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High-Resolution Observations of a Destructive Macroburst

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  • 1 aDepartment of Atmospheric Science, Colorado State University, Fort Collins, Colorado
  • | 2 bAtmospheric and Environmental Research, Inc., Bellevue, Nebraska
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Abstract

Shortly after 0600 UTC (midnight MDT) 9 June 2020, a rapidly intensifying and elongating convective system produced a macroburst and extensive damage in the town of Akron on Colorado’s eastern plains. Instantaneous winds were measured as high as 51.12 m s−1 at 2.3 m AGL from an eddy covariance (EC) tower, and a 50.45 m s−1 wind gust from an adjacent 10-m tower became the highest official thunderstorm wind gust ever measured in Colorado. Synoptic-scale storm motion was southerly, but surface winds were northerly in a postfrontal air mass, creating strong vertical wind shear. Extremely high-resolution temporal and spatial observations allow for a unique look at pressure and temperature tendencies accompanying the macroburst and reveal intriguing wave structures in the outflow. At 10-Hz frequency, the EC tower recorded a 5-hPa pressure surge in 19 s immediately following the strongest winds, and a 15-hPa pressure drop in the following 3 min. Surface temperature also rose 1.5°C in less than 1 min, concurrent with the maximum wind gusts, and then fell sharply by 3.5°C in the following minute. Shifting wind direction observations and an NWS damage survey are suggestive of both radial outflow and a gust front passage, and model proximity soundings reveal a well-mixed surface layer topped by a strong inversion and large low-level vertical wind shear. Despite the greatest risk of severe winds forecast to be northeast of Colorado, convection-allowing model forecasts from 6 to 18 h in advance did show similar structures to what occurred, warranting further simulations to investigate the unique mesoscale and misoscale features associated with the macroburst.

Significance Statement

A macroburst occurred in the early morning hours of 9 June in Akron, Colorado, causing extensive damage and widespread power outages. The macroburst and related wavelike features were captured at high resolution by several weather observation stations in the surrounding area and reveal unprecedented oscillations in both temperature and pressure on scales of seconds. The strongest convective wind gusts in Colorado history were also recorded at a 10-m tower outside of Akron, in excess of 50 m s−1. Atmospheric soundings show low-level moisture and dry air aloft, but a sharp low-level temperature inversion that is not typical of a downburst profile. Observational and modeling analyses reveal the potential for gravity wave interactions in addition to strong radial outflow from the macroburst and motivate further work to investigate the complex small-scale processes of this event.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Samuel J. Childs, sam.childs20@alumni.colostate.edu

Abstract

Shortly after 0600 UTC (midnight MDT) 9 June 2020, a rapidly intensifying and elongating convective system produced a macroburst and extensive damage in the town of Akron on Colorado’s eastern plains. Instantaneous winds were measured as high as 51.12 m s−1 at 2.3 m AGL from an eddy covariance (EC) tower, and a 50.45 m s−1 wind gust from an adjacent 10-m tower became the highest official thunderstorm wind gust ever measured in Colorado. Synoptic-scale storm motion was southerly, but surface winds were northerly in a postfrontal air mass, creating strong vertical wind shear. Extremely high-resolution temporal and spatial observations allow for a unique look at pressure and temperature tendencies accompanying the macroburst and reveal intriguing wave structures in the outflow. At 10-Hz frequency, the EC tower recorded a 5-hPa pressure surge in 19 s immediately following the strongest winds, and a 15-hPa pressure drop in the following 3 min. Surface temperature also rose 1.5°C in less than 1 min, concurrent with the maximum wind gusts, and then fell sharply by 3.5°C in the following minute. Shifting wind direction observations and an NWS damage survey are suggestive of both radial outflow and a gust front passage, and model proximity soundings reveal a well-mixed surface layer topped by a strong inversion and large low-level vertical wind shear. Despite the greatest risk of severe winds forecast to be northeast of Colorado, convection-allowing model forecasts from 6 to 18 h in advance did show similar structures to what occurred, warranting further simulations to investigate the unique mesoscale and misoscale features associated with the macroburst.

Significance Statement

A macroburst occurred in the early morning hours of 9 June in Akron, Colorado, causing extensive damage and widespread power outages. The macroburst and related wavelike features were captured at high resolution by several weather observation stations in the surrounding area and reveal unprecedented oscillations in both temperature and pressure on scales of seconds. The strongest convective wind gusts in Colorado history were also recorded at a 10-m tower outside of Akron, in excess of 50 m s−1. Atmospheric soundings show low-level moisture and dry air aloft, but a sharp low-level temperature inversion that is not typical of a downburst profile. Observational and modeling analyses reveal the potential for gravity wave interactions in addition to strong radial outflow from the macroburst and motivate further work to investigate the complex small-scale processes of this event.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Samuel J. Childs, sam.childs20@alumni.colostate.edu

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