CAREER: Linking cryospheric processes across scales to model non-linear albedo feedback

地球？S冰冻圈拥有通过反照率反馈改变行星反射率和地表温度的巨大能力，这在一定程度上解释了北极气候的高度敏感性。然而，反照率反馈实际上是由多种机制组成的，这些机制的时间尺度从几个小时到几千年不等，既有积雪的情况，也有冰冻圈覆盖变化的情况。该项目旨在填补我们在以下理解上的关键空白：1)像黑碳(BC)这样的吸光杂质如何通过它们对雪变质的影响来影响反照率反馈；2)反照率反馈中十年至十年变化的来源，并解释为什么气候模型普遍低估了1979？2008年的北部地区反照率反馈(比遥感观测低~2倍)。目标(1)的动机来自于辐射模拟和观测表明，BC扰动可见反照率在粗粒(老化)雪中更多，这在近红外光谱中也较暗，表明BC在加速雪变质时至少触发了两种正反照率反馈机制。然而，杂质加热对变质作用的影响尚不确定。PI将使用一种新的近红外光学传感设备，测量在有无BC的情况下产生的积雪中有效雪粒尺寸的演变，以确定BC对一种环境中变质作用的影响。目标(2)的动机是在模型反馈中观察到的偏差，以及在瞬变气候模拟中显示出大的多十年反馈变异性的初步分析。为了分离对反照率反馈的贡献，并查明反馈中模型观测偏差和数十年变化的原因，他将把诊断冰冻圈辐射强迫项纳入NCAR共同体地球系统模型(CESM)，并利用遥感和气候模型数据进行额外分析。对与冰冻层覆盖率减少相关的反照率反馈长期下降的预期引发了这样的问题：反照率反馈是否已经达到峰值？如果不是，什么时候可能会，为什么？实现项目目标将能够在不同尺度上更一致地模拟反照率反馈，更自信地量化人为气溶胶排放对气候和水文的影响，并为全球变暖中反照率峰值的时间提供机械解释。研究将与本科生和研究生教育紧密结合。雪杂质/变质实验和光学测量将成为密歇根大学生物站(UMBS)为大一新生设计的周末研究体验的一部分。学生将接触冬季野外方法、仪器、站塔能量和微量气体通量的测量，以及数据解释。学生团队将部署在道格拉斯湖和UMBS的其他地形上，以测量雪的特性并表征颗粒大小的空间变异性。这项活动将针对早期本科生，以影响学位，专业选择和对科学职业道路的兴趣。参加课程的高年级学生可以设计、建造和应用现场仪器来获得工程学课程学分。冰冻圈-气候相互作用的研究生研究将被纳入其中一门皮？S课程的课程项目中。该项目产生的基础研究将加强对重要的气候反馈过程、工业排放如何影响这些过程以及它们可能如何影响水资源的理解。将这项研究与教育活动相结合，将有助于培训下一代地球科学家和工程师。

Earth?s cryosphere holds large capacity to alter planetary reflectance and surface temperature through albedo feedback, partially explaining the high sensitivity of Arctic climate. Albedo feedback, however, is actually composed of multiple mechanisms operating on timescales ranging from hours to millennia, both within snowpack and via altered cryospheric cover. This project aims to fill crucial gaps in our understanding of: 1) how light-absorbing impurities like black carbon (BC) influence albedo feedback via their effect on snow metamorphism, and 2) sources of decadal to multi-decadal variability in albedo feedback and explanation of why climate models universally underestimate 1979?2008 boreal albedo feedback (by ~2-fold) relative to remote sensing observations. Motivation for goal (1) arises from radiative modeling and observations showing that BC perturbs visible albedo more in coarser-grained (aged) snow, which is also darker in the near-infrared spectrum, indicating that BC triggers at least two positive albedo feedback mechanisms when it accelerates snow metamorphism. The influence of impurity heating on metamorphism, however, is uncertain. The PI will measure the evolution of snow effective grain size in snowpacks generated with and without BC, using a new near-infrared optical sensing device, to determine BC influence on metamorphism in one environment. Goal (2) is motivated by the observed bias in model feedback and a preliminary analysis showing large multi-decadal feedback variability in transient climate simulations. To isolate contributions to albedo feedback and identify causes of model-observation discrepancy and multi- decadal variability in feedback, he will incorporate diagnostic cryosphere radiative forcing terms into the NCAR Community Earth System Model (CESM) and conduct additional analyses with remote sensing and climate model data. Expectation of a long-term decline in albedo feedback associated with reduced cryospheric cover prompts the questions: Has albedo feedback peaked? If not, when might it, and why? Attaining the project objectives will enable modeling albedo feedback more consistently across scales, quantifying climate and hydrological influences of anthropogenic aerosol emissions more confidently, and providing a mechanistic explanation for the timing of peak albedo feedback in a warming world.Research will be tightly integrated with undergraduate and graduate education. Snow impurity/metamorphism experiments and optical measurements will be part of a weekend research experience designed for freshmen at the University of Michigan Biological Station (UMBS). Students will gain exposure to winter field methods, instrumentation, measurements of energy and trace gas fluxes at station towers, and data interpretation. Student teams will deploy on Lake Douglas and other terrain at UMBS to measure snow properties and characterize spatial variability in grain size. This activity will be targeted toward early undergraduate students to influence degree major choices and interest in science career paths. Participating seniors may design, build, and apply field instruments for engineering course credit. Graduate studies of cryosphere?climate interactions will be incorporated into a course project in one of the PI?s courses. The fundamental research resulting from this project will enhance understanding of important climate feedback processes, how industrial emissions influence these processes, and how they may impact water resources. The integration of this research with educational activities will contribute to the training of the next generation of geoscientists and engineers.