A Method to Retrieve the Complex Refractive Index and Single Scattering Optical Properties of Dust Deposited in Mountain Snow Cover

Abstract:

Dust on snow can have regionally important climatic and hydrologic impacts resulting from direct reduction of surface albedo and indirectly from the initiation of snow albedo feedbacks. Modeling the radiative impacts of dust deposited in snow requires knowledge of the optical properties of both components. Here we present an inversion technique to retrieve the effective optical properties of dust deposited in mountain snow cover from measurements of hemispherical dust reflectance and particle size distributions using radiative transfer modeling. First, modeled reflectance is produced from single scattering properties modeled with Mie theory for a specified grain size distribution over a range of values for the imaginary part of the complex refractive index (k=-0.00001 to -0.1). Then, a multi-step look-up table process is employed to retrieve the imaginary part of the complex index of refraction and single scattering optical properties by matching measured to modeled reflectance between 0.35 and 2.5 µm at 10 nm resolution. The real part of the complex refractive index for dust aerosols ranges between 1.5 and 1.6 and a sensitivity analysis shows the method is relatively insensitive to the choice of n within this range, 1.525 was used here. Using the values retrieved by this method to update dust optical properties in a snow+aerosol radiative transfer model reduces errors in spring time albedo modeling by 50% to 70%, matching measured albedo to within 2% on average in the visible wavelengths and 5% over the full range of snow reflectance.