Scalar turbulent flux observations in the entrainment layer and assessment of current parametrizations

Abstract

The entrainment zone (EZ) capping the convective boundary layer (CBL) is documented using lidar turbulent-scale observations. Sensible and latent heat fluxes have been measured during one hundred hours in CBL temperate and arid regions in free-cloud meteorological conditions. The EZ fluxes have been confronted with scalar gradients, integral scales of turbulence, wind shear, and surface sensible heat flux.  In agreement with former observations in stratified surface layer or with EZ large eddy-simulation (LES) studies, Lidar observations show that the buoyant oscillation period associated to the vertical velocity variance are found to be universal relevant parameters for EZ scalar fluxes and gradients. Attempts to introduce non-local CBL scales like the turbulent vertical velocity scale and height, the scalar interfacial layer jumps and the surface sensible heat flux globally increases the scatter of the data. Non-local parametrization particularly failed for EZ latent heat flux compared to sensible heat flux and for weak surface heat flux as it is usually the case in temperate region. Investigating a possible non-local parametrization of EZ vertical velocity variance, it was found that EZ wind shear and then EZ gradient Richardson number play a negligible role. Rather, observations often showed an excess of EZ vertical velocity variance with respect to CBL similarity law that may be explained by a significant contribution of wave-turbulence interaction in the entrainment layer.