Empirical Orthogonal Function Analysis of the Weakly Convective Atmospheric Boundary Layer. Part II: Eddy Energetics

Abstract

Three-dimensional empirical orthogonal functions (E0Fs), calculated from a large-eddy simulation of a weakly convective, planetary boundary layer (PBL), are used to decompose statistics for PBL turbulence into contributions from individual structures. The most energetic E0Fs, corresponding largely to boundary-layer-spanning eddies, together are responsible for about one-half of the turbulent kinetic energy (TKE) throughout the boundary layer, although they carry a substantial amount of the momentum and heat fluxes only near mid-PBL. Examination of the flux profiles also reveals coupling between large roll structures and inversion-borne gravity waves. By filtering the fields through the EOFs, skewness and intermittency (kurtosis) associated with the different vertical scales are determined. Positive skewness around mid-PBL is found to be attributable to the boundary-layer spanning eddies. lntermittency, however, cannot be attributed to either large- or small-scale structures: it results from interscale interactions. Finally, equations for the flux and energy budgets of individual structures are derived. The budget analyses show clearly that the main source of TKE for large roll structures is shearing stress, while the main loss mechanism is transfer to smaller scales. The inversion-borne gravity waves gain TKE from interscale transfers and buoyant acceleration and lose TKE to shearing effects.