Measurements of the Turbulence Master Length Scale Profile in Summertime Eastern North Atlantic Marine Boundary Layer Clouds

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Description

Scalar transports in the marine boundary layer result from boundary layer turbulence that is quantified by the mass-weighted, or specific, Turbulent Kinetic Energy (TKE). Forecasting these transports requires the prognostic equation for TKE, which contains turbulent covariance terms often parameterized using first-order closure, or K-Theory (down-gradient diffusion). First-order closure relates the turbulent covariances to large scale gradients predicted in models through coefficients, K, often termed as eddy viscosities or exchange coefficients. Hence, turbulent transports of heat, moisture, and momentum in models using this formalism rely on the specification of the K’s, and many boundary layer models employ parameterizations of the K’s suggested by Mellor and Yamada (1982) as the underlayment of their boundary layer turbulence physics. Mellor and Yamada postulated that the K’s are products of a stability function, TKE, and a turbulence master length scale, L, which is the TKE dissipation length scale. Typically, values of L computed from Large Eddy Simulations (LES) in different conditions range in magnitude from 10-150 m with small values found at the top of the surface layer and the largest values found at the top of the mixed-layer. On the basis of these limited simulations, a profile of L suggested by Blackadar (1962) is typically assumed (Bretherton and Park, 2009, for example). Mellor and Yamada (1982) state that “The major weakness of all of the turbulent closure models probably relates to the turbulent master length scale and, most important, to the fact that one sets all process scales proportional to a single scale”. Thus, three facts are well established: the profile of L is a critical determinant of the mixed layer fluxes in many boundary layer parameterizations, the profiles of L used in boundary layer models are drawn from a limited number of LES simulations and the Blackadar (1962) parameterization, and, perhaps most importantly, the physical mechanisms that determine L are unknown. Measurements from the ENA Doppler Lidar (DL) and other ARM sites enable direct measurements of L and, thus, provide a means to determine its variability and, hopefully, the underlying physics that determine its magnitude. In this study, we compute half-hour profiles of L in single layer marine stratocumulus clouds at ENA during the summers of 2016 and 2017 and compare them to LES estimates and to the Blackadar (1962) profile.