Daily evolution in dust and black carbon content, snow grain size, and snow albedo during snowmelt, Rocky Mountains, Colorado

S Mckenzie Skiles,Thomas Painter

doi:10.1017/jog.2016.125

S Mckenzie Skiles, Thomas Painter

Open Access

https://doi.org/10.1017/jog.2016.125

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Abstract

ABSTRACTLight absorbing impurities (LAI) initiate powerful snow albedo feedbacks, yet due to a scarcity of observations and measurements, LAI radiative forcing is often neglected or poorly constrained in climate and hydrological models. To support physically-based modeling of LAI processes, daily measurements of dust and black carbon (BC) stratigraphy, optical grain size, snow density and spectral albedo were collected over the 2013 ablation season in the Rocky Mountains, CO. Surface impurity concentrations exhibited a wide range of values (0.02–6.0 mg g−1pptw) with 98% of mass being deposited by three episodic dust events in April. Even minor dust loading initiated albedo decline, and the negative relationship between dust concentrations and albedo was log-linear. As melt progressed, individual dust layers coalesced and emerged at the snow surface, with minimal mass loss to meltwater scavenging. The observations show that the convergence of dust layers at the surface reduced albedo to 0.3 and snow depth declined ~50% faster than other years with similar depth but less dust. The rapid melt led to an unexpected reduction in both grain size and density in uppermost surface layers. BC concentrations co-varied with dust concentrations but were several orders of magnitude lower (<1–20 ppb).

Highlights

Radiative forcing by dust deposition in the Colorado Rocky Mountains has been shown to advance melt by 1–2 months, shift timing and intensity of peak runoff, and reduce total water yield (Painter and others, 2007a; Painter and others, 2010; Skiles and others, 2012; Deems and others, 2013)
The powerful impact on snow albedo has been documented with field, in-situ and remote sensing measurements (Painter and others, 2012a, b; Painter and others, 2013), and ongoing research indicates that this may be a major factor contributing to operational river runoff forecast errors in the Colorado River Basin (CRB) (Bryant and others, 2013)
The total number of dust events and the timing of those events were not abnormal relative to other years, the amount of dust deposited in WY13 was more than 30 times the amount of dust deposited in the lowest dust year (2005) and 5–10% greater than the amount deposited in other highdust years (2009, 2010)

Summary

Introduction

Radiative forcing by dust deposition in the Colorado Rocky Mountains has been shown to advance melt by 1–2 months, shift timing and intensity of peak runoff, and reduce total water yield (Painter and others, 2007a; Painter and others, 2010; Skiles and others, 2012; Deems and others, 2013). The powerful impact on snow albedo has been documented with field, in-situ and remote sensing measurements (Painter and others, 2012a, b; Painter and others, 2013), and ongoing research indicates that this may be a major factor contributing to operational river runoff forecast errors in the Colorado River Basin (CRB) (Bryant and others, 2013) This suite of studies, focusing on understanding the impacts of dust on snow in the hydrologically sensitive CRB, have utilized a semi-empirical determination of dust radiative forcing based on changes in surface reflectance (Painter and others, 2007a). Useful to constrain the relationship between dust and albedo, and investigate its impact on runoff, it is of interest to represent these processes explicitly in physically-based radiative transfer and snowmelt models This requires measurements that have been previously unavailable; an extended high-resolution dataset of snow physical and optical properties to force, validate and calibrate models. Another recent study found BC (atmosphere and snow) processes to be second only to CO2 in climate forcing effectiveness (Bond and others, 2013)

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