NSF-BSF: Quantitative Evaluation of Aerosol Impacts on the Microphysical Composition, Electrification and Radiative Forcing of Deep Tropical Convective Clouds

大气气溶胶颗粒的大小从几纳米到几十微米不等，既有自然来源，也有人为来源。气溶胶粒子充当云凝结核（CCN），因此气溶胶粒子丰度的变化可以改变云滴浓度，进而改变云的反射率和寿命以及相关的降水。热带深层对流云（DCC）对地球的全球能量平衡和水文循环具有重要意义，并能引起具有显著降水和闪电的高影响天气。然而，气溶胶颗粒对DCC特性的影响，包括降水的开始和数量、垂直发展、通电和相关的闪电，以及相关的高空冰云的范围和寿命，仍然缺乏量化。在这个项目中，气溶胶颗粒对这些天气和气候相关现象的影响进行了研究，解决了一个主要的开放性问题，即人为引起的气溶胶颗粒污染是否以及如何改变了天气现象（例如，可能将热带阵雨转变为雷暴）和气候。全球气候模型正被用来为世界各地的社会经济政策决策提供信息，但本项目所研究的过程实际上并不存在于这些模型中。因此，这项研究的结果可能对我们对人为气溶胶对气候预测的影响的理解产生广泛的影响，进而对正在制定的减缓和适应政策产生影响。该项目将通过国际合作在大气基础物理和化学方面培训研究生和本科生，并发展他们在分析大型多变量数据集、大气三维计算机模型和卫星遥感技术方面的技术技能。热带海洋地区可能对起CCN作用的气溶胶颗粒的额外输入最为敏感。40纳米大小的超细气溶胶颗粒（UAP）通常不被认为是CCN，但它们实际上可能在DCC中形成云滴核。在这样的清洁地区，缺乏对UAP和CCN的观测，而这些地区需要检验关于气溶胶对深层对流云的影响及其对气候的相关影响的假设。为了解决这些限制，这个由华盛顿大学（UW）、以色列耶路撒冷希伯来大学（HUJI）和合作者组成的NSF-BSF联合项目，利用i) DCC微物理遥感来约束影响单个对流事件的CCN和UAP， ii)在UAP和CCN的化学输运模型预测的指导下，分析15年的全球热带雷击场。iii)受卫星遥感观测、闪电观测以及UAP和CCN的化学传输模式预测约束的关键域的云解析建模。该项目将评估CCN（包括UAP）如何干扰热带深层对流云的微物理，以及这种扰动在多大程度上影响闪电和云的辐射效应，并提供一个假设的测试，即自工业化前时代以来，由于与燃料燃烧相关的人类活动，CCN和UAP的增加通过深层对流云对气候产生了积极的辐射强迫（变暖）。这种气候强迫将与负辐射强迫相反，负辐射强迫中CCN的增加影响低云反照率和寿命，并将改变我们对气候敏感性的理解。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Atmospheric aerosol particles range in size from a few nanometers to tens of micrometers and have both natural and anthropogenic sources. Aerosol particles act as cloud condensation nuclei (CCN), and therefore changes in aerosol particle abundance can alter the cloud drop concentrations and subsequently cloud reflectivity and lifetime as well as associated precipitation. Tropical deep convective clouds (DCC) are important to Earth’s global energy balance and hydrologic cycle and can induce high impact weather with significant precipitation and lightning. However, the effects of aerosol particles on DCC properties, including the onset and amount of precipitation, vertical development, electrification and associated lightning, and the extent and lifetime of associated high altitude ice clouds, remain poorly quantified. In this project, the effect of aerosol particles on such weather and climate related phenomena is studied, addressing a major open question of whether and how human-induced aerosol particle pollution has altered weather phenomena (e.g., potentially moving tropical showers to thunderstorms), and climate. Global climate models are being used to inform socioeconomic policy decisions around the world, but the processes investigated in this project are practically absent from these models. Therefore, the outcome of this study could have broad impacts on our understanding of the anthropogenic aerosol impacts on climate predictions and in turn upon policies being developed for mitigation and adaptation. The project will train graduate and undergraduate students through international collaboration in the fundamental physics and chemistry of the atmosphere and developing their technical skills in the analysis of large multivariable datasets, 3-dimensional computer models of the atmosphere, and satellite remote sensing technology. Tropical marine regions are likely most sensitive to the additional input of aerosol particles which act CCN. Ultrafine aerosol particles (UAP) 40 nm in size are not typically considered CCN, but they may in fact nucleate cloud droplets in DCC. Observations of UAP, specifically, and CCN, are lacking in such clean regions needed to test hypotheses about the aerosol impacts on deep convective clouds and associated effects on climate. To address these limitations, this joint NSF-BSF project between University of Washington (UW), The Hebrew University Jerusalem Israel (HUJI), and collaborators, leverages i) remote sensing of DCC microphysics to constrain CCN and UAP impacting individual convective events, ii) analysis of 15 years of global tropical lightning stroke fields guided by chemical transport model predictions of UAP and CCN, and iii) cloud resolving modelling of key domains constrained by the satellite remote sensing insights, lightning observations, and chemical transport model predictions of UAP and CCN. The project will evaluate how CCN, including UAP, perturb the microphysics of tropical deep convective clouds and to what extent such perturbations affect lightning and cloud radiative effects, and provide a test of the hypothesis that increases in CCN and UAP since preindustrial time, due to human activities associated with fuel combustion, have induced a positive radiative forcing (warming) on climate through deep convective clouds. Such a climate forcing would be in opposition to the negative radiative forcing in which CCN increases affects low cloud albedo and lifetime and would alter our understanding of climate sensitivity.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.