Identification of Convective Boundary Layer Depth over the Great Lakes Region Using Aircraft Observations: A Comparison of Various Methods

This study evaluates the methods of identifying the height z_i of the top of the convective boundary layer (CBL) during winter (December and January) over the Great Lakes and nearby land areas using observations taken by the University of Wyoming King Air research aircraft during the Lake-Induced Convection Experiment (1997/98) and Ontario Winter Lakeeffect Systems (2013/14) field campaigns. Since CBLs facilitate vertical mixing near the surface, the most direct measurement of z_i is that above which the vertical velocity turbulent fluctuations are weak or absent. Thus, we use z_i from the turbulence method as the “reference value” to which z_i from other methods, based on bulk Richardson number (Ri_b), liquid water content, and vertical gradients of potential temperature, relative humidity, and water vapor mixing ratio, are compared. The potential temperature gradient method using a threshold value of 0.015 K m²¹ for soundings over land and 0.011 K m²¹ for soundings over lake provided the estimates of z_i that are most consistent with the turbulence method. The Ri_b threshold-based method, commonly used in numerical simulation studies, underestimated z_i. Analyzing the methods’ performance on the averaging window z_avg we recommend using z_avg 5 20 or 50 m for z_i estimations for lake-effect boundary layers. The present dataset consists of both cloudy and cloudfree boundary layers, some having decoupled boundary layers above the inversion top. Because cases of decoupled boundary layers appear to be formed by nearby synoptic storms, we recommend use of the more general term, elevated mixed layers.