Atmospheric Undular Bores
Observations from the 2004 North American Monsoon Experiment (NAME) were used in conjunction with numerical modeling experiments (using WRF) to investigate atmospheric undular bores in the region of the North American monsoon.
Atmospheric bores had not previously been identified in this region. It is important to identify and understand atmospheric bores in this region for several reasons. First, circulations produced by bores (and other similar phenomenon) may be capable of directly triggering convection. Second, accelerated and increased depth of the flow behind the leading edge of a bore may be responsible for increased moisture transport into the southwestern United States, providing the fuel for enhanced convection
Two atmospheric bores were identified during NAME 2004 by vertically pointing wind profilers and one studied in depth and compared with a simple gravity wave. Surface variables show a strong signal with the passage of the bore as shown below. The bore propagated along a low-level stable layer and was trapped in the lower atmosphere by an elevated stable layer.
One-minute resolution observations from the surface meteorological station at Bahia Kino, 31 Jul 2004: (a) pressure anomaly (green) (hPa), temperature (red) (°C), and dewpoint temperature (blue) (°C) and (b) wind speed (dotted) (m s−1) and wind direction (solid).
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. A cross section of the simulated bore is shown below.
Model cross section at 0400 UTC of potential temperature (K) (solid contours) and relative humidity (shaded above 70%) from 0 to 5 km.