Disentangling the Large-Scale and Local Factors That Determine Low Cloud Properties during Cold Air Outbreaks

Low-level clouds in the midlatitudes play an important role in the Earth’s energy budget. These clouds are commonly formed by cold air outbreaks (CAOs), which occur when cold advection induces surface moisture and heat fluxes from a warm sea surface. Despite the clear understanding of the circulation and surface conditions required to generate the clouds, questions remain regarding what determines the characteristics of the clouds formed. As such, this analysis utilizes MODIS cloud data and reanalysis for dynamics and thermodynamics to test the relationships between the cloud properties and the local and large-scale environment. Our analysis finds that vertical motion has a strong, linear relationship with cloud top pressure: ascent coincides with high clouds and descent coincides with low clouds. This relation mimics the cloud and vertical motion relationship within extratropical cyclones, and a cyclone association analysis confirms the importance of the cyclones for clouds in CAOs. As such, the next component of the study focuses on three locations that were selected based on their position in the North Atlantic storm track: the Gulf Stream at the storm track entrance, the Azores at the equatorward exit, and Bear Island at the poleward exit. For all locations, during times of subsidence, the surface turbulent heat flux has a linear relationship with cloud optical depth. Given that these CAOs are convective cloud regions the majority of the time, the result suggests that stronger surface fluxes are providing more water, which leads to optically thicker the clouds. However, the relationship between subsidence and cloud top pressure varies significantly for the three regions. This suggests that the upper-level circulation of the cyclones alone can only partially explain the cloud behavior. Related to this, the low-level atmospheric stability and cloud top temperature have a negative correlation during CAOs, and the relationship is consistent across the three locations. This result can be interpreted in two ways: either the low-level stability dictates cloud top height, or the two properties are coupled because sea surface temperatures have minimal variability and so variability in low-level stability only reflects temperature variability near the cloud top. We attempt to determine the correct interpretation using a conditional sorting analysis based on sea surface conditions. In total, this work provides new insights and multiple benchmarks regarding the relationships between low-level clouds properties, large-scale subsidence, and local stability during CAOs.