Effects of Aerosols on the Convective Boundary-Layer Dynamics and Land-Atmosphere System

In this study we show that the dynamics of the convective boundary layer (CBL) strongly depends on the atmospheric aerosol loading and properties. We combine a comprehensive set of observations and numerical experiments to demonstrate that aerosols impact on the CBL dynamics mainly through the response of the surface energy budget (SEB), but also by modifying the heat budget at the top of the CBL.

The observational dataset contains radiation, chemistry, surface and (thermo)dynamic variables observed at CESAR (Cabauw Experimental Site for Atmospheric Research). We perform our analysis for two typical clear-sky days characterized by (i) a residual layer above a strong surface inversion and (ii) a continuously growing CBL. Aerosol information from the European Integrated Project on Aerosol, Cloud, Climate, and Air Quality Interactions (IMPACT/EUCAARI) campaign are also available for (i) and are extrapolated for (ii). Constrained by the CESAR and EUCAARI data sets we design numerical experiments using a large-eddy simulation (LES) model and a mixed-layer (MXL) model. Both LES and MXL models are coupled to a broadband radiative transfer code and a land-surface model, and are able to reproduce satisfactorily the surface and upper-air observations for both days under study. Given the good agreement, we use the MXL model to perform a sensitivity analysis aiming to explore the aerosol effect on the land-atmosphere system for a wide range of optical depths and single scattering albedos. Our results show that higher loads of aerosols decrease the net irradiance, imposing an energy restriction at the surface. Over the studied well-watered grassland, aerosols reduce the sensible heat flux more than the latent heat flux. This effect delays the break-up of the inversion layer (up to 4.5 hours) and subsequent onset of the morning CBL and promotes the earlier decay of turbulence hastening its afternoon collapse (up to 2.5 hours). If also present above the CBL during the morning transition, aerosols maintain a persistent surface inversion. After the aerosols are entrained in the CBL, we observe a strong dependence of the afternoon CBL evolution on the single scattering albedo. Moderately to strongly absorbing aerosols increase the CBL heating rate contributing positively to increase the afternoon CBL height, potential temperature and to decrease the Bowen ratio. In contrast, scattering aerosols are associated with smaller heating rates and cooler and shallower CBLs. Our findings advocate the need for accounting for the aerosol influence in reproducing surface and boundary-layer dynamics.

Current research focus on the feedback induced by inorganic aerosol formation and its vertical distribution on the CBL (thermo)dynamics and shortwave radiation field. Aiming to investigate the impact of heterogeneous surface emissions on the CBL dynamics we perform a sensitivity analysis on the land-use changing the surface temperature/moisture, roughness length and emissions mimicking an idealized rural/urban setup. Our preliminary results corroborate that under colder and wetter conditions (rural patch) more outgassing happens and therefore more aerosols are formed at the top of the CBL.