Abstract

Field measurement results from a novel optical cloud droplet monitoring sensor designed for icing conditions monitoring are presented. The sensor has been demonstrated at two sites in northern Finland; first at Global Atmosphere Watch Station in Pallas together with a reference icing sensor and secondly mounted on a wind turbine nacelle in eastern Finland in 2017. Test runs in an icing wind tunnel have been made where more severe icing conditions were generated.

The ICEMET sensor measurement principle is based on capturing the images of cloud droplets and ice particles. Droplet properties, such as droplet size distribution (DSD) and median volume diameter (MVD), are acquired by means of image analysis of the captured images. The images and the calculated features (size, location, shape descriptors) of all the found particles are saved in a database.

A volume of 0.5 cm³ is imaged in a single frame. The liquid water content (LWC) is calculated based on this known sample volume in combination with the droplet data acquired from the image analysis of the found and filtered particles (droplets only).

The sensor is typically freely rotating — it aligns itself against the wind by a wing on the backside. In the rotating configuration, the maximum sampling rate is 3 cm³/s. The movement of the particles inside sample volume is frozen in the images by a nanosecond scale light flash, making the sample volume independent of the wind speed. The maximum wind speed tested in a wind tunnel with the sensor is 40 m/s. The cloud droplet sizes from 5 to 200 microns are measured by the ICEMET sensor.

In this paper LWC and MVD measurement results from the field tests and the wind tunnel tests with the sensor are presented and discussed.

The webpages for the sensor can be found at https://www.oulu.fi/icemet.