Separating radiative forcing by aerosol–cloud interactions and rapid cloud adjustments in the ECHAM–HAMMOZ aerosol–climate model using the method of partial radiative perturbations

Journal Article · 17 December 2019 · Atmospheric Chemistry and Physics (Online)

DOI: https://doi.org/10.5194/acp-19-15415-2019 · OSTI ID:1580593

Mülmenstädt, Johannes  ^[1]; Gryspeerdt, Edward  ^[2]; Salzmann, Marc  ^[3]; Ma, Po -Lun  ^[4]; Dipu, Sudhakar  ^[3]; Quaas, Johannes  ^[3]

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1. Univ. Leipzig, Leipzig (Germany); Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
2. Imperial College London, London (United Kingdom)
3. Univ. Leipzig, Leipzig (Germany)
4. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)

Using the method of offline radiative transfer modeling within the partial radiative perturbation (PRP) approach, the effective radiative forcing by aerosol–cloud interactions (ERF_aci) in the ECHAM–HAMMOZ aerosol climate model is decomposed into a radiative forcing by anthropogenic cloud droplet number change and adjustments of the liquid water path and cloud fraction.The simulated radiative forcing by anthropogenic cloud droplet number change and liquid water path adjustment are of approximately equal magnitude at –0.52 and –0.53 W m^–2, respectively, while the cloud-fraction adjustment is somewhat weaker at –0.31 W m^–2 (constituting 38 %, 39 %, and 23 %of the total ERFaci, respectively); geographically, all three ERF_aci components in the simulation peak over China, the subtropical eastern ocean boundaries,the northern Atlantic and Pacific oceans, Europe, and eastern North America (in order of prominence). Spatial correlations indicate that the temporal-mean liquid water path adjustment is proportional to the temporal-mean radiative forcing, while the relationship between cloud-fraction adjustment and radiative forcing is less direct. While the estimate of warm-cloud ERF_aci is relatively insensitive to the treatment of ice and mixed-phase cloud overlying warm cloud, there are indications that more restrictive treatments of ice in the column result in a low bias in the estimated magnitude of the liquid water path adjustment and a high bias in the estimated magnitude of the droplet number forcing. Since the present work is the first PRP decomposition of the aerosol effective radiative forcing into radiative forcing and rapid cloud adjustments, idealized experiments are conducted to provide evidence that the PRP results are accurate. The experiments show that using low-frequency(daily or monthly) time-averaged model output of the cloud property fields underestimates the ERF, but 3-hourly mean output is sufficiently frequent.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Biological and Environmental Research (BER)
• Grant/Contract Number: AC05-76RL01830
• OSTI ID: 1580593
• Report Number(s): PNNL-SA-140892
• Journal Information: Atmospheric Chemistry and Physics (Online), Vol. 19, Issue 24; ISSN 1680-7324
• Publisher: European Geosciences UnionCopyright Statement
• Country of Publication: United States
• Language: English