Non-local Mixing Of Momentum In The Convective Atmospheric Boundary Layer

Thermally-driven organized convection is the most prominent structural feature of the daytime convective boundary layer(CBL).Different from inertial-subrange turbulent eddies,organized convection is vertically coherent at the boundary layer scale,and contributes to nonlocal vertical transport of mass and energy within the CBL.In this study,we investigate turbulent mixing of momentum in the CBL,with a focus on the role of organized convection in momentum transport.Then we compare and evaluate momentum flux parameterizations in current boundary layer schemes.Based on modeled CBL with high resolution large-eddy simulations(LESs),organized convection and background turbulence are separated using three selected methods including the Fourier transform,the Proper Orthogonal Decomposition and the Empirical Mode Decomposition.The nonlocal and local fluxes associated with organized convection and background turbulence are computed.It is found that nonlocal flux constitutes a significant portion of the total momentum flux,and dominates the transport of momentum in the mixed layer.How organized convective structures affect the transport of momentum is investigated based on momentum cospectrum and phase-spectrum.Analyses reveal that convective cells formed under strong buoyancy forcing are inefficient at transporting momentum.In the near neutral environment with strong wind shear,convection organizes into horizontal roll vortices,which reduce the phase difference between vertical and horizontal streamwise perturbation velocities,thus improving the efficiency of momentum transport.Through analysis of the turbulent momentum flux budget,it is also found that the buoyancy production term,the pressure correlation term and the turbulent diffusion term are all dominated by organized convection.Finally,momentum mixing models from the different planetary boundary layer schemes of the Weather Research and Forecasting model are compared with the benchmark LES results based on both a priori analysis and posteriori simulations.Results show that gradient-diffusion-based local schemes severely underestimate the momentum fluxes in the convective mixed layer,whereas those with nonlocal momentum flux parameterization are more effective.The parameterized vertical profiles of nonlocal momentum flux are consistent with the results obtained by the three filtering methods.This study suggests that planetary boundary layer schemes must include nonlocal momentum flux corrections,whose parameterization should consider the influence of bulk CBL stability on momentum transport efficiency.