Horizontal Convective Rolls in Cold Air over Water: Buoyancy Characteristics of Coherent Plumes Detected by an Airborne Radar

Qiong Yang Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming

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Bart Geerts Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming

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Abstract

Aircraft and airborne cloud radar data are used to describe the vertical structure of the convective boundary layer (CBL) during cold-air outbreaks over Lake Michigan in January 2004. Two days with mesoscale cloud street structure and a day with cellular organization are contrasted. The radar reflectivity and vertical velocity structure of the CBL, as well as the radar-inferred topography of the CBL inversion, are collected along flight legs normal to the cloud streets. High-resolution horizontal and vertical transects of the dual-Doppler airflow field capture horizontal convective roll circulations on one day.

Coherent structures within the CBL are analyzed as echo plumes, updraft plumes, and CBL domes. Only updraft plumes have the characteristics of buoyant thermals. Updrafts are narrower, weaker, and less buoyant on the no-roll day, but the differences in characteristics between two cloud street days are larger than those between the no-roll day and the two cloud street days. The lack of a clear buoyancy signal in echo plumes and under CBL domes is attributed to a temporal phase shift between maximum buoyancy, maximum ice particle size, and maximum overshooting in thermals, and the transience of convective updrafts.

Corresponding author address: Bart Geerts, Department of Atmospheric Sciences, University of Wyoming, Laramie, WY 82071. Email: geerts@uwyo.edu

Abstract

Aircraft and airborne cloud radar data are used to describe the vertical structure of the convective boundary layer (CBL) during cold-air outbreaks over Lake Michigan in January 2004. Two days with mesoscale cloud street structure and a day with cellular organization are contrasted. The radar reflectivity and vertical velocity structure of the CBL, as well as the radar-inferred topography of the CBL inversion, are collected along flight legs normal to the cloud streets. High-resolution horizontal and vertical transects of the dual-Doppler airflow field capture horizontal convective roll circulations on one day.

Coherent structures within the CBL are analyzed as echo plumes, updraft plumes, and CBL domes. Only updraft plumes have the characteristics of buoyant thermals. Updrafts are narrower, weaker, and less buoyant on the no-roll day, but the differences in characteristics between two cloud street days are larger than those between the no-roll day and the two cloud street days. The lack of a clear buoyancy signal in echo plumes and under CBL domes is attributed to a temporal phase shift between maximum buoyancy, maximum ice particle size, and maximum overshooting in thermals, and the transience of convective updrafts.

Corresponding author address: Bart Geerts, Department of Atmospheric Sciences, University of Wyoming, Laramie, WY 82071. Email: geerts@uwyo.edu

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