The 8th BSRN Workshop and Scientific Review Meeting, 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, Saskatchewan 2 Toronto, Ontario

The 8th BSRN Workshop and Scientific Review Meeting, July 2004 British Met Office, Exeter, UK Experimental Design The study is the result of questions arising from the ARM 1 st 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

The 8th BSRN Workshop and Scientific Review Meeting, July 2004 British Met Office, Exeter, UK 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).

The 8th BSRN Workshop and Scientific Review Meeting, July 2004 British Met Office, Exeter, UK Initial Instrument Configuration CM21 s/nResponsivity (µV/ Wm -2 ) Calibration DateVentilation System Sept. 13, 2001K&Z CV Sept. 13, 2001high volume NBB fan, modified base Oct. 10, 2001original fan, modified base Oct. 10, 2001medium volume NBB fan, modified base Nov. 15, 2001original fan, original base (IOP)

The 8th BSRN Workshop and Scientific Review Meeting, July 2004 British Met Office, Exeter, UK BSRN Instrument Configuration Type, s/nResponsivity (µV/ Wm -2 )Application CM South tracker diffuse CM North tracker diffuse CM Global table west CM Global table east PIR South tracker IR PIR North tracker IR CG Global table IR

The 8th BSRN Workshop and Scientific Review Meeting, July 2004 British Met Office, Exeter, UK Instrumentation BSRN Global Table Kipp and Zonen CG4 with CV2 shield and ventilator BSRN tracker with diffuse and Eppley PIR

The 8th BSRN Workshop and Scientific Review Meeting, July 2004 British Met Office, Exeter, UK Operational Ventilation Settings The experiment was divided into four time periods, based on changes in the ventilation regimes. These periods were: February : no ventilation (control) February 26 - March 15 : ventilation systems on (no heat on CV2) March : 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.

The 8th BSRN Workshop and Scientific Review Meeting, July 2004 British Met Office, Exeter, UK 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, Their technique suggests the use of an equation of the form: os = b 0 + b 1 NetIR + b 2 DC 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 b 0 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.

The 8th BSRN Workshop and Scientific Review Meeting, July 2004 British Met Office, Exeter, UK 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

The 8th BSRN Workshop and Scientific Review Meeting, July 2004 British Met Office, Exeter, UK Instrument offsets during the first ventilation period. Four of the five instruments show little change. Number shows increased scatter (medium volume fan), and is similar to the two BSRN instruments that show scatter. Ventilated (1) CV2High-vol MSC (mod)Med-volMSC BSRN

The 8th BSRN Workshop and Scientific Review Meeting, July 2004 British Met Office, Exeter, UK 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) CV2High-vol MSC (mod)Med-volMSC BSRN

The 8th BSRN Workshop and Scientific Review Meeting, July 2004 British Met Office, Exeter, UK 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 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-volMSC (mod) Med-vol MSC BSRN

The 8th BSRN Workshop and Scientific Review Meeting, July 2004 British Met Office, Exeter, UK 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 CV2High-vol MSC (mod)Med-volMSC BSRN

The 8th BSRN Workshop and Scientific Review Meeting, July 2004 British Met Office, Exeter, UK 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) CV2High-vol MSC (mod)Med-volMSC BSRN

The 8th BSRN Workshop and Scientific Review Meeting, July 2004 British Met Office, Exeter, UK 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 and March 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.

The 8th BSRN Workshop and Scientific Review Meeting, July 2004 British Met Office, Exeter, UK CV2High-volMSC (mod)Med-volMSC Ventilation Effects on Frost Accumulation Frost accumulation on March 13 (top) and March 18 (bottom, CV2 heater on)

The 8th BSRN Workshop and Scientific Review Meeting, July 2004 British Met Office, Exeter, UK The CM21 and net IR data for March 13 are plotted here against time. The irregularity of 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 13 th. Offset Time Dependence

The 8th BSRN Workshop and Scientific Review Meeting, July 2004 British Met Office, Exeter, UK 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.

The 8th BSRN Workshop and Scientific Review Meeting, July 2004 British Met Office, Exeter, UK 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.

The 8th BSRN Workshop and Scientific Review Meeting, July 2004 British Met Office, Exeter, UK 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.

The 8th BSRN Workshop and Scientific Review Meeting, July 2004 British Met Office, Exeter, UK 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.