## 1 Introduction  
The increased use of wind turbines as a source of renewable energy is an international trend, as most European countries saw a significant increase in new installed wind energy production capacity from the previous year [1; 2]. Of the new installations due before 2020, the overwhelming majority will be installed in remote, cold areas. Although the prospects of renewable energy development are promising in this area, ice accumulation on turbine blades still stands as a major operational issue. Excessive ice mass on the turbine blades, as shown in Figure 1, can lead to damage or complete destruction of the nacelle gearbox or the generator. Typical ice profiles at rotor blades consist of so-called rim-ice [3; 4]. Ice can also be non-uniformly distributed on the blades, which leads to asymmetric rotation due to imbalance and can have detrimental consequences, such as the reduced lifetime of gears and bearings. Ice accumulation also changes the profile of the blade airfoil and yields energy losses. There is also the danger of a wind turbine throwing ice from the spinning blades, referred to as \"ice throw\", which can be fatal even with a mere 200 g of ice or 40 J of energy, respectively [5]. The goal of this paper is the identification of changes in the modal properties of a glass fibre-reinforced plastic (GFRP) rotor and a better understanding of how ice accumulation affects the profile of the composite surface, and consequently to describe the relationships between ice accumulation, ice mass, and stiffness distribution. In this way, a more precise control of rotor operations due to different icing conditions and other climate events can be enabled. A novel rotor test rig is developed that simulates various environmental conditions under different rotational speeds and ice distributions. A laser scanning profiler is used to determine the various icing conditions, such as thickness and distribution of accumulating ice. The findings can be used to identify distinct features in sensor data without the need to properly determine the ice accumulation. This also enables the proper determination and, furthermore, simulation of icing events that can be used in monitoring systems [6; 7]. The influence of damage initiation and propagation at GFRP rotors has already been extensively investigated and a non-monotonic shift of the eigenfrequencies has been observed, due to non-linear mechanical properties of the rotors and the damage type and extent [8]. However, experimental investigations for ice accumulation on composite rotors considering the modal properties are relative few [9; 10]. Specifically, typical icing conditions on GFRP rotor blades are reported on the scope of ice detection and identification at wind turbines [11]. Other applications in the field of wind turbine monitoring are based on the physical principle, that ice accumulation changes the vibration behaviour response of a structure [12; 13]. Specifically, most applications use the mass changes due to icing, which causes changes in the natural frequencies. Such methods can achieve a measurement resolution for the thickness of the ice in the millimetre range. The advantage lies in the fact that they can be used during operation as well as in static conditions, which allows for measurements to be taken before starting the turbine, thus ensuring proper operation [9]. Currently available industrial systems use accelerometers as well to detect  
Figure 1: Typical icing conditions on the rotor blade of a glass fibre-reinforced plastic (GFRP) wind turbine,(**a**) top view and (**b**) rear view with ice accumulation. changes in modal properties and, thus, prevent damage to gearboxes, engine bearings etc. due to dynamic loads. The principle is based on the fact that different vibrations are characteristic for certain causes [14; 15].