弛豫振荡器对流涌现的简单模型
A simple model for the emergence of relaxation-oscillator convection
AI总结：
本文结合零浮力整体羽流模型与准平衡热机模型，提出高地表温度下热机平衡被破坏从而产生弛豫振荡器对流的机制，并经一维和三维模拟验证。
摘要：
地球热带的特征是准稳定降水，降水围绕平均值小幅振荡，这促使人们提出湿对流处于准平衡（QE）状态的假说。相比之下，对地球热带对流的极暖模拟则呈现出类似弛豫振荡器（RO）的降水特征：短时对流风暴和暴雨以固定间隔形成并消散，其间降水极少甚至完全没有。我们通过将整体羽流对流的零浮力模型与 QE 热机模型相结合，建立了一个湿对流模型，并用其表明在高地表温度下 QE 会被破坏。我们假设，当对流热机的平衡条件被违反时，即当加热率乘以热力学效率超过功可被执行的速率时，RO 状态就会涌现。我们利用一维和三维数值模拟检验该假设，发现它能准确预测 RO 对流的起始。所提出的由 QE 崩溃导致 RO 涌现的机制不依赖于可凝结物质，可应用于任何发生湿对流的行星大气。迄今为止，RO 状态仅在三维对流解析模拟中得到证明，这使人们似乎认为 RO 状态的物理过程需要能够显式解析云羽与其环境之间三维相互作用的模拟。我们证明，在采用参数化对流的辐射—对流平衡单柱模拟中也存在 RO 状态，尽管其处于不同的地表温度范围，并具有长得多的无风暴间隔。
英文摘要：
Earth's tropics are characterized by quasi-steady precipitation with small oscillations about a mean value, which has led to the hypothesis that moist convection is in a state of quasi-equilibrium (QE). In contrast, very warm simulations of Earth's tropical convection are characterized by relaxation-oscillator-like (RO) precipitation, with short-lived convective storms and torrential rainfall forming and dissipating at regular intervals with little to no precipitation in between. We develop a model of moist convection by combining a zero-buoyancy model of bulk-plume convection with a QE heat engine model, and we use it to show that QE is violated at high surface temperatures. We hypothesize that the RO state emerges when the equilibrium condition of the convective heat engine is violated, i.e., when the heating rate times a thermodynamic efficiency exceeds the rate at which work can be performed. We test our hypothesis against one- and three-dimensional numerical simulations and find that it accurately predicts the onset of RO convection. The proposed mechanism for RO emergence from QE breakdown is agnostic of the condensable, and can be applied to any planetary atmosphere undergoing moist convection. To date, RO states have only been demonstrated in three-dimensional convection-resolving simulations, which has made it seem that the physics of the RO state requires simulations that can explicitly resolve the three-dimensional interaction of cloudy plumes and their environment. We demonstrate that RO states also exist in single-column simulations of radiative-convective equilibrium with parameterized convection, albeit in a different surface temperature range and with much longer storm-free intervals.
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