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。气溶胶粒子起着云凝结核(CCN)的作用，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。热带深对流云(DCC)对全球能量平衡和水文循环具有重要意义，可以引发强影响天气，并伴有明显的降水和闪电。然而，气溶胶粒子对DCC性质的影响，包括降水的开始和数量、垂直发展、带电和伴随的闪电，以及伴随的高海拔冰云的范围和寿命，仍然缺乏量化。在这个项目中，研究了气溶胶颗粒对这种天气和气候相关现象的影响，解决了人类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，可能使热带阵雨变成雷暴)和气候这一重大悬而未决的问题。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，但这个项目中研究的过程实际上没有出现在这些模型中。因此，这项研究的结果可能会对我们理解人为气溶胶对气候预测的影响，进而对正在制定的缓解和适应政策产生广泛影响。该项目将通过在大气基础物理和化学方面的国际合作培训研究生和本科生，并培养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能。热带海洋区域可能对起CCN作用的气溶胶粒子的额外输入最为敏感。40 nm大小的超细气溶胶粒子(UAP)通常不被认为是CCN，但它们实际上可能在DCC中形成云滴。特别是UAP和CCN的观测，缺乏这样干净的区域，需要检验关于气溶胶对深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，这一由华盛顿大学(UW)、以色列耶路撒冷希伯来大学(HUJI)及其合作者联合开展的NSF-BSF项目，利用i)DCC微观物理遥感来约束CCN和UAP影响个别对流事件，ii)在UAP和CCN化学传输模型预测的指导下，对15年的全球热带闪电场进行分析，iii)对受UAP和CCN卫星遥感见解、闪电观测和化学传输模型预测约束的关键领域进行云分辨模拟。该项目将评估包括UAP在内的CCN如何扰乱热带深对流云的微物理，以及这种扰动对闪电和云辐射效应的影响程度，并对自工业化前以来由于与燃料燃烧有关的人类活动而增加的CCN和UAP已通过深对流云对气候产生正辐射强迫(变暖)的假设进行检验。这样的气候强迫将与负辐射强迫相反，在负辐射强迫中，CCN增加会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。这一裁决反映了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。

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.