18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Thursday, 2 August 2001: 1:01 PM
Factors Influencing Atmospheric Internal Bore Formation Resultiing from Colliding Boundaries (formerly paper P5.17)
David E. Kingsmill, DRI, Reno, NV
The atmospheric boundary layer plays a crucial role in the initiation and evolution of convection, especially in weakly forced synoptic regimes. In these cases, warm, moist air in the boundary layer provides the fuel to energize convection. To release this energy, however, air parcels must be lifted to their level of free convection. Over the last fifty years it has been discovered that boundary layer convergence lines (often referred to as boundaries), and their associated regions of forced ascent, are important factors in this process.

When two or more boundaries collide, the amount and intensity of convection is often enhanced. The increased likelihood of convection often associated with boundary collisions has been related to a favorable balance of low-level horizontal vorticity on both sides of the collision interface, which results in more erect (i.e., vertically oriented) updrafts. Boundary collisions may also modulate kinematic, dynamic, and thermodynamic fields as well as convection development by producing internal bores. In contrast to a density current, which is a form of mass transport, the bore is a type of gravity wave that can form and propagate within a stably stratified layer of the atmosphere. Bores exhibit some characteristics, however, that are similar to density currents, such as a wind shift in the direction of their movement, a peak in convergence and vertical velocity at their leading edge, and the capability of initiating convection.

An important unresolved question is why some boundary collisions produce bores while others do not. To address this issue, fourteen boundary collision cases observed during the 1991 CaPE experiment are analyzed, five of which generated bores. Doppler radar, sounding, and surface mesonet data indicate that those boundaries with the most similar kinematic/thermodynamic characteristics and the most similar orientations are most likely to generate bores. In this presentation, I will show specific examples of these interactions and how they can be explained in the context of hydraulic theory.

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