The decomposition of cloud-aerosol forcing in the UK Earth System Model (UKESM1)
The decomposition of cloud-aerosol forcing in the UK Earth System Model (UKESM1)
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DOI：
10.5194/acp-20-15681-2020
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发表时间：
2020-12-17
影响因子：
6.3
通讯作者：
Carslaw, Kenneth S.
中科院分区：
文献类型：
--
作者：
Grosvenor, Daniel P.;Carslaw, Kenneth S.
北大西洋的气候变化影响着飓风活动和干旱等过程。全球模式模拟已经确定气溶胶-云相互作用（ACI）是通过表面气溶胶强迫海表温度变化的重要驱动力。然而，气候变化指数是气候强迫不确定性的一个主要原因;因此，在解释来自粗分辨率、高度参数化的全球模式的结果时需要谨慎。在英国地球系统模式（UKESM 1）的大气版本中，我们分离和量化了由气溶胶引起的地表短波有效辐射强迫（ERF）的分量并根据观测结果评估云的特性及其辐射效应。我们集中在北大西洋（NA）的北方地区的层积云为主（表示为北方NA地区）和南部地区的贸易积云和破碎的层积云为主（南部NA地区）。气溶胶强迫诊断使用一对模拟，其中的气象学是近似固定的，通过轻推分析，一个模拟工业化前（PI）和一个有现在的（PD）气溶胶排放。该模式在对流参数化中不包括气溶胶效应（但气溶胶确实会影响与消散相关的云），因此应该注意的是，对流气溶胶强迫的表示是不完整的。云分数（f（c））、云中液态水路径（LWPic）和液滴数浓度（N-d）的变化对地面ERF的贡献被量化。在北半球北方地区，N（d）（）和LWPic的增加是主要的强迫。这可能是因为已经很高的f（c）降低了f（c）进一步大幅增加的机会，并允许云增亮作用于更大的区域。在北大西洋南部地区，由于额外的气溶胶对降雨的抑制，f（c）的增加占主导地位。气溶胶驱动的云的宏观物理特性（LWPic和f（c））的增加将依赖于边界层参数化的响应，沿着来自云微物理方案的输入，这是高度不确定的过程。PI和PD中存在低空云的模式网格框主导了两个区域的强迫。在北半球北方，完全多云的低云场景（100%的云覆盖，可能是层积云）的增亮贡献最大，而在北半球南部，来自PI晴朗天空的f（c）约为20%的云的形成是最大的单一贡献者，这表明贸易积云的形成是对气溶胶增加的响应。近阴天的云的创建也很重要，正确的空间格局，覆盖范围和云的属性是很重要的气溶胶强迫的大小确定，所以我们也评估了模拟的PD云对卫星观测的真实性。我们发现，该模型再现了所有观测到的云变量的空间格局，但有偏见。短波大气顶层（SWTOA）通量在北半球北方地区被高估了5.8%，在南半球被高估了1.7%，我们认为这主要是由于低空f（c）的正偏差。N-d在北方NA中过低-20.6%，在南方NA中过高21.5%，但对主要SWTOA偏差贡献不大。多云天空液态水路径主要显示斯堪的纳维亚半岛北部的偏差，达到50%到100%之间，并主导该地区的SWTOA偏差。高度不确定的宏观物理调整对UKESM 1模式中气溶胶强迫的巨大贡献表明，需要进一步的有针对性的观测来评估降雨形成过程，它们如何依赖于气溶胶和模式对降水的响应，以减少气候预测的不确定性。
Climate variability in the North Atlantic influences processes such as hurricane activity and droughts. Global model simulations have identified aerosol-cloud interactions (ACIs) as an important driver of sea surface temperature variability via surface aerosol forcing. However, ACIs are a major cause of uncertainty in climate forcing; therefore, caution is needed in interpreting the results from coarse-resolution, highly parameterized global models.Here, we separate and quantify the components of the surface shortwave effective radiative forcing (ERF) due to aerosol in the atmosphere-only version of the UK Earth System Model (UKESM1) and evaluate the cloud properties and their radiative effects against observations. We focus on a northern region of the North Atlantic (NA) where stratocumulus clouds dominate (denoted the northern NA region) and a southern region where trade cumulus and broken stratocumulus dominate (southern NA region). Aerosol forcing was diagnosed using a pair of simulations in which the meteorology is approximately fixed via nudging to analysis; one simulation has pre-industrial (PI) and one has present-day (PD) aerosol emissions. This model does not include aerosol effects within the convective parameterization (but aerosol does affect the clouds associated with detrainment) and so it should be noted that the representation of aerosol forcing for convection is incomplete.Contributions to the surface ERF from changes in cloud fraction (f(c)), in-cloud liquid water path (LWPic) and droplet number concentration (N-d) were quantified. Over the northern NA region, increases in N(d)( )and LWPic dominate the forcing. This is likely because the already-high f(c) there reduces the chances of further large increases in f(c) and allows cloud brightening to act over a larger region. Over the southern NA region, increases in f(c) dominate due to the suppression of rain by the additional aerosols. Aerosol-driven increases in macrophysical cloud properties (LWPic and f(c)) will rely on the response of the boundary layer parameterization, along with input from the cloud microphysics scheme, which are highly uncertain processes.Model grid boxes with low-altitude clouds present in both the PI and PD dominate the forcing in both regions. In the northern NA, the brightening of completely overcast low cloud scenes (100 % cloud cover, likely stratocumulus) contributes the most, whereas in the southern NA the creation of clouds with f(c) of around 20 % from clear skies in the PI was the largest single contributor, suggesting that trade cumulus clouds are created in response to increases in aerosol. The creation of near-overcast clouds was also important there.The correct spatial pattern, coverage and properties of clouds are important for determining the magnitude of aerosol forcing, so we also assess the realism of the modelled PD clouds against satellite observations. We find that the model reproduces the spatial pattern of all the observed cloud variables well but that there are biases. The shortwave top-of-the-atmosphere (SWTOA) flux is overestimated by 5.8 % in the northern NA region and 1.7 % in the southern NA, which we attribute mainly to positive biases in low-altitude f(c). N-d is too low by -20.6 % in the northern NA and too high by 21.5 % in the southern NA but does not contribute greatly to the main SWTOA biases. Cloudy-sky liquid water path mainly shows biases north of Scandinavia that reach between 50 % and 100 % and dominate the SWTOA bias in that region.The large contribution to aerosol forcing in the UKESM1 model from highly uncertain macrophysical adjustments suggests that further targeted observations are needed to assess rain formation processes, how they depend on aerosols and the model response to precipitation in order to reduce uncertainty in climate projections.