Minimize Atmospheric Boundary Layer Rolls

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East China Sea East China Sea

Latitude: 28° 01' N - Longitude: 122° 29' E

Boundary layer rolls in the Taiwan Strait. The line which divides the image into two sections of different textures represents the boundary between water masses of different temperatures. In winter a cold water mass stretches along the east coast of China as is evident also from the AVHRR infrared image of 7 Dec. 1989 shown below. Over the warm water, cold wind blowing from the north generates atmospheric boundary layer rolls, because here the air is heated from below which gives rise to an unstable air-sea interface. The streaky texture of the right-hand section of the image results from imprints of these atmospheric boundary layer rolls on the sea surface. Over the cold water no boundary layer rolls are generated. Note also the numerous black patches in the image which are due to mineral oil spills in this sea area with heavy ship traffic.
Orbit Frame(s) Satellite Date Time Location
18196 3033-3051 ERS-1 07-Jan-1995 02:26 East China Sea

If you have any comments on this image please write an e-mail to alpers@ifm.uni-hamburg.de.

Introduction

Atmospheric boundary layer rolls are helical circulation patterns in the atmospheric boundary layer which are superimposed on the mean wind field, i.e., the primary flow (see figure below). They can be generated either by thermal instability (Rayleigh-Bénard instability) when the layer is heated from below or cooled from above, or by dynamic instability (inflection point stability) when the wind velocity changes with the height in such a way that an inflection point occurs in the wind component normal to the roll axis. The axes of the boundary layer rolls are oriented between the directions of the mean surface wind and the geostrophic wind above the boundary layer. Usually the boundary layer is capped by an inversion so that the depth of the boundary layer and the roll layer coincide. In the case of a thermal instability, the aspect ratio, i.e., the horizontal wavelength of the roll pattern, l, divided by the roll height, h, is 2.8 according to the linear Rayleigh-Bénard convection. The most frequently observed values range between 2 and 6, but values of up to 15 have been observed [Miura, 1986].

If the moisture conditions are favorable, cloud streets may be formed in the upward rising branches of the roll circulation. Atmospheric boundary layer rolls contribute significantly to the vertical exchange of momentum, heat, and moisture in the atmosphere. At higher altitudes their relative contribution to the total vertical fluxes in the atmosphere can even be larger than the turbulent fluxes. [Alpers et al., 1995].

Atmospheric boundary layer rolls

Schematic plot of secondary flow pattern associated with atmospheric boundary layer rolls. Panel A: Perspective view of the three-dimensional flow; Panel B: Variation of the vertical component uz of the wind velocity along the y direction; Panel C: Variation of the horizontal components ux and uy (in the x-y plane).

Ancillary information

Temperature map

Temperature map derived from NOAA-11 AVHRR data of 7 Dec. 1989, showing that in winter a band of cold water is stretching along the east coast of China. Further to the east the water is warmer.

References

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