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1. Introduction
In-cloud icing (also referred to as rime icing) is a weather phenomenon that occurs when an unheated structure is exposed to liquid cloud droplets at a temperature T below the freezing point. It is usually most pronounced in exposed mountainous terrain where the cloud base is frequently located lower than the terrain height such that the mountain peaks are in cloud. In many locations in-cloud icing can persist for many days, or even weeks, and in combination with strong winds the accumulated ice load can become extremely high and cause instabilities, faults, and damages to infrastructure. In the literature several examples are found of such damages to overhead power lines (e.g., Thorkildson et al. 2009; Qiang et al. 2005), reduced efficiency and fatigue damages on wind turbines (Frohboese and Anders 2007; Jasinski et al. 1998), and collapse of telecommunication towers (Mulherin 1998). Also ground structures such as ski lifts, measurement masts, meteorological instruments (Makkonen et al. 2001b), and buildings are often subject to in-cloud icing on the top or near the top of exposed mountains and hills. At a coastal mountainous site in Norway more than 300 kg m−1 of ice were measured in April 1961 on a 66-kV power line (Fikke et al. 2008). Ice loads like that clearly demonstrate that the frequency and magnitude of in-cloud icing need to be carefully taken into account in the design of all ground-based structures in cold regions, both for the mechanical strength of the construction and for functionality and performance under icing conditions. However, long and reliable time series of icing measurements sufficient to assess the icing climatology are rarely available. From a wind energy point of view, the assessment of icing frequency is extremely important as many of the potential onshore wind farm sites in the Nordic countries are located on hilltops and elevated areas. In addition to favorable wind conditions, such sites are very often also exposed to in-cloud icing, and the production loss caused by icing has potentially enormous consequences for the profitability of a wind power plant. Therefore, it is of huge interest for the industry to develop new methods and modeling tools to assess the frequency and intensity of in-cloud icing. In-cloud icing furthermore causes a safety...