OBSERVATION OF GRAVITY-WAVES IN A BOREAL FOREST

In this paper we report the results of the analysis of two 60-min wave events that occurred in a boreal aspen forest during the 1994 BOREAS (Boreal Ecosystems-Atmosphere Study) field experiment. High frequency wind and temperature data were provided by three 3-D sonic anemometer/ thermometers and fourteen fine-wire thermocouples positioned within an d above the forest. Wave phase speeds, estimated from information reve aled by spectral analysis and linear plane wave equations, are 2.2 and 1.3 m s(-1) for the two events. The wavelengths are 130 m and 65 m re spectively and are much larger than the vertical wave displacements. T here is strong evidence from the present analysis and from the literat ure supporting our postulate that these waves are generated by shear i nstability. We propose that wind shear near the top of the stand is of ten large enough to reduce the gradient Richardson number below the cr itical value of 0.25 and thus is able to trigger the instability. When external conditions are favorable, the instability will grow into wav es.