Understanding mesoscale organization of closed-cell marine stratocumulus using large-eddy simulation and observations from the ARM Eastern North Atlantic Site

Poster PDF

Description

This study investigates the mechanisms that promote mesoscale closed cellular organization in well-mixed non-drizzling stratocumulus-topped marine boundary layers. A theory is developed using large-eddy simulation (LES) over an approximately 60x60 km2 periodic domain, which is verified using observations at the Eastern North Atlantic (ENA) Site operated by the US Department of Energy Atmospheric Radiation Measurement program. LES Simulations show that the mesoscale closed-cell stratocumulus organization is driven by positive feedback from cloud-induced mesoscale perturbations of longwave radiative cooling that amplifies boundary-layer humidity perturbations. A conceptual model for closed-cell stratocumulus as a mesoscale wavelength hydrodynamic instability in which mesoscale moist and dry anomalies spontaneously grow is presented. The cell structure is visualized with a compositing approach based on sorting grid columns by their mesoscale-smoothed total water path. A thermally direct mesoscale circulation pattern develops in the interior of the boundary layer with buoyant mesoscale updrafts, thicker cloud, and a slightly higher capping inversion in the moister columns. There is a mesoscale flow of above-inversion air down the slightly sloping capping inversion from the moist to the dry regions, reinforced by cloud-top radiative cooling. This strengthens the mesoscale anomalies by preferentially cooling and drying the already dry regions. Observations of closed-cell cases selected from the ENA site show remarkable positive correlations between mesoscale water vapor path anomalies and mesoscale perturbations of liquid water path, cloud top height, cloud depth, and column maximum reflectivity, which confirms the governing role of humidity anomalies in promoting mesoscale cloud variability.