Impact of Aerosol Shortwave Radiative Heating on Entrainment in the Atmospheric Convective Boundary Layer: A Large-Eddy Simulation Study

Abstract

Entrainment is critical to the development of the atmospheric convective boundary layer (CBL), but little is known about how entrainment is impacted by the aerosol radiative effect. An aerosol radiation transfer model is used in conjunction with large-eddy simulation (LES) to quantify the impact of aerosol shortwave radiative heating on entrainment and thermodynamics of an idealized dry CBL under aerosol-loading conditions. An entrainment equation is derived within the framework of a zero-order model (ZOM) with the aerosol radiative heating effect included; the equation is then examined against the LES outputs for varying aerosol optical depths (AODs) and free-atmosphere stratification scenarios. The results show that the heat flux profiles become more nonlinear in shape as compared to the case of the clean (no aerosol pollution) CBL, with the degree of nonlinearity being highly dependent on the AOD of the layer for the given type of radiation-absorbing aerosols. As AOD increases, less solar radiation reaches the surface and thus the surface heat flux becomes smaller, and both actual (LES) and ZOM-derived entrainment flux ratios decrease. This trend is opposite to the clean CBL where the LES-predicted flux ratios show an increasing trend with diminishing surface heat flux, while the ZOM-calculated flux ratio remains constant. The modified dimensionless entrainment rate closely follows the −1 power law with a modified Richardson number. The study suggests that including the aerosol radiative effect may improve numerical air quality predictions for heavy-air-pollution events.

Transition Periods in the Diurnally-Varying Atmospheric Boundary Layer Over Land

“…Aerosols in the boundary layer may have large effects on radiative heating or cooling when the aerosol loading is significant or the aerosols are strongly absorbing. Aerosol layers within or above the boundary layer reduce the net radiation reaching the surface (Barbaro et al 2014;Liu et al 2019), thereby affecting transition timing. Aerosols within the boundary layer or the residual layer can directly heat those layers enough to affect timing as well.…”

Empirical values and assumptions in the convection of numerical models

“…For instance, Wang et al (2007) improved the simulated diurnal cycle over land and ocean by increasing the entrainment/detrainment rates for deep and shallow convection used in the Tiedtke scheme, which tends to simulate convective precipitation too early in the day and with an unrealistic amplitude over land. Thus, the choice of a convective scheme impacts the diurnal cycle (Bechtold et al, 2004;Wang et al, 2007), as well as the simulation of monsoon precipitation in climate models (Mukhopadhyay et al, 2010), the MJO (Lin et al, 2006), the ENSO (Wu et al, 2007;Neale et al, 2008), and the ITCZ configuration (Liu et al, 2019). This topic has profound practical effects: it has also been shown that choices in the convective parameterization affect the prediction of track, intensity and associated rainfall of tropical cyclones (Mohandas and Ashrit, 2014).…”

The Stove, Dome, and Umbrella Effects of Atmospheric Aerosol on the Development of the Planetary Boundary Layer in Hazy Regions

“…Recent studies have found that in addition to high primary emissions and rapid secondary formation of atmospheric aerosol (Pan et al, 2016;Zhang, Wang, Guo, et al, 2015), the feedback mechanisms between aerosol and planetary boundary layer (PBL) are crucial in promoting the outbreak of haze episodes (Ding et al, 2013(Ding et al, , 2016Liu, Fedorovich, et al, 2019;Liu, Xin, et al, 2019;Petäjä et al, 2016;Wang, Huang, et al, 2018;Wang, Yu, et al, 2018;Wang et al, 2019;Wilcox et al, 2016;Zhao, Xin, et al, 2019;Zou et al, 2017). This feedback process can be described in two aspects, related to surface cooling and atmospheric heating.…”