Large scale drivers versus local processes impacts on post-cold frontal cloud properties in observations and CAM6

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Description

Using observations collected at the ARM Eastern North Atlantic (ENA) site, we examine the relationship between the large-scale environment and the properties of low-level clouds that occur in conditions of subsidence. When these conditions occur in the wake of a cold front, the surface wind speed and subsidence strength are anomalously strong, causing the boundary layer to be unstable and shallow convection to develop. Therefore, when compared to similar but more quiescent conditions, (i.e. without the influence of an extratropical cyclone), the post-cold frontal clouds have boundaries often times at higher altitude, with a top above the melting level. The cloud boundary locations correlate well with the difference in potential temperature between the 800 hPa level and the surface, a measure of the degree of boundary layer instability. A preliminary analysis on a small sample of southern oceans data suggests a similar correlation in that part of the world as well. Moreover, consistent relationships are found between near-surface stability, surface energy fluxes, and cloud fraction, optical thickness, and top temperature in various regions of strong post-cold frontal activity. Because many general circulation models underestimate cloud amounts in the cold sector of extratropical cyclones, we use the CAM6 model to test whether this might be because of issues with the cloud parameterization, with the representation of the large-scale environment or with the relation between the environment and clouds. While the model produces the correct relationship between the potential temperature contrast variable and cloud properties at the ENA site, because its boundary layer is more stable, cloud tops are warmer than in observations. To help understand these model issues, we use the Weather Research Forecast (WRF) model to explore post-cold frontal clouds with a case study. The modeled cloud properties are sensitive to the interactions between the shallow convection and the boundary layer parameterizations. We will report how this sensitivity is related to boundary layer decoupling, vertical shear in the horizontal winds at cloud top, and drizzle. We will test the robustness of these conclusions by analyzing a perturbed initial conditions ensemble using WRF. A comparison of the perturbed physics and the perturbed initial condition ensembles will explore the relative impact of circulation changes and physical processes on low-level cloud in the model.