TURBULENCE NEAR THE FOREST FLOOR OF AN OLD-GROWTH DOUGLAS-FIR STAND ON A SOUTH-FACING SLOPE

This paper reports the results of the analysis of measured turbulence regimes near the forest floor in an old-growth Douglas-fir stand on a south-facing slope in northern Vancouver Island, Canada. Primary instr umentation included one eddy correlation unit, which consisted of a th ree-dimensional sonic anemometer, a krypton hygrometer and a fine wire thermocouple, and four home-made hot wire anemometers. The general fe atures of the turbulence regimes near the forest floor within this sta nd were similar to those observed previously in other stands with an o pen trunk space. The high value of the ratio of the wind speed inside the stand to that outside (0.42) suggested the existence of a secondar y maximum in the stand wind profile. The wind speed near the forest fl oor was approximately a logarithmic function of height with an effecti ve roughness length of 0.012 m. The average turbulence intensity was 0 .86. Power spectra for the streamwise and lateral velocity components exhibited a bimodal distribution in contrast with a unimodal distribut ion for the spectrum of the vertical component. Near the forest floor, latent heat and sensible heat generally flowed down humidity and temp erature gradients, respectively. Some unique features were also observ ed, namely the suppression of the vertical velocity variance by the mo derate to strong temperature inversion in the daytime and the occurren ce of a very small eddy diffusivity for sensible heat flux.