Observation of Cloud Properties and Interaction with Large Scale Environment in Post-Cold Frontal Regions

In recent years, significant attention has been paid to cloud cover underestimates in general circulation models, notoriously in the southern oceans. This bias was found to be most acute in the cold sector of extratropical cyclones and more specifically in the region to the west of the cold front. This bias is also found in the northern hemisphere cyclones but interestingly not in the summer. To better understand the root cause of this bias, a more thorough examination of clouds that occur in post-cold frontal (PCF) conditions is necessary. Most of the clouds in PCF regions are low-level clouds, so both convective and boundary layer processes are important for their accurate representation. However, because of the presence of the parent cyclone, the region is dynamically different from more quiescent subtropical regions of low-level clouds.

Here we use observations from the Eastern North Atlantic (ENA) site of the Atmospheric Radiation Measurement Program to characterize the statistical properties of PCF clouds and their relationship with the large-scale environment in post-cold frontal regions. In addition, we contrast these cloud and environmental properties to those of more quiescent periods (identified as periods of subsidence with northeasterly winds).

We developed a method to detect cold front passages at the ENA site using both an existing database of extratropical cyclones and fronts and the change in wind direction at the site. Clouds occur two-third of the time post-cold frontal conditions are identified and cloud base heights are found below 3 km more than 80% of the time a cloud is detected. Subsidence and surface winds are strong when these conditions occur, an inversion can be detected about 85% of the time, and lower tropospheric stability is relatively weak. The cloud base and top height distributions reveal deeper clouds and boundary layers in PCF vs. quiescent conditions. We find that both PCF and non-PCF clouds are driven by static stability, but PCF clouds show a much stronger sensitivity to surface winds. These results suggest that the large scale dynamics plays a non-negligible role for these low-level clouds.

To add context to these results for the ENA region, we use reanalysis and MODIS cloud observations to examine clouds and their relation to large scale conditions in subsidence conditions in different regions: the ENA, the Gulf-Stream region and the north-east Atlantic area. This work reveals that cloud optical thickness in PCF regions over the Gulf Stream are much larger than those over the ENA site. Also, PCF clouds in the Gulf Stream region tend to occur under stronger subsidence conditions and above stronger surface fluxes. In a set of conditional subsetting analyses, we show that both the forcing from the upper-troposphere and the surface forcing contribute to the difference in cloud optical thickness for the Gulf Stream as compared to the ENA site. The analysis of low-clouds in the north-east Atlantic offers a unique result as compared to the Gulf Stream and ENA region, which is related to the large amount of advection of low clouds into the region.