Large Eddies Regulate Turbulent Flux Gradients in Coupled Stable Boundary Layers

Abstract

Due to strong mean wind shear, the stable boundary layer (SBL) becomes vertically coupled. In a coupled SBL, large turbulent eddies enhance cross-layer mixing and vertically mix turbulent kinetic energy. However, large gradients in momentum and heat fluxes are frequently observed, degrading the performance of Monin-Obukhov similarity theory. It is shown that increased vertical gradients of mean variables (i.e., wind speed and potential temperature) can cause flux gradients. This process is operated upon by downward penetrating large eddies with altered phase difference, contributing unevenly to fluxes and thus causing flux gradients. In the coupled SBL with small gradients in mean variables, large eddies are vertically synchronized, contributing evenly to fluxes across layers and thus causing small flux gradients. As turbulent production becomes weak and the cross-layer difference in turbulent flux transport increases, large eddies become less vertically synchronized, contributing unevenly to fluxes across layers and thus causing large flux gradients.