Observations of longitudinal roll vortices during arctic cold air outbreaks over open water

Abstract

An evolving convective Arctic planetary boundary layer (PBL) containing longitudinal roll vortices (rolls) was observed with aircraft data during the 1983 Marginal Ice Zone Experiment and the 1984 Arctic Cyclone Experiment.The PBL is observed to grow rapidly as the very cold and dry air flows off the ice over the relatively warm water. There is very large sensible heat flux, a result of the large surface-air temperature differences. Coherent structures were identified in these PBL's by use of power, coherence squared and phase spectra of the data. A systematic method of separating the rolls from organized thermal plumes was devised, based on theoretical characteristics for roll circulations and the resulting modified mean wind profile. The rapid mixing by the rolls aids in the establishment of equilibrium and an observed adiabatic modified mean Ekman layer. Rolls that form in a thermally neutral atmosphere over ice have different characteristics than those that appear in the unstable stratification over water. The rolls become increasingly more convective in character with distance from the ice edge. They have aspect ratios (wavelength/PBL height) that decrease with distance from the ice edge in agreement with linear theory. This is in contrast to the cloud street wavelength to inversion height ratio which is observed to increase downwind from the ice edge.

Numerical simulation of the development of the convective boundary layer during a cold air outbreak

“…Contrary to the above, some new observations by Walter and Overland (1984) under near neutral stratification over ice-covered Bering Sea, and by Mtiller et al (1985) over Northern Germany, have implied that inflectionpoint instability is the only cause, but in these cases, observed heat fluxes have been much smaller than during a typical CAO. The possible enhancement of dynamically initiated vortex rolls by buoyancy forces during boundary-layer modification over the arctic marginal ice zone has been pointed out by Hein and Brown (1988).…”

Possible observation of horizontal roll vortices over the Adriatic Sea during bora

“…the height of the upper ABL flight leg, zi = 680 m. This leads to the roll aspect ratio of L/zi = 1.5, which is somewhat smaller than those usually reported in the literature (e.g., Young et al, 2002). The spatial structure of rolls yields specific phase relationships between different wind components (e.g., Hein and Brown, 1988;Alpers and Brummer, 1994). In an idealized system for verticallyupright rolls, the expected phase angles are φ(v,w) = ± 90 • near the top and bottom, and φ(u,w) = ± 180 • throughout (Chen et al, 2001;Hein and Brown, 1988).…”

“…The spatial structure of rolls yields specific phase relationships between different wind components (e.g., Hein and Brown, 1988;Alpers and Brummer, 1994). In an idealized system for verticallyupright rolls, the expected phase angles are φ(v,w) = ± 90 • near the top and bottom, and φ(u,w) = ± 180 • throughout (Chen et al, 2001;Hein and Brown, 1988). LeMone and Pennell (1976) also show that the longitudinal vortices can be tilted with height, which leads to v and w being in phase in the middle of the roll layer (alternatively, v and w could be 180 out of phase, if the tilt is in the opposite direction).…”

Occurrence of roll circulations in a shallow boundary layer

“…They are believed to be caused by a combination of convective and dynamic instability. Horizontal rolls seem to be a common phenomenon over the Arctic Sea as summarized by Hein and Brown (1988). Figure 6 shows a schematic picture of horizontal roll vortices inclined about 20" to the wind.…”

“…For the runs with -zilL > 5, they are fulfilled in all but two cases. Hein and Brown (1988) used spectra and cospectra from aircraft data to identify roll circulations, whereas Sykes and Henn (1989) draw the conclusion, from large eddy simulations, that the ratio u*Iw* is the crucial parameter. They conclude that rolls will form when z&w* > 0.35.…”