Editorial Type:
Article
Article Type:
Research Article

Transitions in Boundary Layer Meso-γ Convective Structures:An Observational Case Study

David A. R. Kristovich Atmospheric Environment Section, Illinois State Water Survey, Champaign, Illinois

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Neil F. Laird Atmospheric Environment Section, Illinois State Water Survey, Champaign, Illinois

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Mark R. Hjelmfelt Institute for Atmospheric Sciences, South Dakota School of Mines and Technology, Rapid City, South Dakota

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Russell G. Derickson Institute for Atmospheric Sciences, South Dakota School of Mines and Technology, Rapid City, South Dakota

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Kevin A. Cooper Institute for Atmospheric Sciences, South Dakota School of Mines and Technology, Rapid City, South Dakota

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Print Publication:
01 Dec 1999
DOI:
https://doi.org/10.1175/1520-0493(1999)127<2895:TIBLMC>2.0.CO;2
Page(s):
2895–2909
Restricted access

Abstract

Boundary layer rolls over Lake Michigan have been observed in wintertime conditions predicted by many past studies to favor nonroll convective structures (such as disorganized convection or cellular convection). This study examines mechanisms that gave rise to transitions between boundary layer rolls and more cellular convective structures observed during a lake-effect snow event over Lake Michigan on 17 December 1983. The purposes of this study are to better understand roll formation in marine boundary layers strongly heated from below and examine the evolution of snowfall rate and mass overturning rate within the boundary layer during periods of convective transition. A method of quantifying the uniformity of convection along the roll axes, based on dual-Doppler radar-derived vertical motions, was developed to quantify changes in boundary layer convective structure. Roll formation was found to occur after (within 1 h) increases in low-level wind speeds and speed shear primarily below about 0.3zi, with little change in directional shear within the convective boundary layer. Roll convective patterns appeared to initiate upstream of the sample region, rather than form locally near the downwind shore of Lake Michigan. These findings suggest that either rolls developed over the upwind half of Lake Michigan or that the convection had a delayed response to changes in the atmospheric surface and wind forcing. Mass overturning rates at midlevels in the boundary layer peaked when rolls were dominant and gradually decreased when cellular convection became more prevalent. Radar-estimated aerial-mean snowfall rates showed little relationship with changes in convective structure. However, when rolls were dominant, the heaviest snow was more concentrated in updraft regions than during more cellular time periods.

Corresponding author address: Dr. David A. R. Kristovich, Illinois State Water Survey, 2204 Griffith Dr., Champaign, IL 61820-7495.

Email: dkristo@uiuc.edu

Abstract

Boundary layer rolls over Lake Michigan have been observed in wintertime conditions predicted by many past studies to favor nonroll convective structures (such as disorganized convection or cellular convection). This study examines mechanisms that gave rise to transitions between boundary layer rolls and more cellular convective structures observed during a lake-effect snow event over Lake Michigan on 17 December 1983. The purposes of this study are to better understand roll formation in marine boundary layers strongly heated from below and examine the evolution of snowfall rate and mass overturning rate within the boundary layer during periods of convective transition. A method of quantifying the uniformity of convection along the roll axes, based on dual-Doppler radar-derived vertical motions, was developed to quantify changes in boundary layer convective structure. Roll formation was found to occur after (within 1 h) increases in low-level wind speeds and speed shear primarily below about 0.3zi, with little change in directional shear within the convective boundary layer. Roll convective patterns appeared to initiate upstream of the sample region, rather than form locally near the downwind shore of Lake Michigan. These findings suggest that either rolls developed over the upwind half of Lake Michigan or that the convection had a delayed response to changes in the atmospheric surface and wind forcing. Mass overturning rates at midlevels in the boundary layer peaked when rolls were dominant and gradually decreased when cellular convection became more prevalent. Radar-estimated aerial-mean snowfall rates showed little relationship with changes in convective structure. However, when rolls were dominant, the heaviest snow was more concentrated in updraft regions than during more cellular time periods.

Corresponding author address: Dr. David A. R. Kristovich, Illinois State Water Survey, 2204 Griffith Dr., Champaign, IL 61820-7495.

Email: dkristo@uiuc.edu

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