Diurnal timescale feedbacks in the tropical cumulus regime
Diurnal timescale feedbacks in the tropical cumulus regime
复制标题
热带积云状况的昼夜时间尺度反馈
DOI：
10.1002/2016ms000713
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发表时间：
2016
影响因子：
6.8
通讯作者：
J. Ruppert
中科院分区：
文献类型：
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作者：
J. Ruppert
虽然日周期在调节云和降水方面的重要性早已被认识到，但其对气候系统在较长时间尺度上的影响仍然难以捉摸。越来越多的证据表明，日周期可能会通过非线性校正对主导气候模态产生重大影响。在这项研究中，一个理想化的云解析模式实验被执行，以隔离在热带暖池上空的浅积云制度的昼夜时间尺度反馈。这种反馈是通过修改（或删除它）的昼夜周期，按比例缩放（或删除）的昼夜热力学强迫云响应隔离。这种日强迫被确定为对流层低层静稳定度和湿度的协变周期，其中最不稳定的条件与每天下午的最大湿度相吻合。这种日强迫产生了更深的云和更大的日平均积云加热，而不是其他情况，反过来又减少了每天的大规模沉降，根据“弱温度梯度”近似。因此，这种日强迫通过加速深对流的发生而表现为时间尺度的反馈。长波云辐射效应被发现放大了这种时间尺度反馈，因为由此产生的云的活跃（增加了上层云的辐射冷却，抑制了下面的冷却）与云的深度（即，光学厚度），因此与周日强迫的大小有关。这些发现强调了迫切需要解决许多气候模式中与日周期有关的长期问题。鉴于气候变率对日变化过程的明显敏感性，这样做可能会在更长时间尺度的气候预测方面取得进展。
Although the importance of the diurnal cycle in modulating clouds and precipitation has long been recognized, its impact on the climate system at longer timescales has remained elusive. Mounting evidence indicates that the diurnal cycle may substantially affect leading climate modes through nonlinear rectification. In this study, an idealized cloud‐resolving model experiment is executed to isolate a diurnal timescale feedback in the shallow cumulus regime over the tropical warm pool. This feedback is isolated by modifying the period of the diurnal cycle (or removing it), which proportionally scales (or removes) the diurnal thermodynamic forcing that clouds respond to. This diurnal forcing is identified as covarying cycles of static stability and humidity in the lower troposphere, wherein the most unstable conditions coincide with greatest humidity each afternoon. This diurnal forcing yields deeper clouds and greater daily‐mean cumulus heating than would otherwise occur, in turn reducing large‐scale subsidence from day to day according to the “weak temperature gradient” approximation. This diurnal forcing therefore manifests as a timescale feedback by accelerating the onset of deep convection. The longwave cloud‐radiation effect is found to amplify this timescale feedback, since the resulting invigoration of clouds (increased upper‐cloud radiative cooling, with suppressed cooling below) scales with cloud depth (i.e., optical thickness), and hence with the magnitude of diurnal forcing. These findings highlight the pressing need to remedy longstanding problems related to the diurnal cycle in many climate models. Given the evident sensitivity of climate variability to diurnal processes, doing so may yield advances in climate prediction at longer timescales.