Cloud adjustments from large-scale smoke–circulation interactions strongly modulate the southeastern Atlantic stratocumulus-to-cumulus transition
Authors: Diamond Michael S. , Saide Pablo E. , Zuidema Paquita , Ackerman Andrew S. , Doherty Sarah J. , Fridlind Ann M. , Gordon Hamish , Howes Calvin , Kazil Jan , Yamaguchi Takanobu , Zhang Jianhao , Feingold Graham , Wood Robert 
Published: September 2022

Abstract. Smoke from southern Africa blankets the southeastern Atlantic Ocean from June to October, producing strong and competing aerosol radiative effects.
Smoke effects on the transition between overcast stratocumulus and scattered
cumulus clouds are investigated along a Lagrangian (air-mass-following)
trajectory in regional climate and large eddy simulation models. Results are
compared with observations from three recent field campaigns that took place
in August 2017: ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES), CLouds and Aerosol Radiative Impacts and
Forcing: Year 2017 (CLARIFY), and Layered Atlantic Smoke Interactions with Clouds (LASIC). The case study is set up around the joint ORACLES–CLARIFY flight that took place near Ascension Island on 18 August 2017. Smoke sampled upstream on an ORACLES flight on 15 August 2017
likely entrained into the marine boundary layer later sampled during the
joint flight. The case is first simulated with the WRF-CAM5 regional climate model in
three distinct setups: (1) FireOn, in which smoke emissions and any resulting
smoke–cloud–radiation interactions are included; (2) FireOff, in which no smoke emissions are included; (3) RadOff, in which smoke emissions and their microphysical effects are included but aerosol does not interact
directly with radiation. Over the course of the Lagrangian trajectory,
differences in free tropospheric thermodynamic properties between FireOn and
FireOff are nearly identical to those between FireOn and RadOff, showing
that aerosol–radiation interactions are primarily responsible for the free tropospheric effects. These effects are non-intuitive: in addition to the
expected heating within the core of the smoke plume, there is also a
“banding” effect of cooler temperature (∼1–2 K) and greatly
enhanced moisture (>2 g kg−1) at the plume top. This banding effect is caused by a vertical displacement of the former continental boundary layer
in the free troposphere in the FireOn simulation resulting from anomalous
diabatic heating due to smoke absorption of sunlight that manifests
primarily as a few hundred meters per day reduction in large-scale subsidence over the ocean. A large eddy simulation (LES) is then forced with free tropospheric fields
taken from the outputs for the WRF-CAM5 FireOn and FireOff runs. Cases are
run by selectively perturbing one variable (e.g., aerosol number
concentration, temperature, moisture, vertical velocity) at a time to better
understand the contributions from different indirect (microphysical),
“large-scale” semi-direct (above-cloud thermodynamic and subsidence
changes), and “local” semi-direct (below-cloud smoke absorption) effects.
Despite a more than 5-fold increase in cloud droplet number concentration when including smoke aerosol concentrations, minimal differences in cloud
fraction evolution are simulated by the LES when comparing the base case with a perturbed aerosol case with identical thermodynamic and dynamic forcings.
A factor of 2 decrease in background free tropospheric aerosol concentrations from the FireOff simulation shifts the cloud evolution from a
classical entrainment-driven “deepening–warming” transition to trade cumulus to a precipitation-driven “drizzle-depletion” transition to open cells,
however. The thermodynamic and dynamic changes caused by the WRF-simulated
large-scale adjustments to smoke diabatic heating strongly influence cloud
evolution in terms of both the rate of deepening (especially for changes in
the inversion temperature jump and in subsidence) and in cloud fraction on
the final day of the simulation (especially for the moisture “banding”
effect). Such large-scale semi-direct effects would not have been possible
to simulate using a small-domain LES model alone.