Articles | Volume 16, issue 9
Research article
04 May 2016
Research article |  | 04 May 2016

Aerosol–radiation–cloud interactions in a regional coupled model: the effects of convective parameterisation and resolution

Scott Archer-Nicholls, Douglas Lowe, David M. Schultz, and Gordon McFiggans

Abstract. The Weather Research and Forecasting model with Chemistry (WRF-Chem) has been used to simulate a region of Brazil heavily influenced by biomass burning. Nested simulations were run at 5 and 1 km horizontal grid spacing for three case studies in September 2012. Simulations were run with and without fire emissions, convective parameterisation on the 5 km domain, and aerosol–radiation interactions in order to explore the differences attributable to the parameterisations and to better understand the aerosol direct effects and cloud responses. Direct aerosol–radiation interactions due to biomass burning aerosol resulted in a net cooling, with an average short-wave direct effect of −4.08 ± 1.53 Wm−2. However, around 21.7 Wm−2 is absorbed by aerosol in the atmospheric column, warming the atmosphere at the aerosol layer height, stabilising the column, inhibiting convection, and reducing cloud cover and precipitation. The changes to clouds due to radiatively absorbing aerosol (traditionally known as the semi-direct effects) increase the net short-wave radiation reaching the surface by reducing cloud cover, producing a secondary warming that counters the direct cooling. However, the magnitude of the semi-direct effect was found to be extremely sensitive to the model resolution and the use of convective parameterisation. Precipitation became organised in isolated convective cells when not using a convective parameterisation on the 5 km domain, reducing both total cloud cover and total precipitation. The SW semi-direct effect varied from 6.06 ± 1.46 with convective parameterisation to 3.61 ± 0.86 Wm−2 without. Convective cells within the 1 km domain are typically smaller but with greater updraft velocity than equivalent cells in the 5 km domain, reducing the proportion of the domain covered by cloud in all scenarios and producing a smaller semi-direct effect. Biomass burning (BB) aerosol particles acted as cloud condensation nuclei (CCN), increasing the droplet number concentration of clouds. However, the changes to cloud properties had negligible impact on the net radiative balance in either domain, with or without convective parameterisation. The sensitivity to the uncertainties relating to the semi-direct effect was greater than any other observable indirect effects. Although the version of WRF-Chem distributed to the community currently lacks aerosol–cloud interactions in parameterised clouds, the results of this study suggest a greater priority for the development is to improve the modelling of semi-direct effects by reducing the uncertainties relating to the use of convective parameterisation and resolution before WRF-Chem can reliably quantify the regional impacts of aerosols.

Short summary
The response of the Weather Research and Forecasting model with Chemistry to forcings by biomass burning aerosol were investigated in high-resolution nested domains over Brazil. The aerosol-layer was found to have a negative direct effect at the top of the atmosphere, but this was largely cancelled by a semi-direct effect which inhibited afternoon cloud formation. The cloud response to the aerosol was found to be highly sensitive to model resolution and the use of convective parameterisation.
Final-revised paper