retrieval algorithms and large-scale heterogeneity, with in situ measurement uncertainty regarded as a minor factor. We compare Moderate Resolution Imaging Spectroradiometer re with in situ observations of marine stratocumulus clouds from three aircraft campaigns using a phase Doppler interferometer probe. Retrieved and in situ re typically agree within uncertainty in both nonprecipitating and drizzling conditions with no apparent systematic bias (mean bias of −0.22 μm, mean relative bias 3%). Agreement depends on the choice of in situ probe as well as microphysical context. We demonstrate that probes must adequately characterize the width of the drop size distribution to avoid systematic underestimation of re.