An Undular Bore and Gravity Waves Illustrated by Dramatic Time-Lapse Photography

Timothy A. Coleman Department of Atmospheric Science, University of Alabama in Huntsville, Huntsville, Alabama

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Kevin R. Knupp Department of Atmospheric Science, University of Alabama in Huntsville, Huntsville, Alabama

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Daryl E. Herzmann Department of Agronomy, Iowa State University, Ames, Iowa

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Abstract

On 6 May 2007, an intense atmospheric undular bore moved over eastern Iowa. A “Webcam” in Tama, Iowa, captured dramatic images of the effects of the bore and associated gravity waves on cloud features, because its viewing angle was almost normal to the propagation direction of the waves. The time lapse of these images has become a well-known illustration of atmospheric gravity waves. The environment was favorable for bore formation, with a wave-reflecting unstable layer above a low-level stable layer. Surface pressure and wind data are correlated for the waves in the bore, and horizontal wind oscillations are also shown by Doppler radar data. Quantitative analysis of the time-lapse photography shows that the sky brightens in wave troughs because of subsidence and darkens in wave ridges because of ascent.

Corresponding author address: Tim Coleman, Department of Atmospheric Science, University of Alabama in Huntsville, NSSTC, 320 Sparkman Drive, Huntsville, AL 35805. Email: coleman@nsstc.uah.edu

Abstract

On 6 May 2007, an intense atmospheric undular bore moved over eastern Iowa. A “Webcam” in Tama, Iowa, captured dramatic images of the effects of the bore and associated gravity waves on cloud features, because its viewing angle was almost normal to the propagation direction of the waves. The time lapse of these images has become a well-known illustration of atmospheric gravity waves. The environment was favorable for bore formation, with a wave-reflecting unstable layer above a low-level stable layer. Surface pressure and wind data are correlated for the waves in the bore, and horizontal wind oscillations are also shown by Doppler radar data. Quantitative analysis of the time-lapse photography shows that the sky brightens in wave troughs because of subsidence and darkens in wave ridges because of ascent.

Corresponding author address: Tim Coleman, Department of Atmospheric Science, University of Alabama in Huntsville, NSSTC, 320 Sparkman Drive, Huntsville, AL 35805. Email: coleman@nsstc.uah.edu

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