Forced Precipitation Response in a Single Column Model with Parameterized Dynamics

倾盆大雨是热带地区的常见景象，从平均意义上讲，热带海面温度（SST）最温暖的大片地区是地球上降雨量最多的地方。对于热带地区人口众多的国家来说，大雨既是一种威胁，也是一种重要资源，而产生大雨的深对流云则是驱动全球大气环流的热机的“锅炉”。因此，对热带降水的研究，包括其对海温的依赖性、对温室变暖的敏感性、对流云的深度以及其他因素，是气候动力学的主要研究领域。热带地区的一个特征是缺乏强烈的温度对比，特别是在地球表面附近产生的湍流运动之上的水平上。大气温度的均匀性激发了基于弱温度梯度（WTG）近似的热带降水理论，其中大尺度大气动力学的净效应是在发生对流和降水的大气柱中施加垂直温度剖面。 在这种假设下，对流和降水可以被理解为在单个大气柱内局部发生的物理和热力学的结果，而不考虑大规模的三维环流。因此，使用 WTG 近似的单柱模型 (SCM) 已成为了解热带对流和降水的重要工具。这里进行的工作开发并使用基于 WTG 近似变体构建的 SCM 来解决热带降水对外部强迫的响应。 一个目标是研究降水对温室变暖的响应理论，该理论以总湿度稳定性（GMS）为框架，这是一种基于柱与其周围环境之间热力学能量交换的稳定性度量。 由于气温升高而使大气湿润而导致的 GMS 减少往往会增加降水，但如果变暖导致对流云变得更高，GMS 也会增加。 更复杂的是，如果较温暖的条件导致云内最强上升气流的高度增加，即使云本身没有变高，GMS也会增加。 这种效应可以在 SCM 中使用 WTG 近似来捕获，但具有不同柱物理表示的 SCM 的结果存在很大差异。 这里检查的另一种形式的外部强迫是柱上方平流层的加热，PI试图协调早期研究中发现的平流层加热的不同降水响应。由于热带降水变化的巨大社会影响，这项工作具有更广泛的影响，因为这里进行的工作与用于预测热带天气和气候的模型的开发直接相关。 PI 还通过将其 SCM 版本作为社区地球系统模型 (CESM) 的一部分提供给更广泛的气候科学界。 SCM 通常用作模型层次结构的一部分，其中全复杂性气候模型用于模拟感兴趣的现象，而更简单的模型用于隔离特定的物理机制并测试有关其在全复杂性模拟中的作用的假设。该项目中开发的 SCM 使用来自 CESM 的柱物理表示，并且与 CESM 软件完全兼容，因此它们可以轻松地合并到为 CESM 开发的模型层次结构中。 此外，这项工作还为研究生提供支持和培训，从而为该研究领域的未来劳动力提供支持。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

Rain falling in sheets and torrents is a common image of the tropics, and the broad regions of warmest tropical sea surface temperature (SST) are, in an averaged sense, the rainiest places on earth. Heavy rain is both a threat and an essential resource for the populous countries of the tropics, and the deep convective clouds that produce it serve as the "boiler" of the heat engine that drives the global atmospheric circulation. The study of tropical precipitation, including its dependence on SST, sensitivity to greenhouse warming, depth of convective clouds, and other factors is thus a primary research area in climate dynamics.A characteristic feature of the tropics is the lack of strong temperature contrasts, particularly at levels above the turbulent motions generated near the earth's surface. The uniformity of atmospheric temperature has motivated theories of tropical precipitation based on the weak temperature gradient (WTG) approximation, in which the net effect of large-scale atmospheric dynamics is to impose a vertical temperature profile in the atmospheric columns where convection and precipitation are occurring. Under this assumption convection and precipitation can be understood as a consequence of physics and thermodynamics occurring locally within a single atmospheric column, without taking the large-scale three-dimensional circulation into account. Thus, single column models (SCMs) using the WTG approximation have become important tools for understanding tropical convection and precipitation.Work performed here develops and uses SCMs built on variants of the WTG approximation to address the response of tropical precipitation to external forcing. One goal is to examine theories of the response of precipitation to greenhouse warming framed in terms of gross moist stability (GMS), a stability measure based on the exchange of thermodynamic energy between the column and its surroundings. Reductions in GMS due to the moistening of the atmosphere with increasing temperature tend to increase precipitation but GMS can also increase if warming causes convective clouds to become taller. A further complication is that GMS increases if warmer conditions cause the height of the strongest updrafts within clouds to increase, even if the clouds themselves do not get taller. This effect can be captured in SCMs using the WTG approximation but there are large discrepancies in results from SCMs with different representations of column physics. Another form of external forcing examined here is heating in the stratosphere above the column, and the PIs attempt to reconcile differing precipitation responses to stratospheric heating found in earlier studies.The work has broader impacts due to the substantial societal impacts of changes in tropical precipitation, as work performed here has direct relevance to the development of models used to predict the weather and climate of the tropics. The PIs also serve the broader climate science community by making their SCM versions available as part of the Community Earth System Model (CESM). SCMs are commonly used as part of a hierarchy of models, in which full-complexity climate models are used to simulate phenomena of interest and simpler models are used to isolate particular physical mechanisms and test hypotheses regarding their roles in the full-complexity simulations. The SCMs developed in this project use column physics representations taken from CESM and are fully compatible with CESM software, thus they can be easily incorporated into the model hierarchy developed for CESM. In addition, the work provides support and training to a graduate student, thereby providing for the future workforce in this research area.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.