Ground-based observations of snow spectral albedo with an autonomous device
Authors: Hannula Henna-Reetta , Pirazzini Roberta , Räisänen Petri 
Published: March 2021

&lt;p&gt;Snow metamorphism is a continuous process affecting the snow albedo on time scales ranging from minutes to days, depending on the weather regime. To understand the complex interactions between snow microstructure, snow surface roughness, and surface albedo, these properties need to be observed at sufficiently high temporal resolution. For this reason, a new device, the SVC-FMI spectro-albedometer, was designed by Spectra Vista Corporation (SVC, USA) in collaboration with the Finnish Meteorological Institute (FMI, Finland) to continuously measure the surface spectral albedo while withstanding the cold and harsh weather conditions typical of polar regions. It consists of a SVC HR-1024i high resolution field portable spectroradiometer, with 3-10nm spectral resolution in the range 350-2500 nm, connected with an optical tube to two integrating spheres, one facing upward and the other facing downward, which collect the irradiance received from the sky and reflected from the surface, respectively. The whole system is enclosed in a weatherproof case which also partially provides thermal stabilization, with ventilated glass domes on the aperture of the integrating spheres, and it is installed in a fixed supporting structure that enables the control of the horizontal alignment of the integrating spheres.&lt;/p&gt;&lt;p&gt;SVC and later FMI calibrated the instrument and characterized the thermal drift of the instrument&amp;#8217;s sensitivity and the deviation from the ideal cosine response. In spring 2019 and 2020 the instrument was installed in Sodankyl&amp;#228; (northern Finland) over a flat wetland area where about 80-110 cm of snow had accumulated during the winter. The measurement campaigns were carried out in the framework of the Academy of Finland project SnowAPP (&amp;#8220;Modelling of the Snow microphysical-radiative interaction and its APPlications&amp;#8221;) with funding also from the H2020 EU project INTAROS (&amp;#8220;Integrated Arctic Observation System&amp;#8221;). The aperture of the downfacing integrating sphere was at 2 m from the snow surface, i.e. high enough to minimize shadows and light obstructions caused by the supporting structure, but low enough to enable easy installation and access to the instrument. Here we illustrate a selection of the collected data, showing all the steps of the data processing, which include the corrections to compensate the temperature drift, the deviation from the ideal cosine response, and shadows and light obstructions. The most complex correction and the one with the largest impact on the data is the deviation from the ideal cosine response. It involves radiative transfer modelling and the measurement (or modelling) of fraction of direct incoming irradiance and snow bidirectional reflectance distribution function. We discuss the different impact of these corrections in case of overcast and clear sky conditions.&lt;/p&gt;&lt;p&gt;Simultaneous measurements of snow properties as well as passive and active microwave signals were carried out, thus these spectral albedo data are very relevant for snow process studies and for the validation of snow products derived from satellite optical sensors.&lt;/p&gt;