Impact of Dry Intrusions on the Marine Boundary Layer

Intrusions of dry air from the upper troposphere were recently suggested to reach the boundary layer and cause its significant deepening. Dry intrusions (DIs) are synoptic-scale slantwise descending airstreams from the midlatitude upper tropospheric jet towards the boundary layer at lower latitudes, thus acting as a circulation type potentially key for understanding boundary-layer cloud occurrence and regime transition. DIs occur mainly during winter over the mid-latitude oceanic storm track regions behind cold fronts trailing from cyclones. These regions are also home to marine boundary clouds that are an important component of the Earth’s radiation budget as they reflect much higher radiation back to the space compared to the ocean surface thereby cooling the Earth’s surface. Although subsidence is generally an inherent feature of the subtropical marine boundary layer, it is unclear how the marine boundary layer reacts to the transient, dynamically distinct DI, differently from the nominal subtropical subsidence resulting from the descending branch of Hadley circulation.

In this study we use the observations made at the Atmospheric Radiation Measurement (ARM) Eastern North Atlantic (ENA) site (39N, 28W) to characterize the impact of dry intrusions on Marine Boundary Layer (MBL) characteristics such as surface fluxes, thermodynamic stabilities and winds. Our analyses are based on measurements from the campaign: radiosondes, surface station data, polarimetric radar, lidar, radar wind profiler, ceilometer among others. Using all identified DI trajectories during the winters of 2016-2018 based on European Center for Medium-range Weather Forecasts (ECMWF) ERA Interim reanalysis data, we distinguish DI days from those before and following DIs, as well as periods with no DIs at all (with and without the occurrence of cold fronts for comparison). We find that during DI events the well-mixed MBL deepens and its vertical structure changes dramatically. Namely, the lower troposphere cools and dries substantially, inducing strong surface sensible and latent heat fluxes, while a strong inversion builds up at the MBL top, all affecting cloud occurrence. Finally, we used the numerical weather prediction (NWP) model COSMO at 2.2 km horizontal resolution to understand the detailed flows and structure in the MBL during DI events.