Accession Number : ADA190217

Title :   Spectral Measurements in a Disturbed Boundary Layer Over Snow,

Corporate Author : COLD REGIONS RESEARCH AND ENGINEERING LAB HANOVER NH

Personal Author(s) : Andreas, Edgar L

PDF Url : ADA190217

Report Date : Nov 1987

Pagination or Media Count : 49

Abstract : The author measured time series of longitudinal (u) and vertical (w) velocity and temperature (t) and humidity (q) fluctuations with fast-responding sensors in the near-neutrally stable surface layer over a snow-covered field. These series yielded individual spectra and u-w, w-t, w-q and t-q cospectra, phase spectra and coherence spectra for nondimensional frequencies (fz/U) from roughly 0.001 to 10. With the exception of the u-w cospectra, all the spectra and cospectra displayed the expected dependence on frequency in an inertial or inertial-convective subrange. All, however, contained significantly more 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 forested hills 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 were also frequently 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; at low frequency the spectra and cospectra reflect a combination of topographically generated turbulence and internal waves. From the measured temperature and humidity spectra and the t-q cospectra, the author computed refractive index spectra for light of 0.55-micrometer and millimeter wavelengths. The refractive index spectra had shapes like the other scalar spectra: excess energy at low frequency and an inertial-convective subrange at high frequency.

Descriptors :   *SNOW COVER, *TURBULENCE, *LIGHT TRANSMISSION, *ATMOSPHERIC MOTION, ATMOSPHERES, BOUNDARY LAYER, COHERENCE, ENERGY, FLOW FIELDS, GRAVITY WAVES, HIGH FREQUENCY, HUMIDITY, INTERNAL WAVES, LOW FREQUENCY, REFRACTIVE INDEX, SCALAR FUNCTIONS, SCALE, SITES, SPECTRA, SPECTROMETRY, STABILITY, STRATIFICATION, TIME SERIES ANALYSIS, VISIBLE SPECTRA, MILLIMETER WAVES, ATMOSPHERIC TEMPERATURE, SPECTRUM ANALYSIS, PHASE, TOPOGRAPHY, FORESTS

Subject Categories : Meteorology
      Optics

Distribution Statement : APPROVED FOR PUBLIC RELEASE