Doppler lidar observations of internal gravity waves, shear instabilities and turbulence during CASES-99

Documentation of the evolution of internal gravity waves, Kelvin-Helmholtz shear instabilities and turbulence events in the stable nocturnal boundary layer (SBL) were principal scientific goals of the CASES-99 field observation program. During its month-long deployment at the CASES-99 field site the NOAA/ETL High Resolution Doppler Lidar (HRDL) provided a wealth of data on the spatial structure and temporal evolution of waves and shear instabilities in the SBL. This paper catalogues these events and summarizes the physical characteristics of the observed structures.

The lidar recorded data on a total of 15 nights between October 4 and October 28, 1999. Periods of wave-like activity were usually episodic, lasting anywhere from several minutes to several hours. The degree of spatial and temporal coherence varied from one observation period to another. Less coherent, shorter wavelength (<~300m) oscillations associated with shear instability below the low-level jet were frequently observed. Shear instability waves of varying amplitude appeared during almost every night of observation. On several nights these short wavelength disturbances persisted for several hours. Highly coherent longer wavelength (>300m) oscillations were observed much less frequently. We have been able to identify only three nights in which such oscillations were observed by HRDL. Of these three nights, waves below the low-level jet were indicated on two nights and waves above the jet were indicated on one night. In addition to documenting the wave events, estimates of coherence length, wavelength, wave amplitude and phase as functions of height for a number of specific cases are presented.