Attribution of Observed Recent Decrease in Low Clouds Over the Northeastern Pacific to Cloud‐Controlling Factors

Abstract

Marine low clouds cool the Earth's climate, with their coverage (LCC) being controlled by their environment. Here, an observed significant decrease of LCC in the northeastern Pacific over the past two decades is linked quantitatively to changes in cloud‐controlling factors. In a comparison of different statistical and machine learning methods, a decrease in the inversion strength and near‐surface winds, and an increase in sea surface temperatures (SSTs) are unanimously shown to be the main causes of the LCC decrease. While the decreased inversion strength leads to more entrainment of dry free‐tropospheric air, the increasing SSTs are shown to lead to an increased vertical moisture gradient that enhances evaporation when entrainment takes place. While the LCC trend is likely driven by natural variability, the trend‐attribution framework developed here can be used with any method in future analyses. We find the choice of predictors is more important than the method.

Machine-Learning Based Analysis of Liquid Water Path Adjustments to Aerosol Perturbations in Marine Boundary Layer Clouds Using Satellite Observations

“…This effect seems to be more pronounced in situations with a lower amount of cloud droplets. Higher SST conditions are associated with increased surface fluxes, a deeper and less stable marine boundary layer, and an increased moisture difference between the marine boundary layer and the troposphere [68,69]. In these conditions, the evaporation-entrainment process may be facilitated [40], leading to a less positive N d -LWP relationship.…”

Earth's Observed Hemispheric Albedo Symmetry by Cloud Type: Climatology, Trends, and Tests of Cloud Adjustment Hypotheses

“…Spatial trends (Fig. S9b-c) clearly show Sc declines in the northeast Pacific (Andersen, Cermak, Zipfel, & Myers, 2022;Loeb, Thorsen, Norris, Wang, & Su, 2018;Stephens et al, 2022), but also show substantial trends of decreasing cloud fraction in the southeast Atlantic and increasing cloud fraction in the southeast Pacific and northeast Atlantic. Sc albedo has been decreasing in almost all regions, with particularly strong trends off the western coasts of North America and southern Africa (Fig.…”

Hidden Large Aerosol‐Driven Cloud Cover Effect Over High‐Latitude Ocean

“…Cao et al (2021) replaced CTRC with RH and showed that these primary cloud-controlling parameters explained up to 97% of the variability in Cf over the United States East Coast. In this study, we further introduce SST as a controlling parameter of cloud coverage due to its domination of Cf (Andersen et al, 2022).…”