Temporal evolution of wavelength and orientation of atmospheric canopy waves

We apply an algorithm to horizontally scanning aerosol lidar images and objectively determine canopy wave wavelength and wave crest orientation. We apply the algorithm (described by Mifsud et al., 2021) to scans from the Raman-shifted Eye-safe Aerosol Lidar (REAL) from the Canopy Horizontal Array Turbulence Study (CHATS). The data set includes 53 episodes of canopy waves distrubed over a three-month period (Mayor, 2017). Episodes ranged from a few minutes in duration to more than an hour in some cases. Therefore, the number of scans (or frames) in an episode ranged from 7 to 148. The algorithm determines a single salient wavelength and orientation angle of the waves from a square subset of pixels that is typically hundreds of meters wide in both horizontal directions in a single lidar scan. By applying the algorithm to all frames in an episode we can efficiently and objectively monitor the wave characteristics over time. An example (episode 19) is shown below. One scan of 31 in the episode is shown in the left panel and the result of the repeated application of the algorithm is shown on the right. It reveals that the wavelength increased from approximately 51 meters at 7:57 UTC to almost 70 meters at 8:01 UTC. At the same time the orientation of the waves ranged from 111 to 115.5 degrees with no perceptible trend. Considering the results of Mifsud et al. (2021), the change in wavelength shown appears to be real and not an artifact of the algorithm. At the time of this writing we have not yet investigated what this change may be related to or caused by. We plan to show how wavelength and wave crest orientation evolve over all episodes and compare those results with in situ measurements from coincident tower-mounted ultrasonic anemometers and other fast-response sensors.

References

Mayor, S. D., 2017: Observations of microscale internal gravity waves in very stable atmospheric boundary layers over an orchard canopy. Agric. For. Meteorol. 244-245, 136-150.

Mifsud, K. E., M. Iqbal, P. Dérian, S. D. Mayor, 2021: Objective determination of wavelength and orientation of atmospheric canopy waves. Poster A45C-1867 at Fall meeting of the American Geophysical Union, 16 December, New Orleans, LA.