Forced Precipitation Response in a Single Column Model with Parameterized Dynamics

倾盆大雨是热带地区常见的景象，而热带海洋表面温度最高的广大地区，平均而言，是地球上降雨最多的地方。对于热带人口稠密的国家来说，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就像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，包括它对海温的依赖性、对温室气体变暖的敏感性、对流云的深度和其他因素，是气候动力学的一个主要研究领域。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。大气温度的均匀性推动了基于弱温度梯度(WTG)近似的热带降水理论，其中大尺度大气动力学的净影响是在发生对流和降水的大气柱中施加垂直温度廓线。在这一假设下，对流和降水可以理解为单个大气柱内局部发生的物理和热力学的结果，而不考虑大范围的三维环流。因此，使用WTG近似的单柱模式(SCM)已经成为理解热带对流和降水的重要工具。本文的工作发展并使用建立在WTG近似变体上的SCM来处理热带降水对外部强迫的响应。一个目标是用总湿度稳定度(GMS)来检验降水对温室气体变暖的响应理论，GMS是一种基于柱子与其周围之间热力能量交换的稳定性衡量标准。随着气温的升高，大气增湿导致的GMS减少往往会增加降雨量，但如果变暖导致对流云变高，GMS也会增加。更复杂的是，如果更温暖的条件导致云中最强上升气流的高度增加，即使云本身不变高，GMS也会增加。这种效应可以在使用WTG近似的SCM中捕捉到，但是不同柱物理表示的SCM的结果有很大的差异。这里考察的另一种外部强迫形式是柱子上方平流层的加热，PI试图协调早期研究中发现的平流层加热对不同降水的响应。由于热带降水变化的重大社会影响，这项工作具有更广泛的影响，因为这里开展的工作与用于预测热带天气和气候的模式的发展直接相关。PI还通过将其SCM版本作为共同体地球系统模型(CESM)的一部分提供给更广泛的气候科学界。SCM通常被用作模型层次的一部分，其中全复杂性气候模型被用来模拟感兴趣的现象，而更简单的模型被用来分离特定的物理机制并测试关于它们在全复杂性模拟中所起作用的假设。本项目中开发的SCM使用来自CESM的柱物理表示法，并与CESM软件完全兼容，因此它们可以很容易地整合到为CESM开发的模型层次结构中。此外，这项工作为研究生提供支持和培训，从而为这一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优势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.