Abstract
Light measurements in the ocean provide crucial information about the energy fluxes in the climate and ecosystem. Currently radiative transfer problems are usually considered in horizontally homogeneous layers although it is known to be a crude assumption in many cases. In this paper, we examine the effects of a horizontally inhomogeneous sea ice layer on the light field in the water underneath. We implemented a three dimensional model, capable to simulate the light field underneath arbitrary surface geometries using ray optics. The results show clear effects of the measurement geometry on measured fluxes obtained with different sensor types, which need to be taken into account for the correct interpretation of the data. Radiance sensors are able to better sense the spatial variability of ice optical properties as compared to irradiance sensors. Furthermore we show that the determination of the light extinction coefficient of water from vertical profiles is complicated under a horizontally inhomogeneous ice cover. This uncertainty in optical properties of the water, as well as the measurement geometry also limits the possibility to correct light measurements taken at depth for the influence of water in between the sea ice and the sensor.
Highlights
Light measurements using radiometers are an important tool in geosciences
A single melt pond was approximated by a circular patch with higher light transmission (Figure 2C) and a part of a classified aerial image enabled for evaluation of a real melt pond geometry with a pond fraction of 12% (Figure 2D)
We developed and implemented a model of geometric radiometry underneath spatially inhomogeneous surfaces and investigated the effects of various simplified geometries on the light field underneath
Summary
Light measurements using radiometers are an important tool in geosciences. They provide crucial input data for various disciplines. Inherent optical properties are often derived from vertical profiles across various disciplines from atmospheric shortwave radiation measurements (e.g., Ohmura et al, 1998) over rain forest ecology (e.g., Nicotra et al, 1999) to ocean optics (e.g., Antoine et al, 2014) In most of those cases the approach of horizontally infinite homogenous layers is a sufficiently good approximation of reality. 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)
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