Breakout Summary Report

ARM/ASR User and PI Meeting

19 - 23 March 2018

Session Title: Broadband Radiometry Group
Session Date: 19 March 2018
Session Time: 4:00 PM - 6:00 PM
Number of Attendees: 30
Summary Authors: Chuck Long

Breakout Description

The Broadband Radiometry Group was founded by request in 2010 and has been operating since under the Charter Statement: "The Instrument and Measurement Focus Group for Broadband Radiometric Measurements will address issues related to producing accurate, reliable, and continuous broadband radiation measurements for scientific use." The purpose of this breakout meeting is to update progress on previously identified issues, reporting and discussion on any new ideas and issues, and presentations on related items of interest such as field campaign activities related to broadband radiometry.

Main Discussion

The Broadband Radiometry Group breakout started with a welcome from the Chair, and then proceeded to hear a report from Ibrahim Reda on the recent Fall 2017 Absolute IR Instrument Campaign held at the SGP. The campaign data from the Absolute Cavity Pyrgeometer (ACP) and InfraRed Integrating Sphere (IRIS) instruments agree within a range of 0.2 to 2.5 Wm-2, within their stated uncertainties of ±3 Wm-2, as did the downwelling LW irradiances calculated by Dave Turner using AERI data. The pyrgeometers calibrated traceable to the World Infrared Standard Group (WISG) are generally lower under clear skies than the average of the ACPs and IRISs by between 4.4 to 6.6 Wm-2 with some dependence on the column integrated water vapor. These findings indicate a low bias of the WISG currently in use.

The discussion then turned to the previously identified pyrgeometer issues. The first issue is how often calibration of ARM PIRs is needed. The second issue is whether to recommend the reprocessing of all ARM PIR data given the findings over the years of the inadequacy of using a 2-coefficient simplified Albrect and Cox formula for calculating the Eppley PIR LW, particularly given the use of a "standard" case-dome temperature difference term coefficient of 4.0 before ARM switched over to using a better 4-coefficient formula with all 4 coefficients actually determined for each individual instrument. The group has now reached consensus to recommend that a calibration frequency of every 2 years is indeed adequate. Associated with the stability of the calibrations and the known errors in ARM's use of the 2-coefficient formula, it is also recommended that all ARM PIR data prior to the switch to using the new 4-coefficient calibration formula and individually determined coefficients be reprocessed to use the more recently determined coefficient in the 4-coefficient formula for all data since the inception of ARM PIR measurements. The broadband instrument mentors will supply the individual 4-coefficient sets for each serial number PIR after the 2018 PIR calibration season.

The group next reviewed reports on action items from last year's meeting given by Mark Kutchenreiter. The instrument mentor recommendations regarding Single Radiometer Configuration are to leave the SkyRad/GndRad systems as is, and to leave the SIRS systems as is with the exception of adding a second PIR to each SIRS system along with the 3-instrument tracker plates and 3-shading-ball arm. This would standardize the facility's surface radiation instrument suite, while allowing the deployment versatility needed for the NSA and Mobile Facilities.

Based on the discussions during the 2017 BBRad group meeting, screens were removed from the SGP BORCAL ventilator fans prior to the start of the first 2017 BORCAL session. Screens were removed from ventilators at the SGP, and OLI sites at the times of radiometer installations following BORCAL calibrations. A brief test will be performed at SRRL prior to the start of the 2018 BORCAL sessions to determine if there is a discernible effect of using or not using fan screens in operation. If there is no discernible effect, we recommend that use of the screens be based on site operator’s judgment of anticipated conditions. Having the tracker plates with the ventilation holes would facilitate screen inspection and cleaning. Site operators were asked for input regarding observed effects of the screen removals during frost/snow/riming, and non-winter conditions.

Mike Ritsche reported that the implementation to install 3-position tracker plates with 5" holes to facilitate improved air access for the radiometer ventilators, along with deployment of the 2nd PIRs to the SIRS systems, and related tasks has been approved, but put on hold due to current budget uncertainties and constraints. The group considers this to be an unfortunate development and hopes that progress will be reinstated soon.

A presentation was given by Chris Cox reporting on the De-Icing Comparison Experiment (D-ICE) currently ongoing at the ARM and NOAA sites at Utqiaġvik (formerly Barrow), and the ARM Oliktok Point site in Alaska. Substantial amounts of data are adversely affected in regions conducive to frost, rime, and snow, such as the polar regions. The D-ICE campaign is aimed at testing strategies developed by research institutes and industry for preventing radiometer icing. Specifically, the aim is to identify a method to be adopted by the research community that is effective at mitigating ice while also minimizing adverse effects on measurement quality, and to serve the needs of the community best, while also being energy efficient. Preliminary analyses indicate that:
- The data supports the hypothesis that aspiration of ambient air using a ventilator is a viable option for ice mitigation
- Additional heating is not a requirement, though it is effective
- Subtleties in the design matter
- ARM ventilation system:
More effective for pyrgeometers than pyranometers
OLI system likely less effective than NSA system

More information on D-ICE is available at:
https://www.esrl.noaa.gov/psd/arctic/d-ice/
https://www.arm.gov/research/campaigns/nsa2017dicexaco

There was also some time to introduce a few other topics by the broadband instrument mentors. These included:
- Testing for differences in the output of radiometers with or without a coil heater to determine if we should use coils in BORCALs for radiometers used in cold region locations.
- Using portable radiometer systems as backup or as primary systems at remote sites where repairs, spare equipment, and onsite capabilities for troubleshooting may be limited.
- Adding EKO WISER for high-resolution spectral measurements at selected sites as a separate datastream. It is noted that NREL is pursuing a partnership between Conner Flynn and NREL to better calibrate and evaluate the SAS spectroradiometer.
- Adding Total UV instruments at selected sites as a separate datastream.

And for OLI:
- Test the use of low-voltage heater pads on the bottom of the PSP ventilator sunshield as an aid for reducing frost and ice accumulation on the dome.
- Test the use of a modified sunshield on one PIR.

Some discussion ensued, but no consensus recommendations were arrived at. The broadband instrument mentors will pursue these ideas as they see fit.

Gary Hodges commented on the frequency of lowering the towers to clean the downward-facing instruments, where he noted the interruptions in data he sees as mentor for the MFRs. In his experience with SURFRAD, the ARM towers are being lowered far more frequently than needed since the downward-facing radiometers tend not to get dirty like the upward facing ones do. After discussion, it was agreed that perhaps trying only a quarterly cleaning was in order, with operators then noting if any of the times they saw obstructions like bee nests or such, and whether the domes appeared to be dirty or not.

Finally, it was announced that the current Chair of the BBRad group will be retiring, and thus stepping down. Whether the BBRad group will continue or perhaps a radiometry science advisory group that includes spectral as well as broadband might be formed is unknown at this time. However, as Chair of the group since its inception in 2010, I wish to express my thanks to all the BBRad group for their help and participation in all the BBRad breakouts we have had. It has been fun, passionate at times, but overall we have managed to end up I believe with some significant accomplishments. Despite my best efforts, but because of all of theirs.

Decisions

The following consensus recommendations are put forth by the BBRad Group:

1) It is recommended that ARM switch to a 2-year cycle of calibrations for the ARM Eppley PIR LW radiometers, rather than calibrating them each year beginning in 2019.

2) It is recommended that all Eppley PIR LW data calculated using the simplified 2-coefficient Albrect and Cox formula be reprocessed using each individual instrument's recently determined 4-coefficient formulation. The broadband instrument mentors will supply each individual instrument's coefficients after the 2018 calibration season.

3) With respect to the Single Radiometer Configuration discussions, it is recommended to leave the SkyRad/GndRad systems as is, and to leave the SIRS systems as is with the exception of adding a second PIR to each SIRS system along with the 3-instrument tracker plates and 3-shading-ball arm.

4) The group recommends proceeding with the implementation to install 3-position tracker plates with 5" holes to facilitate improved air access for the radiometer ventilators, along with deployment of the 2nd PIRs to the SIRS systems, and related tasks.

5) The group recommends decreasing the lowering of the towers and cleaning of the downward-facing radiometers to quarterly (4 times per year) and monitoring the condition of the domes and radiometers by the operators at those times. The instrument mentors will coordinate.