Boundary Layer Clouds in the Midlatitudes: Environmental Controls in Post-Cold Frontal Regions

Cloud cover in the cold sector of extratropical cyclones has been the subject of active research in recent years because of significant biases in GCMs. Improvements in the representation of the microphysics of these clouds, the boundary layer and the convection schemes all produce a reduction in bias but it remains significant. Focusing more specifically in the region in the wake of the cold fronts, we used ground-based cloud observations to explore the sensitivity of the macrophysical properties of these clouds to the large-scale environment. This is done to both provide an observational constraint for model evaluation and help pinpoint aspects of the models that might require more attention.

Using observations from the ARM Eastern North Atlantic (ENA) site in the Azores, we find that clouds in the post-cold frontal regions display sensitivities to large-scale drivers such as surface winds, subsidence strength, inversion strength or environmental moisture, similarly to clouds in more quiescent conditions. However, the extratropical cyclones produce a dynamically active environment that causes clouds to be thicker and higher than their quiescent counterparts. Cloud base and top heights and temperatures show a strong correlation with the strength of the cold air advection, measured with the Marine Cold Air Outbreak parameter M, because of a strong sensitivity to both air-sea contrast in temperature, and inversion strength.

In order to generalize these results to the midlatitudes overall, we will present a similar analysis based on southern ocean observations. The site is located on Macquarie Island, around 50°S, an area where extratropical cyclones are ubiquitous. At this location, the storms are stronger, but the environmental moisture and temperature lower. Consequently, the clouds in post-cold frontal regions are colder than at the ENA site, while being supercooled a lot of the time.

We will discuss the implications for their representation in general circulation models.