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°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 Cold air Outbreaks 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 warm-air intrusions in terms of
stratification of moist static energy 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 between 2-4 km.
The mostly open-cellular convection may contain substantial ice and
produce intermittent moderate precipitation at the observational site,
notwithstanding the low precipitable water vapor. In contrast, WAIs are
marked by high values of precipitable water vapor and integrated vapor
transport. WAI clouds are stratiform, with cloud tops often exceeding 6
km, sometimes layered, and generally producing persistent precipitation
that can be heavier than in CAOs.