Insights into the low-level single-layer stratiform cloud optical depth feedback based on two decades of observations at the North Slope of Alaska

Abstract. A 21 year dataset of cloud optical depth (COD) over Utqiagvik, AK is retrieved from longwave (LW) Atmospheric Emitted Radiance Interferometer (AERI) observations. Cloud property changes with temperature, time, and other meteorological variables are investigated. We find that the COD of low-level, single-layer optically thin, stratiform, Arctic clouds at pressures greater than 680 hPa increases with warming for column averaged atmospheric temperatures between -16 °C and -4 °C, while the COD decreases with warming between -4 °C and 0 °C , with little change below -16 °C. This aligns with current literature. COD changes are driven by liquid water content (LWC) variations with temperature, with additional contributions from cloud physical thickness (CPT) changes and bulk cloud phase shifts. The COD of summer and winter clouds decreases with warming at all observed temperatures, while the COD of spring and autumn clouds increases with warming. We extend existing literature with a cloud controlling factor (CCF) analysis, finding that the atmospheric temperature at 850 hPa; the surface windspeed; and sea salt, sulfur dioxide and hydrophilic organic carbon concentrations are important controls on the COD. The length of the dataset allows us to perform novel trend investigation, with statistically significant positive trends found in the total (0.0824 year^-1) and liquid (0.0719 year^-1) CODs. Evidence is provided for a negative surface cloud feedback. Our results and associated dataset represent an avenue for the evaluation and improvement of model representations of Arctic cloud microphysical processes.