Large-eddy simulations of thermally forced circulations in the convective boundary layer. Part II: The effect of changes in wavelength and wind speed

Abstract

This paper extends previous large-eddy simulations of the convective boundary layer over a surface with a spatially varying sensible heat flux. The heat flux variations are sinusoidal and one-dimensional. The wavelength is 1500 or 4500 m (corresponding to 1.3 and 3.8 times the boundary-layer depth, respectively) and the wind speed is 0, 1 or 2 m s^-1.

In every case the heat flux variation drives a mean circulation. As expected, with zero wind there is ascent over the heat flux maxima. The strength of the circulation increases substantially with an increase in the wavelength of the perturbation. A light wind weakens the circulation drastically and moves it downwind. The circulation has a significant effect on the average concentration field from a simulated, elevated source.

The heat flux variation modulates turbulence in the boundary layer. Turbulence is stronger (in several senses) above or downwind of the heat flux maxima than it is above or downwind of the heat flux minima. The effect remains significant even when the mean circulation is very weak. There are effects too on profiles of horizontal-average turbulence statistics. In most cases the effects would be undetectable in the atmosphere.

We consider how the surface heat flux variations penetrate into the lower and middle boundary layer and propose that to a first approximation the process resembles passive scalar diffusion.