Implications of Warm Rain in Shallow Cumulus and Congestus Clouds for Large-Scale Circulations
Implications of Warm Rain in Shallow Cumulus and Congestus Clouds for Large-Scale Circulations
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DOI：
10.1007/s10712-017-9429-z
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发表时间：
2017-09
影响因子：
4.6
通讯作者：
L. Nuijens;K. Emanuel;H. Masunaga;T. L’Ecuyer
中科院分区：
文献类型：
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作者：
L. Nuijens;K. Emanuel;H. Masunaga;T. L’Ecuyer
星载观测显示，在冰点以下的海洋对流云中，有20-40%会产生降雨。在本文中，我们推测暖雨的盛行对热带海洋上的对流和大尺度环流可能意味着什么。本文介绍了非旋转球体上流体静力学非线性流动的两柱辐射-对流模型的结果，其中对流和辐射参数化，并回顾了目前在高分辨率暖雨建模和观测方面的研究进展。模式试验研究了对流和环流对柱间海表温度梯度的响应，以及对控制云凝结物向雨转化的参数变化的响应。冷海洋上的对流坍缩为顶部在850 hPa附近的浅模态，而顶部在600 hPa附近的密集模态可以在较小的海温差下发展，此时暖雨的形成更有效。在这里，相互作用的辐射和环流的响应是至关重要的：随着拥挤，一个更深的潮湿层形成，这导致更少的低层辐射冷却，柱之间的浮力梯度更小，因此环流更弱，在寒冷的海洋上下沉更少。在密集模式下，沉降柱地表降水较多，深对流柱地表降水较少。对于较冷的海洋上的浅模态，环流也会随着更有效的暖雨形成而减弱，但只是轻微减弱。在这里，更多的暖雨减少了对流顶部和边界层深度——类似于大涡模拟（LES）的研究——从而降低了整体浮力梯度。阐明暖雨的影响可以从大范围的高分辨率模拟和观测中受益。地面云和雨剖面的配置可能会限制暖雨的参数化，而包括全球降水测量（GPM）和即将到来的Aeolus任务在内的星载传感器的配置可能会揭示沉降和对流环流分支中对流和雨的同步变化。
Space-borne observations reveal that 20–40% of marine convective clouds below the freezing level produce rain. In this paper we speculate what the prevalence of warm rain might imply for convection and large-scale circulations over tropical oceans. We present results using a two-column radiative–convective model of hydrostatic, nonlinear flow on a non-rotating sphere, with parameterized convection and radiation, and review ongoing efforts in high-resolution modeling and observations of warm rain. The model experiments investigate the response of convection and circulation to sea surface temperature (SST) gradients between the columns and to changes in a parameter that controls the conversion of cloud condensate to rain. Convection over the cold ocean collapses to a shallow mode with tops near 850 hPa, but a congestus mode with tops near 600 hPa can develop at small SST differences when warm rain formation is more efficient. Here, interactive radiation and the response of the circulation are crucial: along with congestus a deeper moist layer develops, which leads to less low-level radiative cooling, a smaller buoyancy gradient between the columns, and therefore a weaker circulation and less subsidence over the cold ocean. The congestus mode is accompanied with more surface precipitation in the subsiding column and less surface precipitation in the deep convecting column. For the shallow mode over colder oceans, circulations also weaken with more efficient warm rain formation, but only marginally. Here, more warm rain reduces convective tops and the boundary layer depth—similar to Large-Eddy Simulation (LES) studies—which reduces the integrated buoyancy gradient. Elucidating the impact of warm rain can benefit from large-domain high-resolution simulations and observations. Parameterizations of warm rain may be constrained through collocated cloud and rain profiling from ground, and concurrent changes in convection and rain in subsiding and convecting branches of circulations may be revealed from a collocation of space-borne sensors, including the Global Precipitation Measurement (GPM) and upcoming Aeolus missions.