Solar Radiation Measurements under Icing Conditions
Jacques Rast1, Alain Heimo1, Bengt Tammelin2, Michel Leroy3
1
MétéoSuisse, CP 316, CH-1530 Payerne, 2 Finnish Meteorological Institute, 3 MétéoFrance
Phone 41 26 662 62 26, jacques.rast@meteosuisse.ch
1. Introduction
Hemispherical global irradiance is a parameter of great importance for meteorology because it
represents the source of energy for the movements of the atmosphere. This important parameter is
monitored at all the automatic meteorological stations in Switzerland. It was therefore interesting to
analyse the behaviour of pyranometers’ heating and ventilation devices under severe icing
conditions, in order to characterize how biased are the results under these conditions, especially as
the Swiss automatic meteorological network ANETZ is presently being renewed.
The EUMETNET/SWS II project gave the opportunity to perform this comparison (beside of other
measurements described in other contributions) during the winter 2001/2002 on the mount Säntis at
an altitude of 2500m and preliminary results obtained during this campaign will be presented. A
detailed report on the complete experiment is to be published at the end of 2002.
2. System description
Four systems were under testing, equipped with two pyranometers types (CM11 and CM21 from
Kipp&Zonen). The difference between the systems is not due to the pyranometer themselves, but
much more to the ventilation and
heating systems used to prevent icing.
Two pyranometers (AV1 and AV2)
were installed with a ventilation
developped at MeteoSwiss, a further
one with a commercially available
system CV2 from Kipp&Zonen
(K&Z) and the last one with a
ventilation used at the Finnish
Meteorological Institute (FMI). One
pyranometer from MeteoSwiss (AV2)
was cleaned regularly by the local
observer and was therefore selected as
reference, even if not allways totally
free of ice. The air inlet from the
MeteoSwiss systems is protected by a
shell oriented against the main wind
direction. The air conditioning box
from the FMI is installed on the side of the pyranometer.
Figure 1 : the pyranometer assembly at the Säntis
The figure 1 shows the assembly with, from back to front, AV1, AV2, K&Z and FMI devices.
The pyranometers’ values were sampled at 10 s intervall and 10 minutes averages were recorded.
The measurement period lasted from 01.10.2001 to 30.04.2002. During this period 26064 10
minutes measurements were recorded with observations.
3. Results
3.1
Icing quantification
A difference is made between meteorological icing (the perturbation), which is a meteorological
event and the cause of icing problems, and instrumental icing (the answer), which is the effect
from the previous one on the instrument. Meteorological icing was quantified by weighting 7 times
a day (no observation at midnight) the amount of ice accreted on a 100 cm2 cylinder provided by
FMI. Instrumental icing was quantified by visual observation: a note between 0 (free of ice) and 3
(totally covered with ice) was attributed 7 times a day by the observer characterising the icing state
of the case and of the sensitive part (the glass dome and filter) of the instruments participating to the
test.
The measurement period lasted 7 months. Meteorological icing (accretion on cylinder) was
observed during 32% of this period, with accretion rates as high as 1 g/cm2/h.
In order to compare the response of the different instruments, the number of instrumental icing
cases (with a note different of zero) relative to the number of meteorological icing cases (ice
accretion on the cylinder) was considered. The result is plotted in figure 2. It may be seen that even
cleaned at the same time as the cylinder swapping, the AV2 system remains at least slightly covered
with ice (class 1) during 70% of the icing event duration. The same system uncleaned (AV1)
remains hindered by ice 120% and the other instruments 150% of the meteorological icing, e.g. the
instruments remained covered with ice after the end of the meteorological icing event. For
comparison the structure (neither heated nor ventilated) supporting the instrumentation remained
iced 195% of the time with meteorological icing.
Sensor Ice / FMI Cylinder Ice
180
160
140
Percent
120
100
80
60
40
20
0
AV1
AV2
K&Z
FMI
Figure 2 : relative icing : the time with non-zero class divided by the time with ice accretion
Figure 3 deals with the instrumental icing only. It displays the observed time of the different icing
classes relative to the total observation time for each instrument. The effect of regular cleaning of
the AV2 resulting in a small number of class 3 observations as well as the protective effect of shell
on the AV1 system are demonstrated. In fact, it was observed that in case of icing, the filter at the
air inlet of the K&Z and of the FMI instruments was rapidly blocked by ice and the air flow
stopped, thus leading to unhindered ice accretion on the sensor.
Healing Radiation
100
class 0
class 1
class 2
class 3
Percent
80
60
40
20
0
AV1
AV2
K&Z
FMI
Figure 3 : the relative residence in the different classes
3.2
Case study
Figure 4 illustrates the effect of an icing event on the different instruments. The left scale refers to
the measured global irradiance [W/m2] while the right one shows the instrumental icing classes. The
different instruments are identified by colours identical for both class status (dotted horizontal line)
and signal (full line). Meteorological icing observations showed a light accretion rate (0.2 g/cm2/h)
on the 12th an 13th february, leading to the following observed instrumental icing : the reference
AV2 instrument (magenta curve) was oscillating between class 0 and class 1 while the other sensors
remained between class 1 and class 3. Sunshine was only recorded for about 5 hours during the last
day. The air temperature remained around –5 oC. The reference AV2 was cleaned on the 12th
between 08h00 and 20h00 (5 times), on the 13th between 02h00 and 14h00 (5 times) and on the
15th at 2h00 (1 time).
It may be noted that the signal is weakened mainly during class 2 and 3 situations.
Pyranometers
800
4
700
600
3
400
2
300
200
1
100
0
0
12-Feb
13-Feb
14-Feb
CM2_AV1
CM2_AV2
CM2_KZ
AV2_MP
KZ_MP
SF_MP
15-Feb
CM1_SF
AV1_MP
Figure 4 : Global radiation and classes 12-15 february 02
Class
W/m2
500
3.3
Scatterplot
Scatterplots performed over the whole measurement period summarize the behavior of the different
sensors compared to the reference instrument under instrumental icing conditions and give a good
illustration of the link between instrumental icing and data perturbation. Figure 5 displays the
scatterplot of the FMI pyranometer relative to the reference (AV2, regularly cleaned). The different
icing classes are plotted with different colours. An hysteresis between ascending and descending
sun - up to now unexplained – may be spotted, leading to a broadening of the curve.
It must be also noted that observed instrumental icing does usually lead to corrupted data, but with a
different magnitude depending on the class situations. In fact, class 2 and 3 situations clearly
produce very low radiation values, as observed on sunny days following icing situations. Corrupted
data under cloudy situations are difficult to recognize due to the very low recorded intensities. This
fact is clearly confirmed by the previous case study.
SWS Säntis, october 01 to april 02
FMI vs AV2 (10 min avg)
1200
Class 0 12589
Class 1 2701
1000
Class 2 2910
Class 3 7799
W/m2
800
600
400
200
0
0
200
400
600
W/m2
800
1000
1200
Figure 5 : scatterplot
4. Conclusion
The measurements performed during the EUMETNET/SWS experiment showed that strong icing
conditions dramatically disturb the global irradiance measurement. None of the tested system was
able to fully withstand the particularly harsh climatic conditions prevailing at the Säntis. Even a
cleaned instrument may show a small amount of class 2 (partial covering) and class 3 (total
covering) events due to the discontinuous presence of the observers.
If the analysis is restricted to class 2 and 3 and if the present winter conditions were representative,
the present study shows that the tested systems record in the mean the following amounts (in time)
of underestimations during the winter season:
-
7% for AV2 for MeteoSwiss (cleaned),
-
24% for AV1 for MeteoSwiss (un-cleaned),
-
40% for both K&Z and FMI systems (un-cleaned).
This experiment shows that technical developments can clearly improve the quality of the results,
but that the presence of an operator remains at the present time the best (when not the cheapest)
solution for good global irradiance measurements in strong icing situations.