MODIS Retrievals of Cloud Effective Radius Exhibit No Significant Bias

Satellites offer the only global-scale characterization of cloud properties such as cloud effective radius r_e and cloud optical depth. From these basic cloud properties, quantities such as cloud drop number concentration and liquid water path may be derived. Retrievals of r_e are frequently validated using aircraft in situ measurements. Past intercomparisons have found a substantial bias toward larger remotely sensed r_e. Explanations for this bias focused on retrieval algorithms and large-scale heterogeneity, with in situ measurement uncertainty regarded as a minor factor. We compare Moderate Resolution Imaging Spectroradiometer (MODIS) r_e with in situ observations of marine stratocumulus from three aircraft campaigns using a phase Doppler interferometer (PDI) probe. Retrieved and in situ r_e agree within uncertainty with no systematic bias (mean bias of -0.22 μm) regardless of meteorological conditions. The discrepancy between previous studies and this one is primarily due to the ability of the PDI to adequately sample drops in the diameter range 50 to 100 μm.