Analysis of a Gravity Wave Train and its Influence on the Nocturnal Boundary Layer Using Direct and Indirect Measurement Systems

This research details utilization of various surface-based remote sensing instruments to characterize a gravity wave complex observed on August 10, 2014 in Oklahoma. Radar data appears to show a density current (outflow boundary) present in the form of a fine line of enhanced reflectivity that began in northern Oklahoma around 0700 UTC. Shortly thereafter, several more fine lines were detected by the radar and are thought to be the result of the density current transitioning into a gravity wave train comprised of a bore and subsequent solitary wave train. A bore is formed by an intrusion of a density current into a stable layer, behind which a gravity wave-train (soliton) can evolve. Bores change the environment through a sustained vertical increase of potential temperature and moisture content in the column, coupled with concurrent positive perturbations in surface temperature and pressure and modification of the surface kinematic environment. The vertical motions on the anterior side of gravity waves have been known to incite moist convection, which was documented in various portions of the state during this specific case.

This case study offers a detailed investigation into the applicability of various remote sensing instruments to observation of wave-like features interacting with the nocturnal boundary layer. While bore characteristics were analyzed, the primary focus was to determine if the instruments could adequately identify modifications to the nocturnal boundary layer. The Oklahoma Mesonet was used to analyze near-surface characteristics of the bore, while an Atmospheric Emitted Radiance Interferometer (AERI) and Lidar were used to characterize the vertical structure of the soliton. Data were available from AERI and Lidar instruments stationed on the roof of the National Weather Center in Norman, OK, while additional AERI data were available from the Atmospheric Radiation Measurement (ARM) facility in Lamont, Oklahoma. Vertical atmospheric profiles derived from these instruments were compared to rawinsonde data, and bore characteristics (e.g. bore strength and propagation speed) were derived and compared to hydraulic theory to determine validity of the observational data.

While the coupling of AERI and Lidar instruments adequately characterized the evolution of the nocturnal boundary layer, atmospheric characteristics above this layer were muddled by decreasing measurement accuracy and vertical resolution of the AERI. Implementation of additional remote sensing instruments (e.g. a microwave radiometer) is suggested to minimize the limitations associated with the AERI and Lidar instruments. The surface Mesonet stations recorded positive pressure perturbations and positive (negative) temperature perturbations associated with bore (density current) passage, but higher resolution ARM surface station data suggest Mesonet-derived perturbation magnitudes to be too small. AERI/LiDAR coupling adequately resolved the low-level wave duct, but failed to resolve the elevated wave duct. A bore propagated on a low-level inversion associated with the nocturnal stable surface layer, while the posterior soliton propagated within an increasingly elevated stably stratified layer induced and maintained by vertical motions of the preceding gravity wave(s).