An Extreme Cold-Air Outbreak over the Labrador Sea: Roll Vortices and Air–Sea Interaction

Abstract

Observational data from two research aircraft flights are presented. The flights were planned to investigate the air?sea interaction during an extreme cold-air outbreak, associated with the passage of a synoptic-scale low pressure system over the Labrador Sea during 8 February 1997. This is the first such aircraft-based investigation in this remote region. Both high-level dropsonde and low-level flight-level data were collected. The objectives were twofold: to map out the structure of the roll vortices that cause the ubiquitous cloud streets seen in satellite imagery, and to estimate the sensible and latent heat fluxes between the ocean and atmosphere during the event. The latter was achieved by a Lagrangian analysis of the flight-level data. The flights were part of the Labrador Sea Deep Convection Experiment, investigating deep oceanic convection, and were planned to overpass a research vessel in the area. The aircraft-observed roll vortices had a characteristic wavelength of 4?5 km, particularly evident in the water vapor signal. Unlike observations of roll vortices in other regions, a roll signature was absent from the temperature data. Analysis of satellite imagery shows the cloud streets had a characteristic wavelength of 7?10 km, indicating a multiscale roll vortex regime. There was a dramatic deepening of the boundary layer with fetch, and also with time. Off the ice edge, surface sensible heat fluxes of 500 W m?2 and surface latent heat fluxes of 100 W m?2 were measured, with uncertainties of ±20%. The very cold air is thought to be responsible for the unusually high Bowen ratio observed.