On the influence of erect shrubs on the irradiance profile in snow

Maria Belke-Brea,Mathieu Barrere,Ghislain Picard,Laurent Arnaud,Florent Domine

doi:10.5194/bg-18-5851-2021

Abstract

Abstract. The warming-induced expansion of shrubs in the Arctic is transforming snowpacks into a mixture of snow, impurities and buried branches. Because snow is a translucent medium into which light penetrates up to tens of centimetres, buried branches may alter the snowpack radiation budget with important consequences for the snow thermal regime and microstructure. To characterize the influence of buried branches on radiative transfer in snow, irradiance profiles were measured in snowpacks with and without shrubs near Umiujaq in the Canadian Low Arctic (56.5∘ N, 76.5∘ W) in November and December 2015. Using the irradiance profiles measured in shrub-free snowpacks in combination with a Monte Carlo radiative transfer model revealed that the dominant impurity type was black carbon (BC) in variable concentrations up to 185 ng g−1. This allowed the separation of the radiative effects of impurities and buried branches. Irradiance profiles measured in snowpacks with shrubs showed that the impact of buried branches was local (i.e. a few centimetres around branches) and only observable in layers where branches were also visible in snowpit photographs. The local-effect hypothesis was further supported by observations of localized melting and depth hoar pockets that formed in the vicinity of branches. Buried branches therefore affect snowpack properties, with possible impacts on Arctic flora and fauna and on the thermal regime of permafrost. Lastly, the unexpectedly high BC concentrations in snow are likely caused by nearby open-air waste burning, suggesting that cleaner waste management plans are required for northern community and ecosystem protection.

Highlights

Due to Arctic warming, erect shrubs are expanding into the tundra biome, replacing low-growing vegetation like grasses, lichen and mosses (Tape et al, 2006; Myers-Smith et al, 2011; Ropars and Boudreau, 2012; Lemay et al, 2018)
light-absorbing particles (LAPs) type and concentrations were determined from log-irradiance profiles measured in shrub-free snowpacks on 8, 22 and 28 November by using the two methods, i.e. the ke analysis (Fig. 4) and the SnowMCML method (Fig. 5)
In ZOI1 the achieved fit had a R2 value of 0.98 and a root mean square error (RMSE) of 1.61 (Fig. 4a) when LAP was set to black carbon (BC)

Summary

Introduction

Due to Arctic warming, erect shrubs are expanding into the tundra biome, replacing low-growing vegetation like grasses, lichen and mosses (Tape et al, 2006; Myers-Smith et al, 2011; Ropars and Boudreau, 2012; Lemay et al, 2018). The vegetation change is transforming natural snowpacks, which originally consisted of snow with impurities, to a mix of snow, impurities and branches (Pomeroy et al, 2006; Loranty and Goetz, 2012). This has a large influence on the snow radiation budget, because branches are much more lightabsorbing than snow in the visible range (Juszak et al, 2014; Belke-Brea et al, 2019). The thermal regime controls snow melt rates in spring and during warm spells in autumn, which is of crucial importance for many bio-geophysical processes

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Conclusion