Black carbon solar absorption suppresses turbulence in the atmospheric boundary layer

Abstract

The introduction of cloud condensation nuclei and radiative heating by sunlight-absorbing aerosols can modify the thickness and coverage of low clouds, yielding significant radiative forcing of climate. The magnitude and sign of changes in cloud coverage and depth in response to changing aerosols are impacted by turbulent dynamics of the cloudy atmosphere, but integrated measurements of aerosol solar absorption and turbulent fluxes have not been reported thus far. Here we report such integrated measurements made from unmanned aerial vehicles (UAVs) during the CARDEX (Cloud Aerosol Radiative Forcing and Dynamics Experiment) investigation conducted over the northern Indian Ocean. The UAV and surface data reveal a reduction in turbulent kinetic energy in the surface mixed layer at the base of the atmosphere concurrent with an increase in absorbing black carbon aerosols. Polluted conditions coincide with a warmer and shallower surface mixed layer because of aerosol radiative heating and reduced turbulence. The polluted surface mixed layer was also observed to be more humid with higher relative humidity. Greater humidity enhances cloud development, as evidenced by polluted clouds that penetrate higher above the top of the surface mixed layer. Reduced entrainment of dry air into the surface layer from above the inversion capping the surface mixed layer, due to weaker turbulence, may contribute to higher relative humidity in the surface layer during polluted conditions. Measurements of turbulence are important for studies of aerosol effects on clouds. Moreover, reduced turbulence can exacerbate both the human health impacts of high concentrations of fine particles and conditions favorable for low-visibility fog events.

Dome effect of black carbon and its key influencing factors: A one-dimensional modelling study

“…Such kind of effect was named as the "dome effect" of BC . Similar effect has also been found for BC over the Indian Ocean (Wilcox et al, 2016), and dust aerosols in northern and eastern China Yang et al, 2016).…”

“…The warming and cooling of different atmospheric levels remarkably altered the stratification, thereby weakening convective motions. Stable stratification combined with decreased sensible heat flux at the ground surface greatly suppressed vertical turbulence in the 15 boundary layer (Wilcox et al, 2016), contributing to a delay of PBL development and an earlier drop as well as a substantial decrease in PBL height, which hindered the air pollutants from being further dispersed vertically.…”

Transport, mixing, and feedback of dust, biomass burning and anthropogenic pollutants in eastern Asia: A case study

“…This two-way interaction between air pollution and meteorological conditions was widely discussed in the PBL (Fan et al, 2015;Gao et al, 2015;Wang et al, 2015;Wilcox et al, 2016). In this case, we proved this interaction could happen in the 5 MLT due to the special vertical distribution and transport of the mixed pollutants.…”

Cloud albedo changes in response to anthropogenic sulfate and non-sulfate aerosol forcings in CMIP5 models

“…Aerosols that are not efficient as CCN contribute less to cloud brightening but may still affect cloud properties. Absorbing aerosols can cause local heating and a reduction of cloud cover, as suggested by Ackerman et al (2000), or reduced turbulence and entrainment and an increase in cloudiness, as suggested by Wilcox (2010) and Wilcox et al (2016). These, and the many additional possible pathways for aerosol influence cloud properties, are difficult to disentangle, but the relative strength of the individual processes and their net effect are dependent on the properties of the underlying aerosol distribution.…”