Analysis of a Gravity Wave Train Using Direct and Indirect Measurement Systems

This case study offers: 1) a detailed investigation into the applicability of surface observation networks and vertical profiling instruments to observation of wave-like features interacting with the nocturnal boundary layer; and 2) insight into influences of bores and solitons on the nocturnal boundary layer using both observations and theory. Hydraulic theory was applied to a gravity wave-train using observationally derived pre- and post-wave boundary layer characteristics. Results of the theoretical analysis were applied to gravity wave characteristics derived from remote sensing observations to corroborate observationally derived conclusions.

Various surface-based remote sensing instruments were utilized to characterize a gravity wave complex observed on August 10, 2014 in Oklahoma. WSR-88D NEXRAD data appear 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 radar and are thought to be associated with a gravity wave train propagating in advance of the convectively generated density current. Surface observations from the Oklahoma Mesonet and Atmospheric Radiation Measurements (ARM) Southern Great Plains (SGP) surface observation network suggest the density current became progressively diffuse, which eventually allowed the gravity wave complex to advance independently. Two Atmospheric Emitted Radiance Interferometer (AERI) devices – one in northern Oklahoma and one in central Oklahoma – and a Doppler Wind Lidar (DWL) located in central Oklahoma documented the vertical evolution of the wave train and its impacts on the nocturnal boundary layer, and suggested the gravity wave complex consisted of a bore and proceeding, elevated gravity wave train.

The observational data suggest the 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). Initially, the gravity wave complex propagated atop the convectively generated density current in northern Oklahoma. As synoptic katabatic flow and hypothesized bore-related turbulence dispersed the density current air mass, the gravity wave complex began to propagate independently. The gravity wave packet progressively decayed as insolation began to destabilize the near-surface layer.

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/DWL coupling adequately resolved a low-level wave duct thought to support bore propagation, but failed to resolve the elevated wave duct necessary for the sustenance of the elevated soliton. This is suggested to be the result of an exponential decrease in AERI resolution with increasing height. It is also suggested the constraints placed on the AERI observations by the AERIoe algorithm are relaxed to determine if the AERI observations could more accurately directly resolve the elevated inversion layer.