Effect of Tropical Nonconvective Condensation on Uncertainty in Modeled Projections of Rainfall
Effect of Tropical Nonconvective Condensation on Uncertainty in Modeled Projections of Rainfall
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DOI：
10.1175/jcli-d-18-0833.1
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发表时间：
2019-09
影响因子：
4.9
通讯作者：
Benjamin A. Stephens;C. Jackson;B. Wagman
中科院分区：
文献类型：
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作者：
Benjamin A. Stephens;C. Jackson;B. Wagman
我们发现，模拟的降水对二氧化碳强迫响应的幅度和模式的部分不确定性是热带凝结造成的，与参数化对流没有直接关系。根据模型细节和参数设置的不同，与大规模凝结相关的热带降雨的比例可能从几个百分点到远超过一半不等。反过来，由于凝结和热带环流之间的耦合，模式假设影响大尺度降雨量的不同方式也影响到个别模式内的响应模式。在基于国家大气研究中心(NCAR)社区大气模式(CAM)3.1版和5.3版的两个单模式集合中，我们发现热带大尺度降水的比例与气候降水和环流以及对CO2强迫的响应之间存在很强的相关性。虽然热带大尺度降水分数增加的影响在某些方面在两个集合中是相反的--例如，在CAM3.1集合中，Hadley环流随着大尺度降水分数的增加而减弱，而在CAM5.3集合中则随着大尺度降水分数的增加而增强--但我们仍然可以从潜热和环流之间的关系来理解这些不同的影响，并对每个集合提出了解释。我们将这些结果与耦合模型比较项目(CMIP5)第五阶段的数据进行了比较，其中一些相同的模式适用。鉴于这种划分的重要性，有必要使用观察值来约束这种不确定性来源。然而，由于“大尺度降雨率”是一个模型构造，目前尚不清楚如何利用观测数据来检验决定这个降雨率的各种模型假设。
We find that part of the uncertainty in the amplitude and pattern of the modeled precipitation response to CO2 forcing traces to tropical condensation not directly involved with parameterized convection. The fraction of tropical rainfall associated with large-scale condensation can vary from a few percent to well over half depending on model details and parameter settings. In turn, because of the coupling between condensation and tropical circulation, the different ways model assumptions affect the large-scale rainfall fraction also affect the patterns of the response within individual models. In two single-model ensembles based on the National Center for Atmospheric Research (NCAR) Community Atmosphere Model (CAM), versions 3.1 and 5.3, we find strong correlations between the fraction of tropical large-scale rain and both climatological rainfall and circulation and the response to CO2 forcing. While the effects of an increasing tropical large-scale rain fraction are opposite in some ways in the two ensembles—for example, the Hadley circulation weakens with the large-scale rainfall fraction in the CAM3.1 ensemble while strengthening in the CAM5.3 ensemble—we can nonetheless understand these different effects in terms of the relationship between latent heating and circulation, and we propose explanations for each ensemble. We compare these results with data from phase 5 of the Coupled Model Intercomparison Project (CMIP5), for which some of the same patterns hold. Given the importance of this partitioning, there is a need for constraining this source of uncertainty using observations. However, since a “large-scale rainfall fraction” is a modeling construct, it is not clear how observations may be used to test various modeling assumptions determining this fraction.