Details:

Journal Title:
Atmospheric Chemistry and Physics
Personal Author:
Diamond, Michael S. ; Saide, Pablo E. ; Zuidema, Paquita ; Ackerman, Andrew S. ; Doherty, Sarah J. ; ... More +
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 Less -
NOAA Program & Office:
OAR (Oceanic and Atmospheric Research) ; CIRES (Cooperative Institute for Research in Environmental Sciences) ; CSL (Chemical Sciences Laboratory) ; CICOES (Cooperative Institute for Climate, Ocean and Ecosystem Studies)
OAR (Oceanic and Atmospheric Research) ; CIRES (Cooperative Institute for Research in Environmental Sciences) ; CSL (Chemical Sciences Laboratory) ; CICOES (Cooperative Institute for Climate, Ocean and Ecosystem Studies) Less -
Description:
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 scatteredcumulus clouds are investigated along a Lagrangian (air-mass-following)trajectory in regional climate and large eddy simulation models. Results arecompared with observations from three recent field campaigns that took placein August 2017: ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES), CLouds and Aerosol Radiative Impacts andForcing: 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 2017likely entrained into the marine boundary layer later sampled during thejoint flight. The case is first simulated with the WRF-CAM5 regional climate model inthree distinct setups: (1) FireOn, in which smoke emissions and any resultingsmoke–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 interactdirectly with radiation. Over the course of the Lagrangian trajectory,differences in free tropospheric thermodynamic properties between FireOn andFireOff are nearly identical to those between FireOn and RadOff, showingthat aerosol–radiation interactions are primarily responsible for the free tropospheric effects. These effects are non-intuitive: in addition to theexpected heating within the core of the smoke plume, there is also a“banding” effect of cooler temperature (∼1–2 K) and greatlyenhanced moisture (>2 g kg−1) at the plume top. This banding effect is caused by a vertical displacement of the former continental boundary layerin the free troposphere in the FireOn simulation resulting from anomalousdiabatic heating due to smoke absorption of sunlight that manifestsprimarily 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 fieldstaken from the outputs for the WRF-CAM5 FireOn and FireOff runs. Cases arerun by selectively perturbing one variable (e.g., aerosol numberconcentration, temperature, moisture, vertical velocity) at a time to betterunderstand the contributions from different indirect (microphysical),“large-scale” semi-direct (above-cloud thermodynamic and subsidencechanges), 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 cloudfraction 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 aclassical 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-simulatedlarge-scale adjustments to smoke diabatic heating strongly influence cloudevolution in terms of both the rate of deepening (especially for changes inthe inversion temperature jump and in subsidence) and in cloud fraction onthe final day of the simulation (especially for the moisture “banding”effect). Such large-scale semi-direct effects would not have been possibleto simulate using a small-domain LES model alone.
Keywords:
[+]
Atmospheric Science
Source:
Atmospheric Chemistry and Physics, 22(18), 12113-12151
DOI:
https://doi.org/10.5194/acp-22-12113-2022
ISSN:
1680-7324
Format:
PDF
Publisher:
Copernicus GmbH
Document Type:
Journal Article
Funding:
Grant no. NA17OAR4320101
License:
https://creativecommons.org/licenses/by/4.0/
Rights Information:
CC BY
Compliance:
Submitted
Main Document Checksum:
[+]
urn:sha256:81830fe8571440e937e3d27956c5292aca19242920eed44cc298adbafb82ea8d
Download URL:
https://repository.library.noaa.gov/view/noaa/49778/noaa_49778_DS1.pdf
File Type:

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