Structure of an Atmospheric Undular Bore Generated from Colliding Boundaries during CaPE

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  • 1 Department of Atmospheric Sciences, University of California, Los Angeles, Los Angeles, California
  • | 2 National Center for Atmospheric Research, Boulder, Colorado
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Abstract

A case study of a sea-breeze front originating from the east coast of Florida colliding with a gust front moving toward the southeast is presented. Single- and multi-Doppler radar analyses combined with serial balloon ascents suggest that the denser sea-breeze flow undercut the cold pool behind the gust front and may have generated a westward-propagating undular bore. In addition, another undular bore was generated by the collision and propagated eastward through the CaPE network. The latter bore propagated upstream, against the southeasterly flow behind the sea-breeze front and was apparent as a narrow band, 5–6 km wide, of near-zero radial velocities embedded within an overall flow that was positive as seen by the CP-4 Doppler radar.

As the eastern bore propagated through the dual-Doppler lobe defined by the NCAR CP-3 and CP-4 radars, a detailed and unique wind synthesis revealed its three-dimensional kinematic structure. Significant along-bore variability was shown in the convergence and vertical velocity fields. Vertical cross sections of the synthesized wind were similar to past studies of wavelike phenomena embedded within an ambient flow. A sounding was launched within the leading edge of the undular bore as it passed over a CLASS site. The thermodynamic profile revealed a change from a frontal stable layer to a well-mixed zone indicative of forced uplift of surface air parcels within, the cold air behind the sea-breeze front. Theoretical calculations of the propagation speed and wavelength of an undular bore closely matched the observations.

Abstract

A case study of a sea-breeze front originating from the east coast of Florida colliding with a gust front moving toward the southeast is presented. Single- and multi-Doppler radar analyses combined with serial balloon ascents suggest that the denser sea-breeze flow undercut the cold pool behind the gust front and may have generated a westward-propagating undular bore. In addition, another undular bore was generated by the collision and propagated eastward through the CaPE network. The latter bore propagated upstream, against the southeasterly flow behind the sea-breeze front and was apparent as a narrow band, 5–6 km wide, of near-zero radial velocities embedded within an overall flow that was positive as seen by the CP-4 Doppler radar.

As the eastern bore propagated through the dual-Doppler lobe defined by the NCAR CP-3 and CP-4 radars, a detailed and unique wind synthesis revealed its three-dimensional kinematic structure. Significant along-bore variability was shown in the convergence and vertical velocity fields. Vertical cross sections of the synthesized wind were similar to past studies of wavelike phenomena embedded within an ambient flow. A sounding was launched within the leading edge of the undular bore as it passed over a CLASS site. The thermodynamic profile revealed a change from a frontal stable layer to a well-mixed zone indicative of forced uplift of surface air parcels within, the cold air behind the sea-breeze front. Theoretical calculations of the propagation speed and wavelength of an undular bore closely matched the observations.

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