9 Shear-Sheltering Theory and Surface Fluxes: New Exciting Developments

Monday, 20 June 2016
Alta-Deer Valley (Sheraton Salt Lake City Hotel)
Gengsheng Zhang, University of Georgia, Griffin, GA; and M. Y. Leclerc, H. B. Nahrawi, and R. Pahari

This paper reports on new exciting developments regarding the validity of a recent theory expounded by Hunt and Durbin in 1999 and subsequently experimentally tested in several studies.

According to the theory, the stable nocturnal boundary layer is often characterized by the presence of low-level jets which block the downward transport of large eddies from the region above the jet and penetrate the jet layer to be transported to the surface impacting surface turbulence and mixing properties. Smedman et al., (2004) tested the theory using wind profiles and eddy-flux measurements at two marine sites in the Baltic Sea and showed a significant suppression of low-frequency components in the spectra in the cases with low-level jets when compared against those without low-level jets. In contrast, Duarte et al. (2012) analyzed data from a previous experiment at a relatively homogeneous quasi-ideal terrestrial site, the DOE ARM site in Lamont, OK, and reported opposite results.

The properties of steady, continuous (non-intermittent within the observed domain) low-level jets and their influence on atmospheric turbulence and eddy fluxes in the nocturnal stable boundary layer at a coastal site near Sapelo Island, GA was investigated during the winter season and wind profiles using a sodar and boundary-layer structure using a ceilometer were collected simultaneously with turbulence and eddy-fluxes data. Results of the spectral analysis suggest a suppression of spectral energy in the velocity components and temperature signals at low frequencies in the presence of low-level jets. This result is in sharp contrast with periods without low-level jets in similar turbulence and wind shear regimes. The contrast between the Duarte et al. and the present study and that of Smedman et al. thus suggest that shear-sheltering theory appears to hold true be for regions of very large horizontal homogeneity such as seas and oceans rather than over terrestrial land.

Key words: Shear-sheltering theory, Low-level jets, Nocturnal stable boundary layer, Atmospheric turbulence and eddy fluxes, Marine boundary layer

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