On small scales, the tropical atmosphere tends to be either moist or very dry. This defines two states that, on large scales, are separated by a sharp margin, well identified by the antimode of the bimodal tropical column water vapor distribution. Despite recent progress in understanding physical processes governing the spatiotemporal variability of tropical water vapor, the behavior of this margin remains elusive, and we lack a simple framework to understand the bimodality of tropical water vapor in observations. Motivated by the success of coarsening theory in explaining bimodal distributions, we leverage its methodology to relate the moisture field's spatial organization to its time evolution. This results in a new diagnostic framework for the bimodality of tropical water vapor, from which we argue that the length of the margin separating moist from dry regions should evolve toward a minimum in equilibrium. As the spatial organization of moisture is closely related to the organization of tropical convection, we hereby introduce a new convective organization index (BLW) measuring the ratio of the margin's length to the circumference of a well‐defined equilibrium shape. Using BLW, we assess the evolution of self‐aggregation in idealized cloud‐resolving simulations of radiative‐convective equilibrium and contrast it to the time evolution of the Atlantic Intertropical Convergence Zone (ITCZ) in the ERA5 meteorological reanalysis product. We find that BLW successfully captures aspects of convective organization ignored by more traditional metrics, while offering a new perspective on the seasonal cycle of convective organization in the Atlantic ITCZ.

中文翻译：

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使用热带湿缘长度来量化对流聚集

在小范围内，热带大气趋向于潮湿或非常干燥。这定义了两个状态，这些状态在很大程度上以锐利边缘分开，并通过双峰热带气柱水蒸气分布的反模式很好地识别了该状态。尽管最近在理解控制热带水蒸气时空变化的物理过程方面取得了进展，但这一余量的行为仍然难以捉摸，并且我们缺乏一个简单的框架来了解观测中热带水蒸气的双峰性。由于粗化理论在解释双峰分布方面取得了成功，因此我们利用其方法论将湿度场的空间组织与其时间演化联系起来。这为热带水蒸气的双峰性建立了新的诊断框架，从中我们可以得出结论，将湿润地区与干燥地区分开的边际长度应朝着平衡的最小方向发展。由于水分的空间组织与热带对流的组织密切相关，因此我们引入一种新的对流组织指数（BLW），用于测量边界长度与周向的平衡形状的比率。使用BLW，我们在辐射对流平衡的理想云解析模拟中评估了自聚集的演化，并将其与ERA5气象再分析产品中大西洋热带辐合带（ITCZ）的时间演化进行了对比。我们发现BLW成功地捕获了对流组织的各个方面，而传统的指标却忽略了这些对流组织，