Aircraft evaluation of MODIS cloud drop number concentration retrievals

Abstract

Cloud drop number concentration (Nd) can be retrieved through passive satellite observation. These retrievals are useful due to their wide spatial and temporal coverage. However, the accuracy of the retrieved values is not well understood. In this study, we seek to understand why the retrievals agree or disagree with in situ measurements by examining the various cloud properties that underlie the retrievals. To do so, we compare satellite Nd derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument with in situ aircraft measurements made using a phase Doppler interferometer on board three flight campaigns sampling marine stratocumulus clouds. Intercomparison of Nd values shows that the discrepancy between retrieved and in situ Nd can be ± 50 % or more. In the mean, there is evidence of an overestimation bias by MODIS retrievals, although the sample size is insufficient for statistical certainty. We find that MODIS Nd is best interpreted as representative of the mid-cloud region, as there is almost always a greater discrepancy from in situ values near the cloud top and cloud base. We also find evidence of cases where Nd is accurately retrieved but the effective radius is not, presumably due to offsetting errors in other retrieval parameters. Vertical profiles of the extinction coefficient β, liquid water content L, and effective radius re measured during sawtooth-pattern flight legs through the cloud top are also compared to implicit MODIS retrieval profiles. For the two cases with Nd agreement, all profiles match well. For the six cases with significant disagreement, there is no consistent underlying cause. The discrepancy originates from one of the following: (a) discrepancy in the re profile, (b) discrepancy in the β and L profiles, or (c) discrepancy in both.