Earth's Cryosphere-Albedo Feedback: From the Global Scale to in Snow Metamorphosis

Abstract

Previous studies report rapid perennial Arctic sea ice-cover decline over the last few decades, but decadal-scale temporal variability of Earth's albedo feedback has not been fully assessed in future climate simulations. Without a complete dynamic treatment of albedo feedback on these timescales, a question that motivates the research presented here is how does the strength in albedo feedback vary on decadal timescales in transient climate? The answers to when the strength in albedo feedback might peak and start to decline in future transient climate simulations is the topic of Chapter 2. On smaller scales, snow internal albedo feedback is a poorly understood source of instability in snowpacks that can affect the surface energy budget. Mechanisms for both positive and negative snow metamorphosis-driven albedo feedback have been proposed, but due to the delicate nature of snowpacks, it can be difficult to study these mechanisms in nature. Chapters 3 and 4 seek to better understand the snow internal albedo feedback on hourly timescales.

Data from the Coupled Model Intercomparison Project Phase 5 (CMIP5) multi-model ensemble of simulations of historical and future transient climate is applied to assess global scale surface albedo feedback (SAF) in 36 global climate models. Time evolving SAF in multiple decades are calculated from surface albedo and temperature linear regressions. Results are meaningful when temperature change exceeds 0.5K. Decadal scale SAF is strongly correlated with century scale SAF during the 21st century. Throughout the 21st century, multi-model ensemble mean SAF increases from 0.37 to 0.42 watts per square meter Kelvin. These results suggest models' mean decadal scale SAFs are good estimates of their century scale SAFs if there is at least 0.5K temperature change. Persistent SAF into the late 21st century indicates ongoing capacity for Arctic albedo decline despite there being less sea-ice.

To examine the snow internal albedo feedback, first, an instrument designed to measure snow specific surface area (SSA) is engineered to operate in situ during subfreezing conditions. To calibrate the Near-Infrared Emitting and Reflectance-Monitoring Dome (NERD), measured bidirectional reflectance factors (BRFs) are compared to snow SSA estimates derived from X-ray microcomputed tomography (X-CT) scans. This comparison contains multiple snow samples of various morphological quantities including snow density, porosity, and SSA ranging from 10 to 70 square meters per kilogram. In general, there is an exponential relationship between 1.30 micro-meter BRFs and snow SSA. These results provide experimental validation of measuring 1.30 micro-meter BRFs to obtain approximate snow SSA.

Second, two NERDs are deployed to measure 1.30 and 1.55 micro-meter BRFs of natural snow and experimental snow plots with added dust and BC. Snow 1.30 (1.55) micro-meter BRFs evolve from 0.6 (0.15) in fresh snow to 0.2 (0.03) after metamorphosis. Hourly-scale time evolving snow surface BRFs and SSA estimates from X-CT reveal more rapid infrared darkening and snow metamorphosis in contaminated versus natural plots. These findings verify experimentally that dust and BC deposition can accelerate snow metamorphosis and enhance positive snow internal albedo feedback in sunny, calm weather conditions.