Mean and Turbulence Statistics in a Wintertime Convectively-mixed Boundary Layer Over Lake Michigan

Abstract

First, second, and third moment statistics for u, v, w, θ ν, and q have been derived from the Lake-Effect Snow Studies (LESS) 20 Hz aircraft data set collected in a northerly flow cold air outbreak over Lake Michigan on 10 January 1984. Vertical profiles of mean and turbulence statistics in a Type I Cloud-Topped Boundary Layer (CTBL) have been presented, discussed, and compared with similar observational case studies during the Air Mass Transformation Experiment (AMTEX) and the Mesoscale Air-Sea Exchange (MASEX), as well as with numerical model simulations of convective boundary layers as presented by Deardorff (1974, 1980) and Moeng (1984). LESS results show the importance of a more dynamically-active cloud layer in the normalized vertical profile of vertical velocity variance, covariance of horizontal and vertical wind components, buoyancy, turbulent kinetic energy, TKE, vertical transport of TKE, and vertical velocity skewness in Type I CTBLs. Turbulence statistics are also examined with respect to the effect of strong heating at the air-lake interface and entrainment effects near the top of the CTBL. Comparison with model simulations continue to show the need for better agreement with observational results. Finally, selected profiles of LESS turbulence statistics are presented and discussed to show the sensitivity (or lack thereof) between results derived from raw as opposed to detrended data.