Transitions in boundary layer meso-gamma convective structures: An observational case study

Boundary layer rolls over Lake Michigan have been observed in wintertime co nditions predicted by many past studies to favor nonroll convective structu res (such as disorganized convection or cellular convection). This study ex amines mechanisms that gave rise to transitions between boundary layer roll s and more cellular convective structures observed during a lake-effect sno w 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 overt urning rate within the boundary layer during periods of convective transiti on. A method of quantifying the uniformity of convection along the roll axe s, based on dual-Doppler radar-derived vertical motions, was developed to q uantify changes in boundary layer convective structure. Roll formation was found to occur after (within 1 h) increases in low-level wind speeds and sp eed shear primarily below about 0.32z(i), with little change in directional shear within the convective boundary layer. Roll convective patterns appea red to initiate upstream of the sample region, rather than form locally nea r the downwind shore of Lake Michigan. These findings suggest that either r olls developed over the upwind half of Lake Michigan or that the convection had a delayed response to changes in the atmospheric surface and wind forc ing. 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 r elationship with changes in convective structure. However, when rolls were dominant, the heaviest snow was more concentrated in updraft regions than d uring more cellular time periods.