Observations of Canopy Waves from CHATS

This presentation will summarize the results of recent data analysis activities aimed at reconciling the spatial and temporal characteristics of patches of relatively "clean" canopy waves in the CHATS data set. The spatial information comes from the horizontally-scanning Raman-shifted Eye-safe Aerosol Lidar (REAL) and includes quantities such as wavelength, orientation of the wave crests and troughs, and the expanse or lateral extent of the episodes (for example, the number of wave crests present). The lidar also enables a measurement of wave propagation speed for cases when the product of the wave speed (0.5 to 2.0 m/s) and the time between lidar frames (11 to 30 s) is less than the wavelength (30 - 100 m). The vertical structure of the waves in the time domain is revealed by the ISFF vertical tower data that penetrated the REAL's horizontal scans. From the in situ data, it is possible to obtain wave period and amplitude. The ISFF data also provides the lapse rate and vertical wind shear --- quantities that in theory control the presence of the waves and their characteristics.

52 episodes of clearly discernible waves were identified in the horizontal lidar scans over 3 months of data. All of the wave cases occurred at night when strong temperature inversions were present and winds were relatively light. The episodes have been show-cased in previous presentations by this author and his students. This new work reports on recent efforts to take a closer look at the episodes and attempts to quantify and relate as many of the variables as possible. The data analysis is incomplete at the time of this writing, but preliminary results suggest poor correlations between many of the quantities that one may expect to be strongly related. For example, wave period can be obtained three ways: (1) from the wavelength and phase speed obtained by lidar; (2) from the time-series of wind and temperature resulting from the tower data; and (3) from the stability profile as Brunt-Väisälä would predict. The lidar and in situ wave periods (which ranged from 20 - 138 s) showed little to no correlation with each other and were substantially larger than the periods predicted by static stability (which ranged from 9.6 to 36.1 s).

In addition to efforts of quantifying the waves and the environments, it has become apparent that many of the cases also exhibit characteristics of turbulence. In some cases, especially apparent in the lidar data, the wave structure is asymmetric and they appear to be deteriorating quickly over time suggesting that they may be breaking.