Characteristics of Decaying Convective Boundary Layers Revealed by Large-Eddy Simulations

Seung-Bu Park,Jong-Jin Baik

doi:10.3390/atmos11040434

Abstract

The decay of the Convective Boundary Layer (CBL) is studied using large-eddy simulations of free and advective CBLs, in which surface heat supply is suddenly cut off. After the cutoff, coherent convective circulations last about one convective time scale and then fade away. In the mixed layer, the decay time scale increases with height, indicating that nonlocal eddies decay slower than near-surface local eddies. The slower decay of turbulence in the middle of CBL than near-surface turbulence is reconfirmed from the analysis of pattern correlations of perturbations of vertical velocity. Perturbations of potential temperature and scalar concentration decay faster and slower than vertical velocity perturbations, respectively. A downward propagation of negative heat flux and its oscillation are found and a quadrant analysis reveals that warmer air sinking events are responsible for the downward propagation. The fourth quadrant events seem to be induced by demixing of air parcels, entrained from above the CBL. The advective CBL simulation with geostrophic wind illustrates that near-surface eddies are mechanically generated and they decelerate flow from the bottom up in the CBL/residual layer. The two-dimensional spectra show the height- and scale-dependent characteristics of decaying convective turbulence again in the free and advective boundary layer simulations.

Highlights

The Planetary Boundary Layer (PBL), the bottom layer of the troposphere in contact with theEarth’s surface, responds to surface forcings “with a timescale of about an hour or less” and has a strong diurnal cycle over land [1]
Convective turbulence decays near sunset, and the mixed layer is transitioned to a residual layer and a shallow stable boundary layer starts to develop near the surface
Brost [10] reported that the decay rate of convective turbulence is the same at all heights, the height dependency of the decay rate is found in this study

Summary

Introduction

The Planetary Boundary Layer (PBL), the bottom layer of the troposphere in contact with the. Sorbjan [11] simulated a decaying CBL using a Large-Eddy Simulation (LES) model He considered gradually decreasing surface heat flux over time and showed that the decay of Turbulent Kinetic Energy (TKE) is governed by two time scales, the external time scale of the surface heat flux change, and the convective time scale t∗ = zi /w∗ where zi and w∗ are, respectively, the inversion height and the convective velocity scale [17]. The length scale of vertical velocity fluctuations remains nearly constant but the length scales of other variables increase over time after surface heat flux is stopped They showed that convective turbulence with shear decays slower than purely buoyancy-driven convective turbulence but inversion strength is less influential.

Model Description and Setup

Free Decaying Convective Boundary Layer

Advective Decaying Convective Boundary Layer

Summary and Conclusions

Methods