FY 1991

Aircraft observations of the mean and turbulent structure of the atmospheric boundary layer during spring in the central Arctic

Walter, B.A., and J.E. Overland

J. Geophys. Res., 96(C3), 4663–4673, doi: 10.1029/90JC02263 (1991)

The NOAA P-3 research aircraft carried out measurements of the mean and turbulence structure of the planetary boundary layer on March 27 and 30, 1989, at 83°N, 10°E in the central Arctic. The ice motion was strongly decoupled from the atmosphere; surface winds were 2–3 m s⁻¹ and winds at 100 m were 10 to 12 m s⁻¹ on March 27 and 8 m s⁻¹ on March 30. Geostrophic drag coefficients were 0.014 and 0.015. The mean vertical structure on both days is characterized by a shallow slightly stable layer 50 m deep at the surface with a 300-m-deep strong inversion above. Speed shear was 0.1 s⁻¹ in the slightly stable layer and direction shear at the top of the layer was 15°–20° on March 27 and 35°–40° on March 30. Profiles of the Richardson number for each day showed that Ri was less than 0.25 in the slightly stable layer and rapidly increased above. A low level jet was located at the top of the slightly stable layer just below the level where the Richardson number became very large. Shear-induced turbulence at the top of the slightly stable layer resulted in a maximum of turbulence kinetic energy just below the wind maximum on March 27 and at the wind maximum on March 30. Heat flux profiles from the gust probe measurements showed a maximum of upward flux at the top of the slightly stable layer of 9 W m⁻² implying that the flux was countergradient. This upward flux was responsible for maintaining the strength of the inversion above this level. Spectra of turbulent vertical velocity showed a peak at a wavelength of 350–400 m. The source of this peak is gravity waves from an upper- level shear layer. It is speculated that the observed countergradient flux was a result of gravity waves and/or wave- turbulence eddy flux. Use of an E−ε turbulence closure model allowed an estimation of the sensible heat flux from the air to the snow surface of 4 W m⁻². This surface flux played only a secondary role in cooling the surface layer and establishing the structure of the boundary layer compared to inversion layer processes.