Evaluating Operational Boundary Layer Cloud Microphysical Properties Obtained from Long-Term ARM Sunphotometer Datasets

The observations of boundary layer clouds, and stratocumulus cloud processes and properties in particular, is especially critical to advancing Earth system modeling. This is because these clouds exhibit extensive coverage and controls on boundary layer dynamics, as well as on the global radiative energy balance. Importantly, improving and interpreting new observational measures of these cloud conditions is key, as small changes in stratocumulus coverage, thickness and coupled droplet properties can impart significant net radiative changes.

For over two decades, the U.S. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) user facility has deployed sunphotometer instruments at its fixed and ARM Mobile Facility (AMF) deployments. Although originally designed to retrieve aerosol optical properties, these instruments were later advanced to implement a “cloud mode” strategy. This mode, performed during sequences where clouds completely block the sun, enables estimates of key cloud properties. As a narrow-beam instrument, the passive retrievals from these instruments are also potentially viable over a wide range of cloud conditions (e.g., broken to overcast cloud conditions). During the previous year, the DOE ARM program has completed an operational implementation for a Sunphotometer Cloud Mode Value-Added-Product (SPHOT, VAP) that estimates cloud optical depth (COD), cloud droplet effective radius (Re), and liquid water path (LWP) -- including associated uncertainties for these quantities. These new products adopt a three-channel (440, 870, and 1640 nm) retrieval algorithm, building on current Aerosol Robotic Network (AERONET) two-channel methods that were applicable only over vegetated surfaces. In this study, we present results from operational outputs for an extended dataset from the ARM archive, focused on overcast warm, low cloud conditions observed at the ARM Southern Great Plains (SGP) and Eastern North Atlantic (ENA) sites (>10 years and >5 year records, respectively). For evaluation purposes, we offer retrieval performance comparisons for the LWP, COD, and Re by evaluating these new single-sensor VAP quantities against similar ARM retrievals found in previous AERONET iterations, as well as the multi-sensor KAZR and MWR-based MICROBASE VAP and MFRSR and MWR-based MFRSRCLDOD VAP.