NRL Participates in International Campaign Investigating Polar Low Phenomena

From late February through early April, the 45-day international field campaign CAESAR, short for Cold-Air outbreak Experiment in the Sub-Arctic Region, focused on cold-air outbreaks that occur as cold Arctic air flows-out over warmer open waters between northern Norway and the Arctic ice edge.

Cold-air outbreaks, or CAOs—one of the most extreme meteorological air mass transformations on Earth—can occur when cold Arctic air flows over frozen land masses or sea ice to over much warmer open ocean waters resulting in the formation of convective boundary layers that produce hazardous winds and seas, and spawn small-scale, intense “polar lows.”

“Despite the profound impact that CAOs have on atmospheric and oceanic circulations in the Arctic, as well as the important implications for Navy operations, surprisingly little is known about the nature of intense surface flux impacts on the atmosphere and ocean boundary-layer structure,” Doyle said. “The nature of the air-sea-ice interaction and cloud processes in CAOs are rapid with abrupt transitions, which have been a roadblock to process understanding and model predictions.”

CAOs pose challenges to Navy operations because of the severe environmental conditions and the rapid changes in the atmosphere and ocean boundary layer properties that impact electromagnetic and acoustic propagation characteristics. The CAESAR mission seeks to investigate the marine atmospheric boundary layer characteristics and shallow, precipitating clouds that form during CAOs, as well as the mesoscale circulations that lead to polar low developments.

“Under favorable conditions the air-sea interaction intensifies, triggering shallow and vigorous convective cells that produce hazardous winds and seas, and under some conditions lead to the genesis of small-scale, intense polar lows,” Doyle said. “Conventional theories and model parameterizations in Arctic CAOs have been lacking this vital data and CAESAR will provide a detailed characterization that will form the basis for NRL boundary layer and coupled modeling studies.”

Based in Kiruna, Sweden, the CAESAR team will utilize the National Science Foundation (NSF)/National Center for Atmospheric Research (NCAR) center’s C-130 Hercules aircraft, with in situ and remote sensors and dropsondes for sampling the Arctic air mass—from the CAO origin at the ice edge through the transformation—as the boundary layer gets modified downstream. The C-130 suite of instruments also include airborne radar, LiDAR, and aerosol and cloud precipitation probes activated during CAO events.

CAESAR will also make use of Norwegian ground-based radars and instruments located on Norway’s Bear Island.

Written by Daniel Parry, U.S. Naval Research Laboratory.