Boundary-layer moisture variability at ARM's Eastern North Atlantic observatory

Science

Marine boundary-layer stratocumulus and shallow cumulus clouds are maintained by boundary-layer turbulence through the transport of water vapor above the lifting condensation level. Hence changes in boundary-layer water vapor critically impact cloud properties. Unlike liquid water path (LWP), water vapor at the site does not present a diurnal cycle, but rather an annual variability that is strongly connected with clouds and precipitation. Further, marine boundary-layer clouds exhibit a distinct mesoscale organization with scales of 20-50 km. In this work we analyze the variability of marine boundary-layer moisture in the Eastern North Atlantic on a seasonal and daily temporal scale and examine its fundamental role in the control of boundary-layer cloudiness and precipitation.

Impact

To our knowledge, the present analysis is the first study to characterize the controls of moisture variability at a subtropical marine location using a long-term data set of ground-based data. The findings of this work are useful for understanding the relative importance of large-scale and local moisture sources and sinks in the region. Additionally, the work provides observational support to the hypothesis of mesoscale moisture self-aggregation.

Summary

Water vapor at the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) user facility's Eastern North Atlantic (ENA) observatory at Graciosa Island, Azores varies on a seasonal and hourly scale.  Five years of ground-based observations and reanalysis data are used to analyze the seasonal total water budget and mesoscale vapor variability. The mixed layer total water budget framework is used to estimate the relative contribution of large-scale advection, local moisture tendency, entrainment, and precipitation to balance the moistening due to ocean latent heat flux on monthly timescales. When marine conditions prevail, advection of colder and dry air from the north acts as an important moisture sink (~ 50% of the overall budget) during fall and winter, driving the seasonality of the budget. Entrainment and precipitation contribute to the drying of the boundary layer (~25% and ~15% respectively) and the local change in moisture contributes to a small residual part.

On a daily temporal scale, moist and dry mesoscale columns of vapor (~10 km) are analyzed during 10 selected days of precipitating stratocumulus clouds. Adjacent moist and dry columns present distinct mesoscale features that are strongly correlated with clouds and precipitation. Dry columns adjacent to moist columns have more frequent and stronger downdrafts immediately below the cloud base. Moist columns have more frequent updrafts, stronger cloud top cooling, and higher LWP and precipitation compared to the adjacent dry columns. This study highlights the complex interaction between large-scale and local processes controlling the boundary-layer moisture and the importance of vapor spatial distribution to support convection and precipitation.