Sensitivity of the light field under sea ice to spatially inhomogeneous optical properties and incident light assessed with three-dimensional Monte Carlo radiative transfer simulations

Abstract

Light transmittance through sea ice is affected by surface cover and ice optical properties in the vicinity of the measurement. We present three-dimensional Monte Carlo simulations of light propagation in sea ice to derive upper bounds on the lateral spread of light. Our results give guidance on equipment design and on the possibility of using one-dimensional light transfer models to describe transmittance. Rules were derived for simple cases of optically homogeneous slabs. In the absence of absorption, 10% and 90% of the flux detected under optically thick, homogeneous ice are incident on the surface within a radius of less than 0.3 and 2.0 times the ice thickness, respectively. Any increase in optical thickness or absorption in the ice will reduce these radii. For example, the wavelength-dependent absorption of ice results in a 20% reduction at 700 nm. Optical anisotropy of the slab was also found to produce potentially significant spatial narrowing of the transmitted light field. In the case of direct sunlight, the photon path is displaced toward the sun relative to the location of the detector. This distortion can reach 1 m or more in optically thick, ponded ice but will be negligible under a surface scattering layer or snow cover. Case studies showed that transmittance of diffuse light in the vicinity of a semi-infinite surface obstruction could be approximated with exponential and error functions. An absorbing cylindrical perturbation of 0.05 m diameter in 1 m thick ice placed 1 m from the point of measurement will absorb less than 1% of the light otherwise registered by the detector. Many results for transmitted light were independent of the mean cosine of the scattering phase function.

Modelling radiative transfer through ponded first-year Arctic sea ice with a plane-parallel model

“…On the other hand, our interior ice has higher scattering. For a review of b eff used in various publications; see Table 2 in Petrich et al (2012).…”

“…For each boundary between pond and bare ice along a transect, the ice thickness h i for bare ice that was closest to a pond edge was picked out, and all measurements that were done within a distance of 2h i of the pond edge were identified as being close to a pond edge. The value of 2h i was chosen based on Petrich et al (2012), who, using a 3-D Monte Carlo radiative transfer model, found that 90 % of the light hitting a sensor at the ice bottom was incident within a radius of twice the ice thickness, for uniform ice.…”

“…Melt ponds dramatically change the optical properties of the sea ice, as ponds have a much lower albedo than the surrounding bare ice, and transmittance of solar radiation is generally higher through ponds than through adjacent ice that is not covered by ponds (e.g. Perovich et al, 1998). On a larger scale, the timing for the onset of melt, and thereby occurrence of melt ponds, has been shown to possibly influence the yearly sea ice area minimum (Markus et al, 2009;Schröder et al, 2014;Liu et al, 2015), as well as the annual budgets of solar energy (Arndt and Nicolaus, 2014).…”

“…Neither did Light et al (2008) or Light et al (2015) consider edge effects, focusing instead on a few select case studies. Petrich et al (2012) used a 3-D Monte Carlo model to estimate that 90 % of the irradiance measured at a point at the ice bottom will be incident in a radius that is twice the ice thickness. In the study by Ehn et al (2011) edge effects were seen up to 4.4 m away from the pond-bare-ice boundary, for ice thicknesses up to 1.77 m. Ponds have also been seen to influence the irradiance field several metres down in the water column below bare ice (Frey et al, 2011;Katlein et al, 2016).…”

Geometric Effects of an Inhomogeneous Sea Ice Cover on the under Ice Light Field

“…In contrast to most other above named cases, the light field underneath sea ice exhibits a high spatial variability on length scales smaller or of similar size as the measurement footprint of the instruments (Perovich, 1990;Petrich et al, 2012b). Horizontal variability on those length scales is usually not considered in the interpretation of light measurements apart from a few exemptions-such as light focusing by waves at the ocean surface (Wijesekera et al, 2005).…”