Direct and semi-direct radiative forcing of biomass-burning aerosols over the southeast Atlantic (SEA) and its sensitivity to absorbing properties: a regional climate modeling study

Abstract

Abstract. Simulations are performed for the period 2000–2015 by two different regional climate models, ALADIN and RegCM, to quantify the direct and semi-direct radiative effects of biomass-burning aerosols (BBAs) in the southeast Atlantic (SEA) region. Different simulations have been performed using strongly absorbing BBAs in accordance with recent in situ observations over the SEA. For the July–August–September (JAS) season, the single scattering albedo (SSA) and total aerosol optical depth (AOD) simulated by the ALADIN and RegCM models are consistent with the MACv2 climatology and MERRA-2 and CAMS-RA reanalyses near the biomass-burning emission sources. However, the above-cloud AOD is slightly underestimated compared to satellite (MODIS and POLDER) data during the transport over the SEA. The direct radiative effect exerted at the continental and oceanic surfaces by BBAs is significant in both models and the radiative effects at the top of the atmosphere indicate a remarkable regional contrast over SEA (in all-sky conditions), with a cooling (warming) north (south) of 10 ∘S, which is in agreement with the recent MACv2 climatology. In addition, the two models indicate that BBAs are responsible for an important shortwave radiative heating of ∼0.5–1 K per day over SEA during JAS with maxima between 2 and 4 km a.m.s.l. (above mean sea level). At these altitudes, BBAs increase air temperature by ∼0.2–0.5 K, with the highest values being co-located with low stratocumulus clouds. Vertical changes in air temperature limit the subsidence of air mass over SEA, creating a cyclonic anomaly. The opposite effect is simulated over the continent due to the increase in lower troposphere stability. The BBA semi-direct effect on the lower troposphere circulation is found to be consistent between the two models. Changes in the cloud fraction are moderate in response to the presence of smoke, and the models differ over the Gulf of Guinea. Finally, the results indicate an important sensitivity of the direct and semi-direct effects to the absorbing properties of BBAs. Over the stratocumulus (Sc) region, DRE varies from +0.94 W m−2 (scattering BBAs) to +3.93 W m−2 (most absorbing BBAs).

An overview of the ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) project: aerosol–cloud–radiation interactions in the southeast Atlantic basin

“…The average boundary layer N a is a good predictor of the vertical mean N c across the four regimes, consistent with the primary source for droplets being aerosols ingested into cloud base (Diamond et al, 2018). Although N c is approximately constant with height for the two Separated regimes and the Contact/N a < 250 cm −3 regime, consistent with many previous studies (Nicholls and Leighton, 1986;Martin et al, 1994;Miles et al, 2000;Painemal and Zuidema, 2011;Wood, 2012), it experiences a significant and unexpected increase with Z N for the Contact/N a > 250 cm −3 case. The highest N c values occur where the MBL has both high N a and is in contact with overlying BB aerosol layers.…”

The CLoud–Aerosol–Radiation Interaction and Forcing: Year 2017 (CLARIFY-2017) measurement campaign

“…The over-strong aerosol-cloud interaction in CLAS-SIC compared to GLOMAP-MODE has been noted in other studies that have used satellite retrievals to assess their validity (e.g. Malavelle et al, 2017).…”

“…Global bulk aerosol models and empirical representations of aerosol indirect effects are being replaced with microphysical aerosol models such as ECHAM5-HAM (Stier et al, 2005) and GLOMAP (also known as UKCA mode) (e.g. Mann et al, 2010;Bellouin et al, 2013), and more explicit representations of cloud and precipitation processes (Hill et al, 2015;Grosvenor et al, 2017) have also been developed. Such schemes require extensive evaluation, which is often achieved through multi-model intercomparison studies (e.g.…”

“…While there is clear evidence of the Twomey effect (Twomey, 1977) from recent comprehensive satellite assessments of ship-tracks and degassing volcanoes (e.g. Malavelle et al, 2017;Christensen et al, 2014;Toll et al, 2017Toll et al, , 2019, there is generally a lack of clear evidence of the cloud lifetime or Albrecht effect (Albrecht, 1989) as the impact appears to be strongly dependent on the atmospheric state (e.g. Chen et al, 2015;Toll et al, 2019).…”

Modeled and observed properties related to the direct aerosol radiative effect of biomass burning aerosol over the Southeast Atlantic

“…In one study, the direct aerosol radiative effect in the region changed from negative to positive as cloud fraction increased above 40% (Chand et al, 2009), assuming a mid-visible aerosol single scatter albedo (SSA) of 0.85 and for cloud optical depths averaging 7.8 (or cloud albedo of 0.5). For aerosol with lower SSA or for higher cloud albedo this transition would occur at a lower cloud fraction (Mallet et al, 2020). The sign and magnitude of the responses to this forcing (i.e.…”