Technical notes: A detailed study for the provision of measurement uncertainty and traceability for goniospectrometers

The measurement uncertainty and traceability of the Finnish Geodetic Institutes¿s field gonio-spectro-polarimeter FIGIFIGO have been assessed. First, the reference standard (Spectralon sample) was measured at the National Standard Laboratory of MIKES-Aalto. This standard was transferred to FGI¿s field reference standard (larger Spectralon sample), and from that to the unmanned aerial vehicle (UAV), reference standards (1 m2 plates). The reflectance measurement uncertainty of FIGIFIGO has been estimated to be 0.01 in ideal laboratory conditions, but about 0.02–0.05 in typical field conditions, larger at larger solar or observation zenith angles. Target specific uncertainties can increase total uncertainty even to 0.1–0.2. The angular reading uncertainty is between 1° and 3°, depending on user selection, and the polarisation uncertainty is around 0.01. For UAV, the transferred reflectance uncertainty is about 0.05–0.1, depending on, how ideal the measurement conditions are. The design concept of FIGIFIGO has been proved to have a number of advantages, such as a well-adopted user-friendly interface, a high level of automation and excellent suitability for the field measurements. It is a perfect instrument for collection of reference data on a given target in natural (and well-recorded) conditions. In addition to the strong points of FIGIFIGO, the current study reveals several issues that need further attention, such as the field of view, illumination quality, polarisation calibration, Spectralon reflectance and polarisation properties in the 1000–2400 nm range

Soot on snow experiment: bidirectional reflectance factor measurements of contaminated snow

In order to quantify the effects of absorbing contaminants on snow, a series of spectral reflectance measurements were conducted. Chimney soot, volcanic sand, and glaciogenic silt were deposited on a natural snow surface in a controlled way as a part of the Soot on Snow (SoS) campaign. The bidirectional reflectance factors of these soiled surfaces and untouched snow were measured using the Finnish Geodetic Institute's Field Goniospectropolariradiometer, FIGIFIGO. A remarkable feature is the fact that the absorbing contaminants on snow enhanced the metamorphism of snow under strong sunlight in our experiments. Immediately after deposition, the contaminated snow surface appeared darker than the natural snow in all viewing directions, but the absorbing particles sank deep into the snow in minutes. The nadir measurement remained the darkest, but at larger zenith angles, the surface of the contaminated snow changed back to almost as white as clean snow. Thus, for a ground observer the darkening caused by impurities can be completely invisible, overestimating the albedo, but a nadir-observing satellite sees the darkest points, underestimating the albedo. Through a reciprocity argument, we predict that at noon, the albedo perturbation should be lower than in the morning or after-noon. When sunlight stimulates sinking more than melting, the albedo should be higher in the afternoon than in the morning, and vice versa when melting dominates. However, differences in the hydrophobic properties, porosity, clumping, or size of the impurities may cause different results than observed in these measurements.Peer reviewe

Soot on snow experiments: light-absorbing impurities effect on the natural snowpack

Abstract. Light-absorbing impurities affect snow and ice via a decrease in albedo and a consequent disturbance to the radiative energy balance. Experimentally, these matters have only been examined in a few studies. Here we present results from a series of experiments in which we deposited different soot concentrations onto natural snow in different regions of Finland, and thereafter monitored the changes of the snowpack through the melting season. Measurements of the particulates in the snow indicated concentrations in the range of thousands of ppb to have clear effects on the snow properties, including the albedo, the physical snow characteristics, and an increased melt rate. For soot concentrations in the hundreds of ppb range, the effects were not as clearly visible, and it was more difficult to attribute the effects solely to the soot on the snow. Comparisons between our experimental data and the widely used Snow, Ice and Aerosol Radiation (SNICAR) model showed a general agreement when the model was specifically tuned to our measurements. This study highlights the importance of additional experimental studies, to further articulate and quantify the effects of light-absorbing impurities on snow. </jats:p