CLIMATOLOGY OF GRAVITY-WAVES IN A FOREST

The objectives of this study are to establish a climatology of gravity waves in a forest and to gain insights into the dynamics of this moti on type. The site is part of the BOREAS (Boreal Ecosystem-Atmosphere S tudy) flux monitoring network. The analysis relies mainly on 5 Hz temp erature time series observed with two arrays of fine-wire thermocouple s deployed in the vertical (up to 40 m above the ground) and horizonta l (separation up to 150 m) from July to November 1996. Supporting meas urements include mean wind speed and direction at two heights above th e forest. Waves of various intensities are found over 40% of the night -time observations, emphasizing the need to understand its role in tra nsporting momentum and masses between the vegetation and the atmospher e. These waves are shear-generated. They propagate in the direction of the mean wind at a speed that is most likely to fall between the mean wind speeds at z/h = 1.2 and 1.8, where z is the height above the gro und and h is the mean tree height. The median values of wave speed, wa velength, wave vertical displacement, and wave frequency are 1.61 m s( -1) 75 m, 10 m and 0.0214 Hz, respectively. The wave motion remains co herent in the horizontal over less than one wavelength but is in gener al persistent in time.