Mechanisms of the Negative Shortwave Cloud Feedback in Middle to High Latitudes
Mechanisms of the Negative Shortwave Cloud Feedback in Middle to High Latitudes
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DOI：
10.1175/jcli-d-15-0327.1
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发表时间：
2016-01-01
影响因子：
4.9
通讯作者：
Webb, Mark J.
中科院分区：
文献类型：
--
作者：
Ceppi, Paulo;Hartmann, Dennis L.;Webb, Mark J.
云光学厚度和液态水路径（LWP）的增加是高纬度全球变暖模式模拟的强大功能，产生负短波云反馈，但机制仍然不确定。在这里的微物理过程的负光学深度反馈的重要性进行评估的微物理方案的两个aquaplanet模型，这两个模型都有单独的预测方程液态水和冰的扰动温度。据发现，大部分的LWP增加与变暖是由抑制冰微物理过程中的混合相云，导致液态水转化为冰和降水的效率降低。扰动对流冷凝物的温度相关的相分离也产生一个小的LWP增加。总的来说，大尺度微物理和对流冷凝物分区的扰动解释了超过三分之二的LWP响应相对于一个参考案例与增加SST，并捕获所有的垂直结构的液态水响应。在支持这些发现，一个非常强大的正相关关系，月平均LWP和温度在CMIP5模式和观测显示只存在于混合相云区。在模型中，历史LWP对温度的敏感性是一个很好的预测强迫全球变暖响应的极向约45度，虽然模型似乎高估了LWP对变暖的响应相比，观察。结果表明，在气候模式中，抑制冰相微物理过程，耗尽云液态水是LWP增加的关键驱动力与变暖和相关的负短波云反馈。
Increases in cloud optical depth and liquid water path (LWP) are robust features of global warming model simulations in high latitudes, yielding a negative shortwave cloud feedback, but the mechanisms are still uncertain. Here the importance of microphysical processes for the negative optical depth feedback is assessed by perturbing temperature in the microphysics schemes of two aquaplanet models, both of which have separate prognostic equations for liquid water and ice. It is found that most of the LWP increase with warming is caused by a suppression of ice microphysical processes in mixed-phase clouds, resulting in reduced conversion efficiencies of liquid water to ice and precipitation. Perturbing the temperature-dependent phase partitioning of convective condensate also yields a small LWP increase. Together, the perturbations in large-scale microphysics and convective condensate partitioning explain more than two-thirds of the LWP response relative to a reference case with increased SSTs, and capture all of the vertical structure of the liquid water response. In support of these findings, a very robust positive relationship between monthly mean LWP and temperature in CMIP5 models and observations is shown to exist in mixed-phase cloud regions only. In models, the historical LWP sensitivity to temperature is a good predictor of the forced global warming response poleward of about 45 degrees, although models appear to overestimate the LWP response to warming compared to observations. The results indicate that in climate models, the suppression of ice-phase microphysical processes that deplete cloud liquid water is a key driver of the LWP increase with warming and of the associated negative shortwave cloud feedback.