Spectral Measurements in a Disturbed Boundary Layer over Snow

Abstract

I have measured time series of the turbulent fluctuations in longitudinal (u) and vertical (w) velocity and in temperature (t) and humidity (q) with fast-responding sensors in the near-neutrally stable surface layer over a snow-covered field. The series yielded individual spectra, u-w, w-t, w-q, and t-q cospectra and phase and coherence spectra for nondimensional frequencies (fz/U) from roughly 0.001 to 10. This is, thus, one of the most extensive spectral sets over collected over a snow-covered surface. With the exception of the u-w cospectra, all of the spectra and cospectra displayed the expected dependence on frequency in an inertial or inertial-convective subrange. All, however, contained significantly move energy at low frequency than the Kansas neutral-stability spectra and cospectra. This excess low-frequency energy and the erratic behavior of the u-w cospectra imply that the forested bills bordering the site on two sides were producing disturbances in the flow field at scales roughly equal to the height of the hills, 100 m. The phase and coherence spectra suggest that internal gravity waves may have frequently been present, since the atmospheric boundary layer generally had slightly stable stratification. Consequently, at this complex site, turbulence alone determines the spectra and cospectra at high frequency, while at low frequency, the spectra and cospectra reflect a combination of topographically generated turbulence and, probably, internal waves. From the measured temperature and humidity spectra and the t-q cospectra, I computed refractive index spectra for light of 0.55 ?m and millimeter wavelengths, the first such spectra obtained over snow. The refractive index spectra had shapes like the other scalar spectra: excess energy at low frequency and an inertial-convective subrange at high frequency. The existence of an inertial-convective subrange allowed computation of the refractive index structure parameter Cn2 for each spectrum. Values of Cn2 ranged from 9 ? 10?17 to 6 ? 10?15 m?? for visible wavelengths and from 8 ? 10?16 to 1.5 ? 10?14 m?? for millimeter wavelengths. From the u, t and q spectra, I also estimated the sensible (Hs) and latent (HL) heat fluxes using the inertial-dissipation technique. The absolute value of Hs, rarely exceeded 10 W m?2, and the absolute value of HL was always less than 10 W m?2. Though Hs, could take on either sign, HL was virtually always positive? the snow was subliming.