Investigating Cold Air Outbreaks over the Western North Atlantic Ocean

Abstract:

Outbreaks of cold continental air are common over the western North Atlantic Ocean (WNAO) with the Gulf Stream current further strongly enhancing the air-sea interactions. Challenges in modeling their complex and quickly evolving cloud morphology are partially motivating the ongoing NASA's Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) project. Here we describe a climatology of visually-identified cold air outbreaks (CAOs) spanning from 2006 to 2020 using MODIS imagery to provide further context for the field campaign. CAOs occur over the WNAO from September through May, but are most frequent from January through March, with an average of 12 days in January and 10 days each in February and March. A first presentation will be made of a composite based on airflow as it moves over the Gulf Stream using coincident datasets from CALIOP, CloudSat, MODIS, and ERA5. A first case study documents organized stratiform clouds prevailing along the cold western side of the Gulf Stream, with cloud top below 1 km. An abrupt increase in the boundary layer height, by 500 m to 1 km, occurs over a horizontal distance of about 100 km as air moves over the Gulf Stream. Perhaps paradoxically, the boundary layer deepening is associated with a strengthened subsidence aloft from the western flank of the cyclone located further north, increasing the entrainment of extremely dry air capable of reaching the surface. About 250 km further, precipitation is evident within CloudSat data, and clouds break up and dissipate. In addition, the ERA5 surface fluxes are evaluated using dropsondes gathered during the 2020 deployment phase of the ACTIVATE field campaign in support of modeling analysis. The dropsondes indicate drier air overall than is depicted by ERA5, and too-warm (cold) near-surface air over the western (eastern) side of the Gulf Stream. These systematically impact the ERA5 surface fluxes.