Dedrick Jeramy L, Pelayo Christian N, Russell Lynn M, Lubin Dan, Mülmenstädt Johannes, Miller Mark
Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92037.
Atmospheric, Climate, and Earth Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99354.
Proc Natl Acad Sci U S A. 2025 Apr;122(13):e2412247122. doi: 10.1073/pnas.2412247122. Epub 2025 Mar 24.
The Twomey effect brightens clouds by increasing aerosol concentrations, which activates more droplets and decreases cloud supersaturation in response to more competition for water vapor. To quantify this competition response, we used marine low cloud observations in clean and smoky conditions at Ascension Island in the tropical South Atlantic during the Layered Aerosol Smoke Interactions with Cloud (LASIC) campaign. These observations show similar increases in droplet number for increased accumulation-mode particles from surface-based and satellite cloud retrievals, demonstrating the importance of below-cloud aerosol measurements for retrieving aerosol-cloud interactions (ACI) in clean and smoky aerosol conditions. Four methods for estimating cloud supersaturation from aerosol-cloud measurements were compared, with cloud scene-based and parcel-based methods showing sufficient variability for a strong dependence on both aerosol accumulation number concentration and cloud-base updraft velocities. Decomposing aerosol-related changes in cloud albedo and optical depth shows the calculated competition response accounts for dampening the activation response by 12 to 35%, explaining the diminished Twomey effect at high aerosol concentrations observed for smoky conditions at LASIC and previously around the world. This result was consistent for independent supersaturation retrievals by cloud scene-based droplet number and cloud condensation nuclei and parcel-based multimode size-resolving Lagrangian methods. Translating aerosol effects to local radiative forcing with clean conditions as a proxy for preindustrial and smoky conditions for present-day showed that the competition response reduces cooling from the Twomey radiative forcing by 12 to 35%, providing an essential process-specific constraint for improving the representation of aerosol competition in climate model simulation of indirect aerosol forcing.
Twomey效应通过增加气溶胶浓度使云层亮度增加，这会激活更多的云滴，并因对水汽的竞争加剧而降低云的过饱和度。为了量化这种竞争响应，我们在热带南大西洋阿森松岛进行的“分层气溶胶与烟雾相互作用云”（LASIC）实验中，利用了清洁和烟雾条件下的海洋低云观测数据。这些观测结果表明，基于地面和卫星云反演的累积模态粒子增加时，云滴数量有类似的增加，这表明在清洁和烟雾气溶胶条件下，云下气溶胶测量对于获取气溶胶-云相互作用（ACI）至关重要。我们比较了四种从气溶胶-云测量中估算云过饱和度的方法，基于云场景和基于气块的方法显示出足够的变异性，强烈依赖于气溶胶累积数浓度和云底上升气流速度。分解与气溶胶相关的云反照率和光学深度变化表明，计算出的竞争响应使激活响应减弱了12%至35%，这解释了在LASIC实验中以及此前全球范围内烟雾条件下高气溶胶浓度时Twomey效应减弱的现象。对于基于云场景的云滴数和云凝结核以及基于气块的多模态尺寸分辨拉格朗日方法进行的独立过饱和度反演，这一结果是一致的。将清洁条件下的气溶胶效应转换为工业化前的代理条件，将烟雾条件转换为当前条件下的局部辐射强迫，结果表明竞争响应使Twomey辐射强迫的冷却作用降低了12%至35%，为改进气候模型模拟间接气溶胶强迫时气溶胶竞争的表示提供了一个重要的特定过程约束。
