Large simulated radiative effects of smoke in the south-east Atlantic

Abstract

Abstract. A 1200 km-square area of the tropical south Atlantic Ocean near Ascension Island is studied with the HadGEM climate model at convection-permitting and global resolutions for a ten-day case study period in August 2016. During the simulation period, a plume of biomass burning smoke from Africa moves into the area and mixes into the clouds. We examine the interaction of the smoke with clouds and find it has substantial instantaneous direct, indirect and semi-direct radiative effects, which vary in magnitude between model configurations. The smoke aerosol is simulated realistically, and is found to affect dynamical, microphysical and radiative properties of the 10 atmosphere across the region. The model captures the large-scale horizontal transport of the aerosol adequately, approximately reproducing a transition from pristine to polluted conditions. However, for some of the simulation, the smoke is around 1km too low in altitude and therefore mixes into the clouds earlier than observed. Fire emissions increase the total aerosol burden by a factor 3.7 and cloud droplet number concentrations by a factor of 3, which is consistent with MODIS observations. Strong localised perturbations to heating and cooling rates due to the smoke affect the dynamics: in the regional model, the inversion relative to a simulation without smoke aerosol, when averaged over the polluted period. This increase is uncertain, and smaller in the global model. It is mostly due to radiatively driven dynamical changes: the reduced entrainment of dry air from above the cloud layer, rather than precipitation suppression by aerosol. 20The smoke has substantial direct radiative effects of +11.4 W m . However, the radiative effects are sensitive to the model set-up: the semi-direct effect is smaller in the global model, and also in a simulation with the Kogan (2013) parameterisation of autoconversion and accretion instead of the default, from Khairoutdinov & Kogan (2002). Furthermore, we simulate a liquid water path that is biased high compared to 25 satellite observations by 22% on average, and this leads to high estimates of the domain-averaged aerosol direct effect and the effect of the aerosol on cloud albedo. With these caveats, we simulate a large net cooling across the region, of −27.6 W m

Simulation of the transport, vertical distribution, optical properties and radiative impact of smoke aerosols with the ALADIN regional climate model during the ORACLES-2016 and LASIC experiments

“…Such values of SW heating due to smoke appear to fall well within the range of values reported by Tummon et al (2010), Gordon et al (2018), Adebiyi et al, (2015) or Wilcox et al (2010), who reported, respectively, additional SW heating due to smoke of 1 (JJAS period), 0.34 (5 days of simulations), 1.2 (for fine AOD > 0.2) and 1.5 °K by day. In addition, Keil and Haywood (2003) estimated a SW heating rate of 1.8°K/day near the coast using a radiative transfer model and observations during SAFARI-2000.…”

The Influence of Elevated Smoke Layers on Stratocumulus Clouds Over the SE Atlantic in the NASA Goddard Earth Observing System (GEOS) Model

“…We are thus making an assumption that the CALIOP extinction profile is suitable for redistributing the aerosol mass profile. This is not strictly correct, as the relative humidity within the free troposphere is generally less than in the MBL, but over SE Atlantic in particular it is frequently high enough (Adebiyi et al, ; Gordon et al, ) that the hygroscopic growth can be similar to what would be experienced in the MBL. Overall, we think the assumption being made is reasonable, but we acknowledge that the mass redistribution does result in an overall decrease in the total column extinction by about 15% near the coast and a somewhat greater reduction (~30%) further west in elevated aerosol case versus the MERRAero control case.…”

“…Aerosol indirect effects might also be relevant in this region, based on the previous space‐based observational studies (Costantino & Bréon, ; Painemal et al, ) and the presence of pollution within the MBL observed in recent in situ measurements (Layered Atlantic Smoke Interactions with Clouds (LASIC), Zuidema et al, ; and ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES), Diamond et al, ). In this context, regional modeling studies by Lu et al () and Gordon et al () evaluated the aerosol indirect or microphysical effects in addition to the aerosol direct and semi‐direct effects using a forecast model configuration run at a high (<5‐km grid spacing) resolution. Lu et al () found the aerosol microphysical effects play the dominant role, larger by about a factor of 2, compared to the aerosol radiative effects in determining the total radiative forcing at the TOA.…”

“…Lu et al () found the aerosol microphysical effects play the dominant role, larger by about a factor of 2, compared to the aerosol radiative effects in determining the total radiative forcing at the TOA. On the other hand, Gordon et al () suggested that the aerosol direct and semi‐direct effects together still overpowered the indirect effects for a 10‐day period case study over a smaller domain (1,200 × 1,200 km 2 ) around Ascension Island in SE Atlantic. The findings of Gordon et al () also suggested that the radiative effects were sensitive to whether the structure of the model was global or regional.…”

“…On the other hand, Gordon et al () suggested that the aerosol direct and semi‐direct effects together still overpowered the indirect effects for a 10‐day period case study over a smaller domain (1,200 × 1,200 km 2 ) around Ascension Island in SE Atlantic. The findings of Gordon et al () also suggested that the radiative effects were sensitive to whether the structure of the model was global or regional.…”