A study of the effect of Lake Erie on deep convective systems

Mesoscale;Boundary Layer;Marine Boundary Layer;Deep Convection;Great Lakes;Climatology;Squall Line

Workoff, Thomas ; Kristovich ; David A.

关键词: Mesoscale;    Boundary Layer;    Marine Boundary Layer;    Deep Convection;    Great Lakes;    Climatology;    Squall Line;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/16480/2_Workoff_Thomas.pdf?sequence=3&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

This study examines the effect that Lake Erie, and its corresponding marine boundary layer (MBL), have on pre-existing convective storms which propagate over the water surface.To do this, data collected from the 2001-2007 period was analyzed to identify trends in storm behavior over Lake Erie based on season, time of day, convective type and changes in surface-based atmospheric instability due to the lake.Additionally, a study of the 26 July 2005 squall line case was conducted to identify processes by which the squall line interacted with a cool MBL, as well the effects on the convection caused by the movement from land to water. Results of the climatological study identified 89 cases of pre-existing convection(>45 dBZ) that moved over Lake Erie during the 7-year period. Of these storms, 24 were classified as ‘clusters’, 20 ‘isolated’, 27 ‘linear’ and 18 ‘complex.’The evolution of each type of convective system, depicted by changes in the storm’s maximum base reflectivity after spending 30 and 60 minutes over the water, was related to environmental parameters characterizing the change in surface conditions over Lake Erie. Analyses revealed that noteworthy changes in the maximum reflectivity of the systems did not occur until 60 minutes over the water.Additionally, cluster and isolated systems tended to weaken in cases where the over-lake environment was convectively unfavorable (i.e. reduced surface-based instability), while linear and complex systems tended to be less affected the lake, regardless of the over-lake conditions.Correlations conducted between several parameters and maximum reflectivity change suggest that no one parameter was the cause of the observed storm behavior over the lake, although linear systems showed high correlation of storm weakening in cases of small low-level (2.5 km) vertical wind shear.The squall line case of 26 July 2005 was examined in order to understand processes by which Lake Erie and its associated MBL can influence a mature squall line.On this date, a squall line developed over eastern Michigan and propagated eastward over the relatively cool surface of Lake Erie.Surface observations show the presence of a cooler, mesoscale airmass located over and north of Lake Erie (~5oC cooler than the surrounding land).The effect of the MBL on the squall line was hypothesized based on previous squall line theory which relates storm intensity to the ratio between squall line’s cold pool intensity and the ambient low-level wind shear.Results show that the squall line’s cold pool would have encountered a MBL which became increasing deep and cooler at the surface as it moved east through southern Ontario toward Lake Erie.This was expected to reduce cold pool strength by reducing the buoyancy gradient at the cold pool’s leading edge.Radar observations show the squall line experienced an increase in maximum reflectivity and a decrease in forward speed as it approached Lake Erie, consistent with previous numerical modeling studies of squall line/cool layer interaction and increasing cold pool strength.As the squall line moved eastward over the lake, the convection interacted with a MBL that became increasingly warmer at the surface and more shallow.This, in combination with the storm experiencing a decrease in surface friction upon movement from land to water, was expected to increase the cold pool strength.Consequently, the squall line was observed to develop two small-scale bow echoes, the convective updraft became tilted toward the rear, and the system experienced an increase in forward speed while over the water.This behavior is agreement with theory regarding the cold pool overwhelming the ambient shear and accelerating forward at the surface.Furthermore, the observed rapid alteration of the system in correlation with movement from land to water suggests the decrease in friction played a larger role in system alteration than surface-based buoyancy changes.