Marine and Continental Stratocumulus Cloud Microphysical Properties Obtained from Routine ARM Cimel Sunphotometer Observations

Abstract. This study investigates marine and continental stratocumulus (Sc) cloud properties obtained from an automated implementation of a multispectral photometer retrieval. Photometer methods simultaneously retrieve cloud optical depth (𝜏) and cloud droplet effective radius (r_e), with estimates for liquid water path (LWP) calculated on the availability of those quantities. These efforts evaluate retrieved cloud properties for Sc identified during a recent 6-year period collected over the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) program sites in Oklahoma, USA (SGP) and in the Azores, Portugal (ENA).

Modest agreement in key quantity retrievals is found between the routine photometer products and multisensor collocated profiling references. Cumulative breakdowns contingent on cloud thickness indicate increases in all retrieved quantities in thicker clouds, with larger discrepancies in the relative performance between the retrievals collected in the presence of drizzle. Under continental cloud conditions, the clouds of a similar thickness and r_e to those sampled under marine conditions report a factor of 1.5 larger 𝜏 and LWP. An R² ≅ 0.65 is found between photometer 𝜏 retrievals and shadowband radiometer measurements, with photometer retrievals reporting a high (relative) bias. The 𝜏 intercomparisons indicate that variability between retrievals is a factor of three larger than errors reported from individual retrieval input perturbation tests. Photometer r_e retrievals suggest a low R² (< 0.1) having a standard deviation ≅ 3 μm when compared to ARM baseline multi-sensor radar/radiometer references (accounting for offsets in the cloud droplet number concentration assumptions of the latter). However, photometer LWP calculations remain relatively unbiased in non-drizzling conditions, with errors O[50 g m^-2] and R² ≅ 0.5 to collocated radiometer and interferometer references. Additional sensitivity tests for island influences on ENA marine Sc properties suggest that while oceanic versus island-influenced winds may promote significant shifts in quantities, this influence is lower than retrieval method uncertainty and/or collocated instrument variability.