Modelled impacts of Amazonia Biomass Burning Aerosols (BBA) on weather during SAMBBA

The South America Biomass Burning Analysis (SAMBBA) took place from 14 September to 3 October 2012 during the biomass burning season. The Met Office Unified Model (MetUM), in a limited area configuration, is used to examine the impact of biomass burning aerosols (BBA) on South American weather during SAMBBA using three sensitivity experiments. The horizontal grid spacing used is 0.1o x 0.1o which corresponds to approximately 11km. Firstly we ran the MetUM without biomass emissions and aerosols, secondly with monthly mean aerosol climatologies and finally with fully prognostic aerosols modelled using the Coupled Large-scale Aerosol Simulator for Studies in Climate (CLASSIC) scheme. The CLASSIC prognostic BBA scheme only was implemented in the LAM; all other aerosol-radiative interactions from other species come from a climatology. The prognostic CLASSIC aerosol scheme has improved BBA spatial representation compared with the observations during SAMBBA. Impacts of BBA on the net radiation at surface for clear sky conditions and the top of the atmosphere are found to be approximately 50 Wm-2 and 16 Wm-2, respectively. This leads to a significant surface and skin temperature cooling of approximately 2oC due to BBA. BBA cool the boundary layer (BL) and warms air above by around 0.2oC due to the absorption of shortwave radiation, reducing BL depth by around 150m. Due to the deeper BL in the east of the domain, this leads to a more cyclonic circulation at 700 hPa with BBA, with winds changing by around 1 ms-1. Locally, on a 150 km scale, changes in the precipitation reach around 4 mm day-1 due to impacts on the location of convection, but these localised changes average out to give little change in total precipitation over the Amazonian region. Case studies simulated at 1km and using the new UKCA aerosol scheme are being evaluated with SAMBBA flight observations and will be used to evaluate BBA impacts in detail for weather events during SAMBBA.