Forced Precipitation Response in a Single Column Model with Parameterized Dynamics

成片的暴雨是热带地区的常见景象，而热带最温暖的海面温度（SST）的广阔区域，平均而言，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，暴雨既是一种威胁，也是一种必不可少的资源，而产生暴雨的深对流云则是推动全球大气环流的热力发动机的“锅炉”。热带降水的研究，包括其对SST的依赖性、对温室效应的敏感性、对流云的深度和其他因素，因此是气候动力学的主要研究领域。热带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以上的水平。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廓线。 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，而不考虑大尺度三维环流。因此，使用WTG近似的单柱模式（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和使用SCM建立在WTG近似的变体，以解决热带降水对外部强迫的响应。 一个目标是研究理论的降水温室效应的响应框架总湿稳定性（GMS），一个稳定性措施的基础上交换的热力学能量之间的列和周围环境。 由于气温升高，大气变湿，GMS减少，这往往会增加降水，但如果变暖导致对流云变高，GMS也会增加。 更复杂的是，如果温暖的条件导致云层中最强的上升气流的高度增加，即使云层本身没有变得更高，GMS也会增加。 这种效应可以在使用WTG近似的SCM中捕获，但是具有不同柱物理表示的SCM的结果存在很大差异。 这里研究的另一种形式的外部强迫是加热在平流层以上的柱，和PI试图调和不同的降水响应平流层加热在早期的研究中发现，由于热带降水变化的重大社会影响，工作有更广泛的影响，因为这里进行的工作有直接关系到用于预测热带天气和气候的模式的发展。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。SCMs通常用作模型层次的一部分，其中全复杂性气候模型用于模拟感兴趣的现象，而较简单的模型用于隔离特定的物理机制并测试关于其在全复杂性模拟中的作用的假设。在这个项目中开发的SCM使用从CESM中提取的柱物理表示，并与CESM软件完全兼容，因此它们可以很容易地被纳入为CESM开发的模型层次结构中。 此外，这项工作提供了支持和培训的研究生，从而提供了未来的劳动力在这一研究领域。这一奖项反映了NSF的法定使命，并已被认为是值得通过评估使用基金会的智力价值和更广泛的影响审查标准的支持。

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.