Structure of an Internal Bore and Dissipating Gravity Current as Revealed by Raman Lidar

Abstract

Detailed moisture observations from a ground-based Raman lidar and special radiosonde data of two disturbances associated with a dissipating gust front are presented. A synthesis of the lidar data with conventional meteorological data, in conjunction with theoretical calculations and comparison to laboratory studies, leads to the conclusion that the disturbances seen in both the lidar and accompanying barograph data represent a weak gravity current and an associated undular bore. The disturbances display excellent coherence over hundreds of kilometers upstream of the lidar site. Bore formation occurs at the leading edge of the gust front coincidentally with the rapid weakening of the gravity current. Analysis suggests that the bore was generated by the collapse of the gravity current into a stable, nocturnal inversion layer, and subsequently propagated along this wave guide at nearly twice the speed of the gravity current. The Raman lidar provided detailed measurements of the vertical structure of the bore and its parent generation mechanism. A mean bore depth of 1.9 km is revealed by the lidar, whereas a depth of 2.2 km is predicted from hydraulic theory. Observed and calculated bore speeds were also found to agree reasonably well with one another (? ±20%). Comparison of these observations with those of internal bores generated by thunderstorms in other studies reveals that this bore was exceedingly strong, being responsible for nearly tripling the height of a surface-based inversion that had existed ahead of the bore and dramatically increasing the depth of the moist layer due to strong vertical mixing. Subsequent appearance of the relatively shallow gravity current underneath this mixed region resulted in the occurrence of an elevated mixed layer, as confirmed with the special radiosonde measurements. A synthesis of the lidar and radiosonde observations indicates that bore-induced parcel displacements attenuated rapidly at the same height as the level of strongest wave trapping predicted from the theory of Crook. This trapping mechanism, which is due to the existence of a low-level jet, results in a long-lived bore, and seems to he a common phenomenon in the environment of thunderstorm-generated bores and solitary waves. Despite the weakening of a capping inversion by this strong and persistent bore, analysis indicates that the 30-min averaged lifting of 0.7 m s?1 was confined to a too shallow layer near the surface to trigger deep convection, and could only produce scattered low clouds as deduced from the lidar measurements.