Abstract
Turbulence statistics and spectra in a radiatively heated convective boundary layer (CBL) under aerosol pollution conditions are less investigated than their counterparts in the clear CBL. In this study, a large-eddy simulation (LES) coupled with an aerosol radiative transfer model is employed to determine the impact of aerosol radiative heating on CBL turbulence statistics. One-dimensional velocity spectra and velocity–temperature cospectra are invoked to characterize the turbulence flow in the CBL with varying aerosol pollution conditions. The results show that aerosol heating makes the profiles of turbulent heat flux curvilinear, while the total (turbulent plus radiative) heat flux profile retains the linear relationship with height throughout the CBL. The horizontal and vertical velocity variances are reduced significantly throughout the radiatively heated CBL with increased aerosol optical depth (AOD). The potential temperature variance is also reduced, especially in the entrainment zone and near the surface. The velocity spectral density tends to be smaller overall, and the peak of the velocity spectra is shifted toward larger wavenumbers as AOD increases. This shift reveals that the energy-containing turbulent eddies become smaller, which is also supported by visual inspection of the vertical velocity pattern over horizontal planes. The modified CBL turbulence scales for velocity and temperature are found to be applicable for normalizing the corresponding profiles, indicating that a correction factor for aerosol radiative heating is needed for capturing the general features of the CBL structure in the presence of aerosol radiative heating.
Highlights
In the atmospheric convective boundary layer (CBL), turbulence is the main mechanism for the transport and mixing of heat, moisture, and air pollutants [1]
An aerosol radiative heating term (∂R/∂z) is added to the filtered large-eddy simulation (LES) heat balance equation for potential temperature, where z is the height above the ground, R is the shortwave radiative flux, which is determined by using the Santa Barbara Discrete Ordinates Radiative Transfer (DISORT) Atmospheric
We focus on how aerosol radiative heating affects the vertical profiles of meteorological variables
Summary
In the atmospheric convective boundary layer (CBL), turbulence is the main mechanism for the transport and mixing of heat, moisture, and air pollutants [1]. The skewness of vertical velocity, which is an indicator of the asymmetry of vertical motion, is mostly positive throughout the CBL, and increases with height away from the surface [28,29] These typical features of the CBL turbulence characteristics have been discussed extensively in many studies, and form a basis of our current understanding of the aerosol-free CBL turbulence structure. Where w∗ is convective velocity scale, g is the gravity acceleration, θ0 is a constant reference potential temperature (290 K in this study), zi is boundary layer depth, θ∗ is temperature scale and Qs is the surface kinematic heat flux Being normalized with these convective scales, the profiles of turbulence statistics in shear-free CBLs appear in a self-similar manner.
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