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Abstract This study evaluates the methods of identifying the heightz_iof 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 Lake-effect Systems (2013/14) field campaigns. Since CBLs facilitate vertical mixing near the surface, the most direct measurement ofz_iis that above which the vertical velocity turbulent fluctuations are weak or absent. Thus, we usez_ifrom the turbulence method as the “reference value” to whichz_ifrom 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 m⁻¹for soundings over land and 0.011 K m⁻¹for soundings over lake provided the estimates ofz_ithat are most consistent with the turbulence method. The Ri_bthreshold-based method, commonly used in numerical simulation studies, underestimatedz_i. Analyzing the methods’ performance on the averaging windowz_avgwe recommend usingz_avg= 20 or 50 m forz_iestimations for lake-effect boundary layers. The present dataset consists of both cloudy and cloud-free 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. Significance StatementThe depthz_iof the convective atmospheric boundary layer (CBL) strongly influences precipitation rates during lake-effect snowstorms (LES). However, variousz_iapproximation methods produce significantly different results. This study utilizes extensive concurrently collected observations by project aircraft during two LES field studies [Lake-Induced Convection Experiment (Lake-ICE) and OWLeS] to assess howz_ifrom common estimation methods compare with “reference”z_iderived from turbulent fluctuations, a direct measure of CBL mixing. For soundings taken both over land and lake; with cloudy or cloud-free conditions, potential temperature gradient (PTG) methods provided the best agreement with the referencez_i. A method commonly employed in numerical simulations performed relatively poorly. Interestingly, the PTG method worked equally well for “coupled” and elevated decoupled CBLs, commonly associated with nearby cyclones.