An Observational and Modeling Study of an Atmospheric Internal Bore during NAME 2004

Abstract

Observations from the 2004 North American Monsoon Experiment (NAME) have been used to identify an atmospheric internal bore that occurred over the Gulf of California (GoC) on 31 July 2004. This bore disturbance was identified at Bahia Kino along the northwest coast of Mexico during the late evening of 31 July. It was hypothesized to have originated from the interaction of a gravity current from a large mesoscale convective system (MCS), which formed along the western slopes of the Sierra Madre Occidental during the afternoon, and a surface stable layer that developed from a sea-breeze circulation. It is suggested that the bore?s energy was trapped at low levels by an elevated stable layer. The vertical structure and undular nature of the bore was initially identified from 915-MHz wind profiler data at Bahia Kino. Results show a series of waves along the bore?s leading edge and turbulent mixing of air from above the stable layer to the surface on the downstream face of the leading undulation. The speed of the bore calculated from satellite imagery and surface observations (approximately 16.8 m s?1) compared favorably with the speed of a bore from hydraulic theory when a reliable estimate of the bore depth was used. A real-data simulation of the event was performed using the Weather Research and Forecasting model (WRF). Results show the model captured both the formation mechanism and structure of the bore, but it was produced too far south compared to observations, as the MCS also developed too far south. Model results indicated that while evidence of a trapping mechanism due to the stability of the atmosphere was present in the simulation, the conditions for trapping were modified by the passage of the bore allowing vertical propagation of wave energy. The bore led to increased moisture in the lowest levels of the atmosphere across the GoC, providing evidence of the possible importance of these features as moisture transport mechanisms in this region.