Cluster analysis of midlatitude oceanic cloud regimes – Part 2: Temperature sensitivity of cloud properties

Abstract. Clouds have a large impact on Earth's radiation budget by reflecting incoming solar radiation and trapping longwave radiation emitted from the surface. The present balance could change as the atmosphere warms from increasing anthropogenic greenhouse gases, thus altering the net radiation flux and mitigating or exacerbating the initial temperature increase. To ascertain the sign and magnitude of cloud-climate feedback, we must better understand the way in which clouds interact with their environment and how temperature modifies cloud and radiative properties. Since global climate models do not consistently and correctly simulate clouds, we undertake an observational analysis of how midlatitude oceanic clouds change with temperature when dynamical processes are held constant (i.e., partial derivative with respect to temperature). For each of the seven cloud regimes identified through k-means clustering of daily satellite data in the companion study, we examine the difference in cloud and radiative properties between warm and cold subsets. To avoid misinterpreting a cloud response to large-scale dynamical forcing as a cloud response to temperature, we require horizontal and vertical temperature advection in the warm and cold subsets to have near-median values in three layers of the troposphere. Across all of the seven clusters, we find that cloud fraction is smaller and cloud optical thickness is mostly larger for the warm subset. Cloud top pressure is higher for the three low-level cloud regimes and lower for the cirrus regime. The net upwelling radiation flux at the top of the atmosphere is larger for the warm subset in every cluster except cirrus, and larger when averaged over all clusters. This implies that the direct response of midlatitude oceanic clouds to increasing temperature acts as a negative feedback on the climate system. Note that the cloud response to atmospheric dynamical changes produced by global warming, which we do not consider in this study, may differ, and the total cloud feedback may be positive.