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

W Choi,A H Goldstein,M Mckay,B Baker,I C Faloona

doi:10.5194/acp-11-6837-2011

Abstract

Abstract. The atmospheric boundary layer (ABL) height (zi) 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 zi values estimated with this technique show very good agreement with observations obtained from balloon tether sondes (2007) and rawinsondes (2009) under unstable conditions (z/L < 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 (h) with respect to the power spectra of the v-wind component and temperature under stable conditions (z/L > 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.

Highlights

The depth of the lower atmospheric mixing layer, or boundary layer height, is one of the most important meteorological parameters, affecting the distribution of reactive atmospheric chemical compounds and playing a key role in air quality assessment at the local or regional scale (Seibert et al, 2000)
The distributions of highly reactive atmospheric species, such as some volatile organic compounds (VOCs), are strongly influenced by atmospheric boundary layer (ABL) turbulence and its vertical extent, zi, because these compounds are in general emitted from the surface, or produced secondarily within the ABL, and may possess lifetimes similar to or shorter than the time scales associated with the largest eddies confined by zi
It is essential to determine the stable nocturnal boundary layer (NBL) height (h), because the impact of dry deposition on chemical species budgets at night is inversely proportional to h, and dry deposition can be a major loss mechanism for many reaction products that are otherwise decomposed by photolysis and/or the reaction with OH in the daytime (e.g., H2CO, O3, peroxy acyl nitrates, VOCs)

Summary

Introduction

The depth of the lower atmospheric mixing layer, or boundary layer height (zi), is one of the most important meteorological parameters, affecting the distribution of reactive atmospheric chemical compounds and playing a key role in air quality assessment at the local or regional scale (Seibert et al, 2000). The distributions of highly reactive atmospheric species, such as some volatile organic compounds (VOCs), are strongly influenced by atmospheric boundary layer (ABL) turbulence and its vertical extent, zi, because these compounds are in general emitted from the surface, or produced secondarily within the ABL, and may possess lifetimes similar to or shorter than the time scales associated with the largest eddies confined by zi. It is not always easy to measure zi at any given site and time due to limited resources It is common in many atmospheric chemistry and transport studies to resort to assumptions about zi based on previous model results or measurements made at other locations with similar surface conditions (Dillon et al, 2002; Day et al, 2008). Spectral analyses and discussions for the stable nocturnal boundary layers (limited to 2009) are presented, and shown to exhibit fair correspondence with the parameterization recently suggested by Zilitinkevich et al (2007), which depends on the Coriolis parameter, friction velocity, Obukhov length, and the buoyancy frequency directly above the NBL

Theory and methods

Observed ABL heights above the canopy of Blodgett Forest

Findings

I I II III IV III II IV I III II