Wind rolls in Greenland

This case from 16 January 1992 provides the possibility to compare boundary layer cloud structure and surface roughness patterns in a synergistic fashion. The NOAA Advanced Very High Resolution Radiometer (AVHRR) image and the ERS-1 SAR image were obtained about 20 minutes apart off the ice edge in the East Greenland Current.
The SAR image to the left is about 200x100 km, while the AVHRR image to the right covers the region between Greenland, northern Norway and Svalbard.

Such structures are reported to be associated with horizontal roll vortices in the atmospheric planetary boundary layer. In close vicinity of the ice edge the rolls are not seen, while a gradual increase in the horizontal roll spacing with distance downwind is observed.
This is explained by the growing boundary layer height, eventually reaching a depth where the available energy decays and the organization of the roll structure breaks down. The mean wavenumber is estimated to be 1.5 km-1, equivalent to a roll spacing of about 5 km. The spacing is reported to range 2-4 times the planetary boundary layer depth. In this case this implies that the boundary layer ranges from 1.25 to 2.50 km in height.
Rolls are frequently observed in infra-red images downwind from the ice edge due to cold air flowing over warmer water, thus leading to unstable stratification in the boundary layer with significant moisture flux. They are also reported to be of substantial importance for the heat flux.
The SAR image reveals the corresponding surface roughness in the open water off the ice edge. One can see clearly reveal a perturbed, streak-like pattern with a mean orientation aligned in the direction of the roll vortices. The estimated wavenumber ranges from 1.5 to 2.0 per km in agreement with the estimate from the infra-red image.

The surface waves that lead to this characteristic pattern are formed primarily in response to the surface wind variations introduced by the horizontal roll vortices (see this figure).
In turn, atmospheric phenomena that induce a varying sea surface wind field, and hence wind stress, are detectable by the SAR. Several bands of dark backscatter are also observed. We interpret this to be areas of new ice, which dampens out the short gravity waves.
The analyzed weather map documents that the pattern is aligned almost along the isobars and consequently the geostrophic wind direction. The wind speed is reported to be 7-8 m/s from a nearby weather station on Jan Mayen Island, while the off-ice direction implies that cold (-16ºC), dry air is advected over warmer water with temperature ranging from -1.5 to +3.0ºC. This in turn leads to strong unstable stratification.

(NERSC, Bergen, Norway)

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry