Ice-Free Sensors - the Eumetnet Sws Ii Project

ICE-FREE SENSORS - THE EUMETNET SWS II PROJECT

Bengt Tammelin*, Alain Heimo** and Michel Leroy***

*Finnish Meteorological Institute, ** Meteo Swiss, *** Meteo France

ABSTRACT

Wind, temperature, humidity and solar sensors were tested during the winter 2001/02 under heavy atmospheric icing conditions at three test sites located in southern France, Switzerland and northern Finland. There are some significant differences between the sensors in respect to operation during icing periods. Some of the sensors operate very successfully under harsh conditions, as some can be improved by improved heating or protection against ice accretion. In this paper the EUMETNET SWS II project is briefly described. Also some examples of analyses of wind sensor data are shown.

1BACKGROUND

Ice and rime accretion upon meteorological sensors due to atmospheric icing is a major problem for accuracy of measurements done in cold climate conditions at high latitudes and high altitudes. Most of present sensors operated e.g. by the national weather services are not ice free sensors, even if they may have some partial heating. The need for ice-free sensors is increasing not only due to meteorological or synoptic purposes, but also due to requirements on accurate measurements at hostile environments concerning applications linked e.g. to structural design, exploitation of renewable energy sources, aviation, power lines, skiing centres etc.

Concerning accuracy and reliability of measurements the requirements introduced by the WMO [ 3] are followed widely. Unfortunately the effect of atmospheric icing is not discussed in this guide. However, traditionally e.g. axial heating or black painted cups for wind sensors [ 1] and special precipitation gauges have been used at sites located in cold climate regions.

Finnish Meteorological Institute has studied icing effect and ice-free sensors especially concerning wind measurements (e.g. [ 1],[ 2], [ 4]). Also other reported experiments/observations on icing effects on sensors are available. The EUMETNET[1] project "Specification of Severe Weather Sensors 1997-1998" summarised the icing effects on different types of meteorological sensors [ 5]. Following the first SWS project the SWS II project was started by the EUMETNET in July 2000 in order to test a number of ice-free sensors, and other measurement arrangements designed for cold climate conditions, at three different types of sites in Finland, France and Switzerland. The final report of the project is expected to be ready by the end of the year 2002.

The objective of the project is to systematically test and analyse ice-free instruments (humidity, temperature, wind speed, wind direction, solar radiation) under harsh icing conditions to produce data for representative scientific studies:

-for intercomparison of ice-free sensors used or preferred by EUMETNET members

-to produce information on errors and the amount of heat needed for sensors to be ice-free

-to start a network providing certification for sensors that are free of icing problems.

This paper describes the SWS II project and some preliminary results concerning wind sensors. Temperature, humidity and solar radiation measurements at these sites will be discussed in three other papers presented at the TECO -2002 (see: [ 8], [ 9], [ 10]).

2TEST SITES

The three test are located in different parts of Europe under various types of cold climate and icing conditions. All these sites are within areas of heavy or strong atmospheric icing (with number of icing days more than 15 d/y [ 6]) and high wind speeds. However, in the far north the winter days are short with very little solar radiation to melt the ice, while close to the Mediterranean daily solar energy significantly reduces the amount of ice upon sensors (depending e.g. on the size, shape and colour of the sensors) but also affects the structure of ice. One reason to have the test sites at such places is to have enough events with atmospheric icing for representative analyses of icing effects upon sensors and measurements

The Mont Aigoual station, which is part of the basic network of Météo-France, is located on the top of Mont Aigoual (altitude 1567 m a.s.l., 44º07´N, 03º35´E). The MeteoSwiss test station at Säntis mountain is close to the MeteoSwiss weather station LPNN 2220. The test platform is at altitude of 2490 m a.s.l. (47º15´N, 9º20´E). The FMI test station is located in northern Finland north of the Polar circle on the top of Luosto fell at 515 m a.s.l. (68º08´N, 26º54´E; LPNN7509/WMO 05841). Luosto is at the northern end of a chain of arctic fells. At each site a platform for sensors was built (Figure 1). Each test site has an automatic data acquisition system providing with 10 minute averages and 3 s min an max values and standard deviations. Mont Aigoual and Säntis are manned stations where the observers also frequently take photos of the status of the sensors. Luosto is a fully automatic station where the status of sensors is monitored by three properly heated (ice-free) video cameras (for more detailed description of the sites see [ 7]).

Figure 1. Test platforms with different sensors at Mont Aigoual (left) and Säntis (right)

Figure 2. Examples of video monitoring at Luosto: on the left the rotating Cam2 watching wind sensors, and on the right the fixed Cam1 watching the ice detectors. Wind sensors in the Cam2 image from the left:Thies, Lamrecht, Metek 55W, Vaisala sonic, Rosemount, Vaisala cups and Irdam.

3SENSORS TESTED

The sensors to be tested during the winter 2001/02 at various sites were chosen according to interest of the EUMETNET members, and the discussions had at the SWS II workshop held in Paris in June 2001. Thus e.g. some wind sensors known to be ice free but not able to meet the WMO requirements concerning accuracy of measurements especially at mountainous sites where e.g. vertical wind component affects strongly the measurements [ 4] were excluded.

Measurements of icing is not actually included in the SWS II project, but as information of rime/ice accretion is required for other analyses also some ice-detectors and observation methods were tested.

According to results from the test period 2000/2001 it was decided that ice/rime upon sensors was not to be removed by observers during the test period. Thus all sensors were performing like at an automatic weather station. The sensors tested at Luosto (L), Säntis (S) and Mont Aigoual (A) were:

A)wind sensors: R.M. Young Wind Monitor-Model 05103-45 (A), Irdam SA WST 7000HS (A,L), Metek USA-1 55 W (L), METEK USA-1 125 W (A, S), Degreane Deolia 300 (A), Gill Windobserver II (S), Firma Kroneis Zamg 263PRH (A), Theodor Friedrichs Type 4035.0000 and Type 4123.0000 (L), A. Thies Ultrasonic 2D (L), Lambrecht Static Wind Sensor M16420 (A, L), BF Goodrich Rosemount M1774W (A, L, S), Meteolabor NOWA (S), FT Technology FT702 (S), Vaisala WAA252 (L, S), Vaisala WAS425 (A, L, S),

B)temperature and humidity: Rotronic ventilated radiation shield (A, L, S), Meteolabor Thygan (A, L, S), Eigenbrodt ventilated sheild with HMP45 (L), and additionally Irdam WST7000HS (A, L), Metek USA-1(A, L, S), GILL Windobserver (S), NOWA (S),

C)solar radiation: Kipp&Zonen CM21 with MeteoSwiss ventilation, cleaned and uncleaned (S), CM11 with FMI ventilation system (S, L), Hänni solarimeters (S),

D)icing: Labko LID-3503 (L, S), BF Goodrich Rosemount ice detector (A, L, S), Vaisala FD12P (L).

The reference sensors at all three sites were chosen to be Rosemount Model 1774W for wind speed and wind direction, and THYGAN for temperature and humidity measurements, as these were seen to operate most correctly at all three sites also during heavy icing. All other sensors were verified to these sensors also using data from non-icing periods.

4PRELIMINARY RESULTS - WIND MEASUREMENTS

Number of icing days, intensity of ice accretion and type of icing varies between the three sites. In South solar radiation helps to remove the ice, but also turns rime to clear ice due to melting and icing, while in North solar radiation has no effect in November-February.

Figure 3. An example of definition of icing classes for some wind sensors (WAS425 above and Young below). Class 1 on the left where there is some ice upon the body and sensor, but the sensor head is free of ice, Class 2 in the middle where ice is detected also upon the sensor head, and on the right Class 3 where the sensor is totally blocked. For WAS425 one of the sensor heads at each picture is shown by a circle.

According to the amount of ice upon sensors (body, shaft, cups etc.) the severity of icing upon each sensor is divided into four classes: 0 = totally clear, 3 = totally blocked. At Luosto these analyses were based on continuous video monitoring. At Säntis the classification was done by the observer, and at Mont Aigoual based on photos taken by the observer

To verify icing effect on wind sensors only open (no obstacles or shade of neighbour instrument) wind sectors were used. To study duration of different ice amounts on sensors all data was used.

In general availability of representative data measured with the "3rd generation[2] " heated ice-free sensors is much higher than with ordinary shaft heated sensors. Thus heated wind sensors available offer improved opportunities for proper wind measurements even at harsh climates. However, there are significant differences between the wind sensors in respect to icing effects on measurements. According to the results from Luosto in Figure 5 Thies, Lambrecht, Irdam and Rosemount are practically ice free all the time, while WAA252 and Metek 55 W require some additional heating. For Thies and Rosemount class 1 (ice upon the body) does not really affect the wind measurements. On the other hand, Irdam and Lambrecht sensors seem to be very sensitive to water droplets carried by hard wind into the sensor, which causes strong overestimation of wind speed. WAA252 is partially affected by strong vertical wind component, which is typical for these sites, and which leads to overestimation of wind speed (see Figure 4 on the right).

Figure 4. An example of verification of wind sensors (Friedrichs on the left, WAA252 on the right) to the Rosemount at Luosto in February 2002. The regression line describes the relation between the sensors during non icing period. On the left all icing classes were detected, on the right only the class 1 is observed. Each dot represents 10 minutes average wind speed.

Figure 5. Duration (% of time) of different types of ice classes (class 0 on the left and class 3 on the right) upon different wind sensors at Luosto in January-February 2002. Sensors from left: Friedrichs, Thies, Lambrecht, Metek 50 W, Handar or WAS425, WAA252, Irdam, Rosemount. As seen for Rosemount the class 1 is dominant (ice upon the body), which, however, does not actually affect the measurements.

5REFERENCES

[ 1] Tammelin, B., 1982. Frost formation on anemometers and frost prevention experiments. Technical report No 26. Finnish Meteorological Institute, Helsinki. 34 p.

[ 2] Tammelin, B., 1992. Experiences of wind measurements on fell peaks. In the proceedings of BOREAS conference 1992 (Ed. B. Tammelin et al.). Finnish Meteorological Institute, Helsinki. p. 241-246.

[ 3] WMO, 1996. Guide to Meteorological Instruments and Methods of Observation. WMO-No. 8. Secretariat of the World Meteorological Organization - Geneva - Switzerland.

[ 4] Tammelin, B., Hyvönen, R. and Peltomaa, A., 1996. The accuracy of wind measurements in hilly regions. In the proceedings of BOREAS III 19-21 Saariselkä, Finland. (Ed. B. Tammelin et al.). Finnish Meteorological Institute, Helsinki. p. 232-250.

[ 5] Tammelin, B., Joss, J. and Haapalainen, J., 1999. Specification of severe weather sensors - Final report of the EUMETNET project. Finnish Meteorological Institute, Helsinki. 154 p. + Annexes.

[ 6] Tammelin, B., Cavaliere, M., Holttinen, H., Morgan, C., Seifert, H. and Säntti, K., 2000. Wind energy production in cold climate. Meteorological Publications No 41. Finnish Meteorological Institute; Helsinki. 41 p.

[ 7] Tammelin, B., Heimo, A., Leroy, M., Rast, J. and Säntti, K., 2001. Meteorological measurements under icing conditions - EUMETNET SWS II project. Reports 2001:6. Finnish Meteorological Institute. 52 p.

[ 8] Leroy, M. et al. Temperature/humidity measurements during icing conditions. To be presented at TECO 2002.

[ 9] Musa, M. et al. Measurements of temperature with wind-sensors during severe winter conditions. To be presented at TECO 2002.

[ 10] Rast, J. et al. Solar radiation measurements under icing conditions. To be presented at TECO 2002.

[1] EUMETNET is a network of 18 National Meteorological Services: those of the EU plus Iceland, Norway and Switzerland;

[2] 1st generation: external heating elements in early 1980s, 2nd generation: heavy internal heating since middle of 1980s, 3rd generation: direct low power heating of sensing parts of the sensor since middle of 1990s.