Optimal estimation of snow and ice surface parameters from imaging
spectroscopy measurements
Abstract
Snow and ice melt processes are a key in Earth’s energy-balance and
hydrological cycle. Their quantification facilitates predictions of
meltwater runoff as well as distribution and availability of fresh
water. They control the balance of the Earth’s ice sheets and are
acutely sensitive to climate change. These processes decrease the
surface reflectance with unique spectral patterns due to the
accumulation of liquid water and light absorbing particles (LAP), that
require imaging spectroscopy to map and measure. Here we present a new
method to retrieve snow grain size, liquid water fraction, and LAP mass
mixing ratio from airborne and spaceborne imaging spectroscopy
acquisitions. This methodology is based on a simultaneous retrieval of
atmospheric and surface parameters using optimal estimation (OE), a
retrieval technique which leverages prior knowledge and measurement
noise in an inversion that also produces uncertainty estimates. We
exploit statistical relationships between surface reflectance spectra
and snow and ice properties to estimate their most probable quantities
given the reflectance. To test this new algorithm we conducted a
sensitivity analysis based on simulated top-of-atmosphere radiance
spectra using the upcoming EnMAP orbital imaging spectroscopy mission,
demonstrating an accurate estimation performance of snow and ice surface
properties. A validation experiment using in-situ measurements of
glacier algae mass mixing ratio and surface reflectance from the
Greenland Ice Sheet gave uncertainties of ±16.4 μg/g_ice and less than
3%, respectively. Finally, we evaluated the retrieval capacity for all
snow and ice properties with an AVIRIS-NG acquisition from the Greenland
Ice Sheet demonstrating this approach’s potential and suitability for
upcoming orbital imaging spectroscopy missions.