Vertical structure of clouds and precipitation during Arctic cold-air outbreaks and warm-air intrusions: Observations from COMBLE

The Arctic is marked by deep intrusions of warm, moist air, alternating with outbreaks of cold air down to lower latitudes. The typical vertical structure of clouds and precipitation during these two synoptic weather extremes is examined at a coastal site at 69 DEG;N in Norway. The Norwegian Sea is a corridor for warm-air intrusions (WAIs) and frequently witnesses cold-air outbreaks (CAOs). This study uses data from profiling radar, lidar, and microwave radiometer, radiosondes and other probes that were collected during the CAOs in the Marine Boundary Layer Experiment (COMBLE) between 1 December 2019 and 31 May 2020. Marine CAOs are defined in terms of thermal instability relative to the sea surface temperature, and WAIs in terms of equivalent potential temperature stratification between the surface and 850 hPa. Cloud structures in CAOs are convective, driven by strong surface heat fluxes over a long fetch of open water, with cloud tops rarely exceeding 6 km above sea level. The mostly open-cellular convection produces intermittent moderately-heavy precipitation at the observational site, notwithstanding the low precipitable water vapor (PWV). In contrast, WAIs are marked by high values of PWV and integrated vapor transport. WAI clouds are synoptically driven, stratiform, with cloud tops often exceeding 5 km, sometimes layered, and generally producing persistent precipitation that can be heavier than in CAOs. Plain Language Summary The Arctic, more so than Antarctica, is marked by deep intrusions of warm, moist air, alternating with outbreaks of cold air down to lower latitudes. Here we examine the typical vertical structure of the updrafts, clouds and precipitation during these two weather types. We use data collected at a coastal site at 69 degrees N in Norway during the Cold air Outbreaks (CAOs) in the Marine Boundary Layer Experiment (COMBLE). COMBLE ran between 1 December 2019 and 31 May 2020 with funding from the United States Department of Energy. The Norwegian Sea is a corridor for warm-air intrusions (WAIs) into the Arctic, and it frequently witnesses CAOs, which in extreme events can be hazardous to maritime activities. The main findings of this study are: (a) the CAO cloud regime is convective, driven by strong surface heat fluxes over a long fetch of open water. Clouds are rather low-topped yet they do produce precipitation. (b) WAIs are marked by enhanced water vapor, high vapor transport, and clouds driven by deep frontal weather systems generally producing persistent precipitation.