Implications of MODIS/in situ microphysical comparisons for southeast Pacific stratocumulus

We investigated the ability of the Moderate Resolution Imaging Spectroradiometer (MODIS) retrievals to represent the microphysical properties of the Chile-Peru stratocumulus deck. We compared MODIS estimates at 10:30 LT (Terra satellite) and 13:30 LT (Aqua satellite) with aircraft observations collected during the Variability of the American Monsoon Systems' (VAMOS) Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx), in October-November 2008. The in situ drop size distributions were constructed from the Cloud Droplet or Gerber probes (drop diameter < 50 micron) and the two-dimensional cloud (2DC) probe. The latter is capable of measuring precipitation drops with diameter up to 640 micron. The MODIS cloud optical depth closely matched the in-situ value. In contrast, the standard 2.2 micron-derived MODIS re was found to exceed the in situ cloud-top effective radius, with offsets of up to 3 micron, increasing with droplet size. The offset is not easily explained by the adiabatic cloud vertical structure typically associated with stratocumulus clouds, nor by three-dimensional radiative transfer effects, and instead an explanation is sought that also accounts for the presence of drizzle. The MODIS re estimates had ramifications for secondarily-derived MODIS liquid water paths that were then found to exceed the in situ liquid water path values. MODIS re and optical depth were also combined to form a cloud droplet number concentration (Nd) estimate assuming an adiabatic vertical structure. We found that MODIS Nd agreed the best of the four MODIS variables with the aircraft observations, with an almost negligible offset and a high linear correlation (r = 0.9). In addition, a decrease in MODIS Nd between the morning and afternoon overpasses noted for selected cases was confirmed by the in-situ Nd values, and does not appear to be a retrieval artifact. Instead, the daytime decrease in satellite-derived cloud droplet number suggests the signature of droplet sedimentation or cloud-top entrainment.