Influence of light-absorbing particles on snow spectral irradiance profiles

Francois Tuzet,Ghislain Picard,Jesus Revuelto,Laurent Arnaud,Maxim Lamare,Fanny Larue,Didier Voisin,Marie Dumont

doi:10.5194/tc-13-2169-2019

Abstract

Abstract. Light-absorbing particles (LAPs) such as black carbon or mineral dust are some of the main drivers of snow radiative transfer. Small amounts of LAPs significantly increase snowpack absorption in the visible wavelengths where ice absorption is particularly weak, impacting the surface energy budget of snow-covered areas. However, linking measurements of LAP concentration in snow to their actual radiative impact is a challenging issue which is not fully resolved. In the present paper, we point out a new method based on spectral irradiance profile (SIP) measurements which makes it possible to identify the radiative impact of LAPs on visible light extinction in homogeneous layers of the snowpack. From this impact on light extinction it is possible to infer LAP concentrations present in each layer using radiative transfer theory. This study relies on a unique dataset composed of 26 spectral irradiance profile measurements in the wavelength range 350–950 nm with concomitant profile measurements of snow physical properties and LAP concentrations, collected in the Alps over two snow seasons in winter and spring conditions. For 55 homogeneous snow layers identified in our dataset, the concentrations retrieved from SIP measurements are compared to chemical measurements of LAP concentrations. A good correlation is observed for measured concentrations higher than 5 ng g−1 (r2=0.81) despite a clear positive bias. The potential causes of this bias are discussed, underlining a strong sensitivity of our method to LAP optical properties and to the relationship between snow microstructure and snow optical properties used in the theory. Additional uncertainties such as artefacts in the measurement technique for SIP and chemical contents along with LAP absorption efficiency may explain part of this bias. In addition, spectral information on LAP absorption can be retrieved from SIP measurements. We show that for layers containing a unique absorber, this absorber can be identified in some cases (e.g. mineral dust vs. black carbon). We also observe an enhancement of light absorption between 350 and 650 nm in the presence of liquid water in the snowpack, which is discussed but not fully elucidated. A single SIP acquisition lasts approximately 1 min and is hence much faster than collecting a profile of chemical measurements. With the recent advances in modelling LAP–snow interactions, our method could become an attractive alternative to estimate vertical profiles of LAP concentrations in snow.

Highlights

Snow is a highly reflective medium in the wavelengths of the visible and of the near infrared where most of the solar energy is available (Warren, 1982)
Most of the uncertainties may be due to uncaught variations of Light-absorbing particles (LAPs) optical properties (Fig. 9) and snow physical parameters (Fig. 11), which is illustrated by the fact that, when subtracting the 15.7 ng g−1 equivalent BC (eqBC) positive bias, all measured LAP concentrations higher than 5 ng g−1 are within the range of uncertainty of the retrieval
This paper presents a unique dataset including two seasons of near-weekly surveys of snow physical properties (SSA, density) associated with measurements of spectral irradiance profiles (SIPs)

Summary

Introduction

Snow is a highly reflective medium in the wavelengths of the visible and of the near infrared (up to 1.4 μm, referred to as NIR) where most of the solar energy is available (Warren, 1982). The amount of solar energy absorbed by snow-covered areas is small compared to other surfaces such as bare soil, vegetation or oceans, making snow a singular component of our climate system (Armstrong and Brun, 2008). Snow optical properties depend on its physicochemical characteristics whose evolution is driven by atmospheric conditions (Colbeck, 1982; Aoki et al, 2006). This dependence involves snow in strong optical feedback loops that are of crucial importance for the snowpack evolution and are still poorly understood (Hall, 2004; Box et al, 2012). Tuzet et al.: LAP impact on snowpack spectral irradiance profile backs, impacting significantly the cryosphere and its evolution under a changing climate (Skiles et al, 2018)

Methods

Results

Discussion

Conclusion