High-resolution forecast modeling

Specific real-time high-resolution forecasts of clouds, dust and biomass burning aerosol (BBA) events were made with the French limited area mesoscale model Meso-NH (Lac et al., 2018) to guide flight planning. The model was run for 84 h starting at 1200 UTC each day, with the ECMWF analysis and forecasts used as initial and boundary conditions were issued. The simulation domain covered the southern tip of Africa (5°-35°E, 13°-36°S). The horizontal grid mesh was set to 5 km to permit the explicit representation of deep convection if needed. The dust prognostic scheme of Grini et al. (2006) was used with the same tuning as for the Fennec campaign over the Sahara (Chaboureau et al., 2016). A passive tracer was used as a proxy for BBA to forecast long-range transport of smoke over Namibia and offshore. Rather than attempting to forecast the BBA injection accurately, the objective was to identify air masses likely to be loaded with BBA. The BBA tracer was injected up to the height of 800 m above ground at locations where fire was observed by the Medium Resolution Imaging Spectrometer instrument in August 2011, i.e., at specific locations in Angola and the eastern part of the domain (Mozambique, South Africa, Zambia, Zimbabwe). An e-folding time of 12 h was applied to slowly decaying the BBA tracer. The dust and BBA prognostic variables at the end of a given 24 h forecast were passed on as initial conditions at the start of the next 24 h forecast. Dedicated vertical cross-sections from the Walvis Bay airport along the northern coast of Namibia and inland towards the Etosha Pan were also provided to the OC forecasters.

Regional dust modelling

The meso-scale atmosphere-aerosol model COSMO-MUSCAT (COSMO: COnsortium for Small-scale MOdelling, MUSCAT: MUltiScale Chemistry Aerosol Transport Model) is used for examining the spatio-temporal activity of dust sources in Namibia, horizontal and vertical distribution of dust concentrations ultimately pointing towards major dust transport routes.

The aerosol model MUSCAT consists of modules describing dust emission as well as dust removal via dry and wet deposition (see Schepanski et al. (2016) for more details). In the framework of AEROCLO-sA, model simulations are performed for the period 15 August to 1 October 2017 at a horizontal grid with a grid spacing of 0.25°.

Regional climate modelling

ALADIN-Climate

The ALADIN-Climate simulations cover the period from 01 August to 30 September 2017. The lateral boundary conditions are provided by ERA-Interim. The possible long-range transport of BBA is not forced at the lateral boundary conditions but rather a large domain is defined encompassing the main biomass-burning sources. The horizontal resolution of the model is 12 km with 91 vertical levels. The land surface is treated using the SURFEX model (Masson et al., 2013). The FMR (RRTM, Mlawer et al., 1997) radiative transfer scheme is used to calculate the shortwave and longwave (SW and LW) radiation.

The biomass-burning emissions from the CMIP6 inventory have been used for BC, OC and sulfur gaseous SO₂. Following the study of Petrenko et al. (2017), an adjustment factor of 2.5 is applied to the biomass burning emissions. Smoke emissions force the model at the first model level following the recommendations from the first phase of AEROCOM (Dentener et al., 2006). The recent aerosol scheme (TACTIC, Tropospheric Aerosols for ClimaTe in CNRM-CM) included in the ALADIN-Climate model accounts for sulfate, organic (OC) and black (BC) carbon, dust and primary sea-salt particles and is described in Nabat et al. (2015) and Michou et al. (2015). Two new tracers have been recently implemented in ALADIN-Climate describing, respectively, the mass concentration of fresh and aged smoke aerosols, following the methodology presented in Mallet et al. (2018). In the absence of an explicit representation of secondary organic aerosols (SOA) production in ALADIN-Climate, a ratio of particulate organic matter (POM) to primary OC has been used for artificially representing SOA formation within the smoke plume. For this ALADIN-Climate simulation, an average POM/OC ratio of 2.3 is applied, as reported by Formenti et al. (2003).

For the primary BC and OC species and secondary sulfates, a bulk approach is applied whereby a fixed aerosol size distribution is assumed for calculating aerosol properties, while for mineral dust and sea salt particles, a more explicit size representation is used based on 3 bins for dust and sea-salt. The TACTIC scheme assumes an external mixture of the different aerosol species. Aerosol DRF at the surface and at TOA (in SW and LW spectral range and for both clear-sky and all-sky conditions) is diagnosed using a double call (with and without aerosols) to the radiation scheme during the model integration.

RegCM

We use the International Center for Thoretical Physics (ICTP) regional climate model RegCM4 (Giorgi, et al., 2012) at a 50 km horizontal resolution. Runs are performed at different resolutions ranging from 12 km for case studies over southern African/eastern Atlantic domain, to 50 km over a larger continental Africa domain.  Boundary conditions are provided by ERA-Interim reanalysis through a 1000 km buffer zone. The Newtonian relaxation to large scale fields applied in the boundary buffer zone is designed to limit as much as possible wave reflections in the domain. Relevant physics options used in this study include the Community Land Model version 4.5 (CLM4.5), the University of Washington turbulence scheme (O’Brien, et al., 2012) and the Emanuel convection scheme (Emanuel, 1991) with enabled tracer transport capabilities. The RegCM4 aerosol scheme includes a representation of anthropogenic sulfates, black and organic carbon from fossil fuel combustion and anthropogenic activity (Solmon et al., 2006), as well as sea-salt and dust aerosol. Specific biomass burning tracers representing fresh and aged smoke mixture of aerosol, following the methodology presented in Mallet et al. (2018), are also implemented. Daily biomass burning emission are prescribed using the Global Fire Emission database Version 4 (GFED4, Randerson et al., 2018).  Following the study of Petrenko et al. (2017), an adjustment factor of 2.5 is applied to the biomass burning emissions. Secondary aerosol formation in the plume is accounted for by artificially enhancing the organic fraction of aged smoke production by a tunable factor set to 2, in line with Formenti et al., 2013. For natural particles, sea salt aerosol emissions are calculated on-line and are represented by two (sub and super-micronic) different bins (Zakey, et al., 2008). The dust emission scheme (Marticorena, et al., 1995), (Zakey, et al., 2006) includes updates of soil texture distribution following (Menut, et al., 2013) and emission size distribution (Kok, 2011; Nabat, et al., 2012). Lateral Boundary conditions for aerosols are prescribed from monthly global outputs of the CAM-Chem model running with ACCMIP emissions. Dusts are represented using 4 bins and impact short- and long-wave radiation transfer (Solmon et al., 2015). All other aerosols impact the RegCM shortwave radiation scheme through pre-calculated optical properties (Solmon, et al., 2006). The first and second indirect effect can be accounted for through simple approaches and applied to hydrophilic species following a method similar to that of Wozniak et al., 2018.