Countergradient momentum flux in the presence of rolls in the atmospheric boundary layer

Abstract

In the atmospheric boundary layer, when surface heat flux is small and mean wind shear is strong, horizontal convective rolls that are elongated along the wind shear are formed. This study attempts to explain the asymmetry of rolls in terms of turbulence using large-eddy simulations. A pressure gradient in the north-south y direction is applied, which results in an east-west geostrophic wind. It is shown that the turbulent kinetic energy components in the x and y directions are not equal when rolls develop. In addition, a countergradient regime is present for vertical momentum flux in the y direction in convective boundary layer with rolls. In the countergradient regime, the wind variance in the y direction is destroyed, contrary to being produced in the x direction. The presence of a countergradient regime for v'w' but not u'w' suggests that the eddy viscosity in the x and y directions would be rather different, and even become negative for v'w'. Thus, the existing parameterization scheme in global atmospheric models may need to be modified. However, the design of an improved parameterization scheme is non-trivial as the countergradient regime is non-stationary when stability decreases, while it does not exist in neutral or stable boundary layers with rolls.