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