Pinning down the growth-rate law of Atmospheric Convective Boundary Layers

Abstract

This project aims to answer a longstanding and controversial issue in weather and climate prediction: what is the growth rate law for the evolution of the daytime atmospheric boundary layer (ABL). For weather, climate, and air quality models, it is of vital importance to correctly forecast the height of the boundary layer as it develops under daytime heating. Turbulence in the ABL mixes heat, momentum, and bio(chemical) species originating from the surface, over the entire depth of the ABL, and any inaccurate prediction of boundary layer height results in flawed predictions of scalar concentrations, e.g., temperature and pollutants.

The most widely employed growth rate law is riddled with controversy: results from atmospheric observations, large-eddy simulations, and laboratory experiments are mutually inconsistent and display substantial scatter rendering an accurate prediction of the boundary layer depth based on these results questionable.

We aim to end this controversy by conducting ground truth Direct Numerical Simulation (DNS) of convective boundary layers. DNS employs no empirical rules as it fully resolves the entire spatial spectrum of turbulence. Of course one cannot simulate the high Reynolds number of atmospheric turbulence, but present computer resources do allow one to faithfully simulate the classical laboratory experiments that gave rise to the existing growth rate laws for the ABL, and more importantly to reach Reynolds numbers ten times higher than the classical experiments. By mapping out the impact of Reynolds number on the ABL growth rate over three decades, many of the existing controversial issues will be answered by this project.