Spatial scales of vertical mixing in the lower atmosphere during EUREC4A/ATOMIC

Abstract:

The fate of low-level clouds in a warmer world remains a pressing question. Climate models suggest low-cloud fraction in tropical environments is sensitive to the amount of moisture exchange between the boundary layer and free troposphere. Yet such mixing in the vertical is almost certainly regulated, at least in part, by the clouds themselves, highlighting the need for observations and simulations that can resolve these smaller scales. Here, we apply new observations from the 2020 EUREC4A and ATOMIC field campaigns to estimate both the amount of moisture exchange between the boundary layer and free troposphere over the western tropical Atlantic and its scale of spatial variability. Using aircraft in situ measurements of water vapor and its isotopic composition, we demonstrate a high degree of horizontal heterogeneity in vertical mixing—noting dramatic variations on the order of 10 km—for a select set of case studies. To begin examining the role of cloud and precipitation processes in controlling this heterogeneity, we map variations in moisture’s length scale—estimated from level-flight legs—to the patterns and organization of clouds observed and use isotopic measurements from the cloud layer to quantify variations in cloud precipitation efficiency. Large reductions in both the degree of vertical moisture exchange and the efficiency of precipitation formation are evident for flights that were affected by Saharan dust compared to those that were not. We also examine the degree to which the horizontal heterogeneity in vertical mixing varies with the regional-mean thermodynamic state of the atmosphere. Specifically, we use isotopic measurements of the free troposphere and Lagrangian back trajectories to reveal how large-scale dynamics and moisture transport set the “background” against which boundary layer-free troposphere exchange occurs.