The Frontal Hydraulic Head: A Micro-α Scale (∼1 km) Triggering Mechanism for Mesoconvective Weather Systems

M. A. Shapiro NOAA/ERL/Wave Propagation Laboratory, Boulder, CO 80303

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Tamara Hampel NOAA/ERL/Wave Propagation Laboratory, Boulder, CO 80303

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Doris Rotzoll Space Science and Engineering Center, University of Wisconsin, Madison, WI 53706

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F. Mosher Space Science and Engineering Center, University of Wisconsin, Madison, WI 53706

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Abstract

Measurements from the NOAA Boulder Atmospheric Observatory (BAO) 300 m tower, the National Center for Atmospheric Research (NCAR) Sabreliner aircraft, and the NOAA GOES-5 satellite, give evidence for the cross-front scale collapse of nonprecipitating surface cold-frontal zones to horizontal distances of ∼1 km or less. The leading edges of these frosts possess the characteristic structure of density current flows: an elevated hydraulic head followed by a turbulent wake. Vertical motions at the frontal heads exceed 5 m s−1 at 300 m (AGL). The ascent at the frontal head may act as a (∼1 km-scale) triggering mechanism for the release of potential instability and the formation of intense squall-line mesoconvection.

Abstract

Measurements from the NOAA Boulder Atmospheric Observatory (BAO) 300 m tower, the National Center for Atmospheric Research (NCAR) Sabreliner aircraft, and the NOAA GOES-5 satellite, give evidence for the cross-front scale collapse of nonprecipitating surface cold-frontal zones to horizontal distances of ∼1 km or less. The leading edges of these frosts possess the characteristic structure of density current flows: an elevated hydraulic head followed by a turbulent wake. Vertical motions at the frontal heads exceed 5 m s−1 at 300 m (AGL). The ascent at the frontal head may act as a (∼1 km-scale) triggering mechanism for the release of potential instability and the formation of intense squall-line mesoconvection.

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