Observations of Canopy Waves Above an Orchard by a Horizontally-scanning Aerosol Lidar

Studies of canopy waves (internal atmospheric gravity waves immediately above forest canopies) to date have primarily been performed using time-series data. In our work we show how advances in active, laser-based, remote-sensing technology have enabled the spatial observation of canopy waves. From March to June 2007, the Raman-shifted Eye-safe Aerosol Lidar (REAL) scanned horizontally above an orchard canopy nearly continuously creating images every 11 s to 30 s. The resulting spatiotemporal dataset enables us to detect the waves more easily and quantify key characteristics that have not been possible to observe until now. For example, the lidar provides a direct measurement of the wavelength and phase speed of the waves. Wavelength can be determined subjectively through visual inspection of the images or objectively through autocorrelation. Phase speed can be calculated from the cross-correlation of two sequential images. When used in conjunction with the REAL data, in situ measurements from a fully-instrumented micrometeorological mast enable a more comprehensive description of the waves and the environment that supports them. From the 3-month dataset we identified 52 episodes of canopy waves. They all occurred at night when the atmosphere from the treetops to the top of the mast (29 m AGL) was very stably stratified. The wavelengths ranged from approximately 40 m to 100 m. The phase speeds ranged from 1 m/s to 2 m/s. The phase speed is always within the range of wind speeds at canopy top (10 m) and 18 m AGL. The periods ranged from 20 s to 60 s. The crests are oriented perpendicular to the mean wind direction and they propagate downstream. The oscillations in vertical-velocity time series are 90 degrees out of phase with both the temperature and horizontal velocity, resulting in near zero heat and momentum flux in the vertical. One square kilometer area from one scan created by the REAL during canopy wave activity.