Estimating the atmospheric boundary layer height over sloped, forested terrain from surface spectral analysis during BEARPEX

Abstract

The atmospheric boundary layer (ABL) height (<i>z_i</i>) over complex, forested terrain is estimated based on the power spectra and the integral length scale of cross-stream winds obtained from a three-axis sonic anemometer during the two summers of the BEARPEX (Biosphere Effects on Aerosol and Photochemistry) Experiment. The <i>z_i</i> values estimated with this technique show very good agreement with observations obtained from balloon tether sondes (2007) and rawinsondes (2009) under unstable conditions (<i>z/L</i> < 0) at the coniferous forest in the California Sierra Nevada. On the other hand, the low frequency behavior of the streamwise upslope winds did not exhibit significant variations and was therefore not useful in predicting boundary layer height. The behavior of the nocturnal boundary layer height (<i>h</i>) with respect to the power spectra of the <i>v</i>-wind component and temperature under stable conditions (<i>z/L</i> > 0) is also presented. The nocturnal boundary layer height is found to be fairly well predicted by a recent interpolation formula proposed by Zilitinkevich et al. (2007), although it was observed to only vary from 60–80 m during the 2009 experiment in which it was measured. Finally, significant directional wind shear was observed during both day and night soundings. The winds were found to be consistently backing from the prevailing west-southwesterlies within the ABL (the anabatic cross-valley circulation) to southerlies in a layer ~1–2 km thick just above the ABL before veering to the prevailing westerlies further aloft. This shear pattern is shown to be consistent with the forcing of a thermal wind driven by the regional temperature gradient directed east-southeast in the lower troposphere.