the 8th bsrn workshop and scientific review meeting, 26-30 july 2004 british met office, exeter, uk...

The 8th BSRN Workshop and Scientific Review Meeting, 26-30 July 2004

British Met Office, Exeter, UK

Pyranometer Thermal Offsets Under Different Ventilation Regimes

David H. Halliwell 1

L. J. Bruce McArthur 2

Experimental Studies Division Meteorological Service of Canada

1 Wilcox, Saskatchewan2 Toronto, Ontario



Experimental Design

• The study is the result of questions arising from the ARM 1st Diffuse IOP, with respect to the performance of the Kipp and Zonen CM21 with the Meteorological Service of Canada (MSC) ventilation system

• Carried out at the Regina BSRN station, Feb. 22 to Apr. 12, 2002 (50 days)

• Set up five Kipp and Zonen CM21s with five different ventilation systems (configured as global sensors)

• Supplemental data from the BSRN data stream, consisting of four CM21s (two global, two diffuse), two Eppley PIR and one Kipp and Zonen CG4 pyrgeometers (co-located within 10 m)

• All data measured once per second, store one-minute statistics



Ventilation Systems

The ventilation systems used in the experiment consisted of the following:

1. Kipp and Zonen CV2 shield and ventilator (with optional heating)

2. Standard MSC shield, fan and base (nominal 35 cfm)

3. Standard MSC fan with modified base (increased hole sizes)

4. Medium volume NBB fan (nominal 90 cfm) with revised base

5. High volume NBB fan (nominal 166 cfm) with revised base

Note: all BSRN instruments use the Standard MSC shield, fan and base (except the CG4, which uses a CV2 shield with heater).



Initial Instrument Configuration

CM21 s/n Responsivity

(µV/ Wm-2)

Calibration Date Ventilation System

920063 10.82 Sept. 13, 2001 K&Z CV2

950248 14.65 Sept. 13, 2001 high volume NBB fan, modified base

950249 12.41 Oct. 10, 2001 original fan, modified base

910003 17.96 Oct. 10, 2001 medium volume NBB fan, modified base

950250 12.61 Nov. 15, 2001 original fan, original base (IOP)



BSRN Instrument Configuration

Type, s/n Responsivity (µV/ Wm-2) Application

CM21 910018 16.32 South tracker diffuse

CM21 920066 12.15 North tracker diffuse

CM21 980481 21.84 Global table west

CM21 920065 11.48 Global table east

PIR 32616 3.64 South tracker IR

PIR 27382 3.82 North tracker IR

CG4 010546 12.61 Global table IR



Instrumentation

BSRN Global Table

Kipp and Zonen CG4 with CV2 shield and ventilator

BSRN tracker with diffuse and Eppley PIR



Operational Ventilation Settings

The experiment was divided into four time periods, based on changes in the ventilation regimes. These periods were:

•February 22 - 26 : no ventilation (control)•February 26 - March 15 : ventilation systems on (no heat on CV2)•March 15 - 27 : ventilation systems on (heat added to CV2)•March 27 – April 12: instruments shifted on ventilators.

The shift meant that each CM21 was exposed to a different ventilation system. The five instruments were removed from the base and moved one position north (with the northmost moving to the southmost position)

The BSRN instruments (four CM21, two PIR, one CG4) operated under standard ventilation throughout the experiment.



Analysis Procedure

Analysis follows the guidelines from Dutton et al (2001) “Measurement of broadband diffuse solar irradiance using current commercial instrumentation with a correction for thermal offset errors” J. Atmos. Ocean. Tech. 18, 297-314. Their technique suggests the use of an equation of the form:

os = b0 + b1NetIR + b2DC

Dutton et al found that the DC term does little to improve the offset correction, which leaves the offset as a linear function of the NetIR term. Dutton et al also suggested that the b0 term can be set to zero (forcing the line through the origin).

In the current study, regression have not been performed on the data. We will examine the data graphically, plotting the pyranometer value against NetIR.



Instrument offsets during the initial “no ventilation” period. Offsets are small and similar amongst all five CM21s for this study. The first four instruments are the BSRN ones, which are ventilated through this period. Two of the four BSRN instruments show greater scatter than the others.

Unventilated

BSRN BSRN BSRN

BSRN



Instrument offsets during the first ventilation period. Four of the five instruments show little change. Number 910003 shows increased scatter (medium volume fan), and is similar to the two BSRN instruments that show scatter.

Ventilated (1)

CV2 High-vol

MSC (mod) Med-vol MSC

BSRN BSRN BSRN

BSRN



The only change from the first ventilation period is that the CV2 ventilation system (centre graph) heater has been turned on. No significant differences are apparent.

Ventilated (2)

CV2 High-vol


BSRN BSRN BSRN

BSRN



The non-BSRN instruments (5-9) have shifted positions, changing ventilation systems. Small differences in position of the scatter plots are noted, but the patterns remain essentially the same. S/n 920063 shows the largest shift, but even the BSRN instruments show minor shifts compared to the previous time period, in spite of no change in ventilation.

Ventilated (3)

CV2

High-vol MSC (mod)

Med-vol MSC

BSRN BSRN BSRN

BSRN



This data for March 12 shows the results of all instruments plotted against the Eppley PIR on the south tracker. The results are slightly dependent on which instrument is used for the net IR signal. The PIR on the north tracker is different, and the CG4 is different again.

IR Instrument Selection

CV2 High-vol


BSRN BSRN BSRN

BSRN



The CM21 data for March 12 are plotted here against the CG4 data. Generally, there is a shift to the right, but the scatter is slightly different, as well. If plotted against the north tracker PIR, the results generally fall between the south tracker PIR and CG4 graphs.

IR Instrument Selection (2)

CV2 High-vol


BSRN BSRN BSRN

BSRN



Accumulation of Frost and Dew on Pyranometer Domes

• The formation of frost or dew on instrument domes and windows is a common problem in certain seasons at the Regina BSRN station.

• The accumulation will often occur even though humidity levels measured at the instrument platform are less than 100%. Alternatively, domes may remain clear under conditions where the ground or other surfaces form frost or dew.

• Ventilation helps reduce this accumulation, and during this study the dome conditions were monitored closely. Heating the air stream also reduces the problem.

• The following slide shows the accumulation of frost on the five instrument domes on the nights of March 12-13 and March 17-18. Both nights saw relative humidity values in the 80-90% range, and overnight temperatures around -15°C to -20°C. The second night was not as cold, and frost accumulation was less severe. The CV2 ventilation system heater had been turned on on March 15, and this effect is evident.



CV2 High-vol MSC (mod) Med-vol MSC

Ventilation Effects on Frost AccumulationFrost accumulation on March 13 (top) and March 18 (bottom, CV2 heater on)



The CM21 and net IR data for March 13 are plotted here against time. The irregularity of 910003 is apparent. There is no obvious point at which frost begins to form on the CM21s. Maximum RH was about 90% the evening before. Temperatures dipped to -20°C with RH=80% at 4am on the morning of the 13th.

Offset Time Dependence



Discussion (1)

• All ventilations systems used in this study show very similar results, with nocturnal offsets generally in the 0 to -3 Wm-2 range for net IR values of 0 to -120 Wm-2. Typically, a linear fit of CM21 flux values against net IR values (e.g., Dutton et al, 2001) will give an intercept of about -1 Wm-2. The slopes are similar in magnitude to those found for CM21s by Dutton et al.

• In the Diffuse IOP, larger offsets were reported for the CM21, but net IR magnitudes were also greater.

• Three of the instruments show significantly greater scatter in general, and these instruments also tend to show larger nocturnal offsets (to -4 or -5 Wm-2). The reason for this is unknown. The PIR Case-Dome temperature correction of Dutton et al has not been examined, and is one possible variable that could be examined to find the cause of the instrument variation.

• Although changing the ventilation system has some effect on nocturnal offsets, there is greater variability amongst individual instruments with no differences in ventilation.

• The relationship between the pyranometer offset and net IR is slightly dependent on the individual IR instrument, but the results are very similar even between two instruments from different manufacturers with significantly different design characteristics. A suitable offset correction to the pyranometer data is probably possible with any reasonable IR instrument that provides a net IR signal.



Discussion (2)

• Greatly increased ventilation rates do not cause large shifts in the offset/IR relationship.

• Increased ventilation rates generally help reduce frost (and presumably dew) accumulation on domes. The data does not, however, suggest that “more is better” – the relationship between frost formation and ventilation rate is more complex than that.

• Heating the air stream (CV2) also helps reduce frost, with no significant effect on the nocturnal offset behaviour. The CV2’s heating circuit is relatively low powered (10W), so even a small amount of heating is beneficial. The heating power should probably be linked to the ventilation rate.

• Characterization of and generalizations about a particular brand or model of pyranometer using observations from a single instrument is a Bad Idea.



Additional Work

The instruments used in this study have extensive history at the Regina BSRN site, having been used at several locations over a number of years. By examining the data from these other periods, we can see the results of the IR offsets over a wider range of seasonal conditions and temperatures.

One location of interest will be the 30m tower, where the instruments are used to measure upwelling fluxes. At night, temperature inversion can approach 10°, so IR signals are often negative (though not as large as instruments exposed to the sky). During the day, as the surface heats, the IR signal is generally positive, however (as indicated by the net IR reading from the pyrgeometers used to measure upwelling IR values). This represents a different set of conditions from the ones encountered when using pyranometers for diffuse or global measurements.



Acknowledgements

• Edmund Wu, Ihab Abboud, and Aaron Ullberg prepared ventilation systems and instruments, and performed calibrations.

• Kipp and Zonen arranged for the loan of a CV2 ventilation shield from Campbell Scientific Canada.

• Ormanda Niebergall maintained the instruments during the experiment, and provided the photographs of dome conditions.

the 8th bsrn workshop and scientific review meeting, 26-30 july 2004 british met office, exeter, uk...

Documents

standard ventilation

different ventilation

ventilation controlfebruary

zonen cv2 shield

base nominal

zonen cm21s

global table eastpir

global table westcm21