National Center for Atmospheric Research
ATD... SHEBA ISFF Flux-PAM Project Report

7.0 Radiometers

  1. Instrumentation
  2. Data Processing
  3. Calibration
  4. Mounting
  5. Locations
  6. Level Sensors
  7. Riming and Ventilation

7.1 Instrumentation

Kipp and Zonen model CM21 pyranometers and Eppley model PIR pyrgeometers were used to monitor the incoming and outgoing, short-wave and long-wave radiation.
Rimed Radiometers
J. Militzer
The pyranometers deployed during SHEBA97 were the standard model produced by Kipp and Zonen. However, the pyrgeometers were specially modified to NCAR specifications by Eppley Laboratories. In order to reduce the reliance upon periodic servicing, the battery powering the temperature compensation circuit was replaced with a voltage standard circuit, mounted on an internal electronics board, fitted inside the radiometer cavity. Two additional thermistors were attached to the inside of the dome to ensure that a more representative dome temperature was monitored. These three dome thermistors were connected in series, and the dome temperature was derived using the total resistance. The values of the response of the thermopile (Rpile) and of the temperatures of both the case (Tcase) and the average temperature of the dome (Tdome) were all recorded separately.

Both types of radiometer were mounted in circular PVC ventilator housings fitted with Eppley radiation shields. Ventilation of the radiometers was provided by circulating air into the plenum of the housing and venting through the annular slot surrounding the radiometer dome. The four housings were attached to an adjustable platform fitted with an electronic electrolytic level to allow the horizontal aspect of all four, two upward- and two downward-viewing, radiometers to be established and monitored. The platform was affixed to the end of a four meter boom canter-levered out from the flux station tripod, two meters above the surface.

7.2 Data Processing

The raw voltage data from the radiometer array corresponding to:

was input to a Campbell CR10 data logger mounted on the flux station tripod. Calibration factors and coefficients loaded in the memory of the data logger enabled these voltages to be converted to engineering units. This data was sent in serial form to the Eve computer at a rate of 1 Hz. The values of four components of the surface radiation budget,, Rsw.out, and Rlw.out can be calculated from the measured parameters.

For the pyranometer, the calibrated response of the thermopile directly yields the measured visible radiation flux.

Rsw = cswVsw

where Rsw is the short-wave radiation flux, csw is a calibration constant and Vsw is the voltage output of the radiometer.

For the pyrgeometer, the situation is more complex because of the sensitivity of the long-wave radiation sensor to thermal effects beyond the simple response of the thermopile. The thermopile voltage Vlw responds to the long-wave radiation flux absorbed by the thermopile,

Rpile = clwVlw

where Rpile is the signal associated with the portion of the long-wave radiation flux absorbed by the thermopile and clw is a calibration constant

However, three corrections are required to this simple relation in order to obtain the long-wave radiation flux. The pyrgeometer thermopile response under-estimates the intercepted infrared radiation because the top surface of the pyrgeometer thermopile re-emits infrared radiation proportional to its absolute temperature. Hence an additional term Cc, dependent upon the temperature of the top surface of the thermopile, must be added to the output of the thermopile in order to obtain the true estimate of energy arriving at the pyrgeometer. For each pyrgeometer the temperature of the case housing the thermopile Tc is measured and Cc is calculated from the Stefan-Boltzman relation,

Cc = sigma Tc4

where sigma is the Stefan-Boltzman coefficient, 5.67 x 10-8, and the temperature Tc is in °K.

The pyrgeometer thermopile views the environment through the pyrgeometer dome. Because the dome is not completely transparent in the long-wave spectral region, the dome's own temperature contributes to the radiation received by the thermopile. A correction Cdc is calculated using the relationship,

Cdc = sigma Adc (Td4- Tc4)

where the value of the coefficient Adc is subject to uncertainty and values from 1.1 to 3.0 have been suggested. Here the dome temperature Td, again in °K, is determined from the average resistance of the three dome thermistors. This contribution, Cdc, must be subtracted from the thermopile output.

Finally, The pyrgeometer has a partial sensitivity to short-wave radiation due to the incomplete optical cut-off of the vacuum-deposited interference filter below 3 µm. This requires a correction.

Cv = Av Rsw

This correction, too, must be subtracted from the pyrgeometer output. Again, there is uncertainty as to the best value for the coefficient Av.

The correction terms applied to the pyrgeometer response yield the true value of the infrared radiation flux.

Rlw = Rpile + Cc - Cdc - Cv

Because all the individual sensor parameters involved are measured and archived, the four radiation components are calculated afresh each time that they are required. This procedure allows the values for the coefficients involved to be set according to preference during post-deployment analysis.

7.3 Calibration

Both pre- and post- deployment calibrations of the radiometers were carried out for all radiometers. To compensate for the variation of radiometer response throughout the duration of the program, simple linear interpolations between the pre- and the post calibration factors were used for the radiometers throughout the program. The differences between the pre-project and the post-project thermopile response were less than 1% for the pyranometers, but for the pyrgeometers the differences ranged from 0.091% to 5.54% with a mean of 2.19%. The changes in the thermistor calibrations were negligible. The internal precision of the group of eight pyrgeometers is estimated to be within 3 W/m², and the overall accuracy to be 5 W/m².

The pyranometer calibration was undertaken at the NOAA/CMDL Solar Radiation Facility (SRF), in Boulder, Colorado, under the supervision of Donald Nelson. The SHEBA pyranometers, together with SRF calibration-standard pyranometers, were exposed to daylight on the roof of the SRF building, for a period of two months, both before and after the SHEBA field program.

The pyrgeometer calibration involved both the black body radiation response of the sensor thermopile and the temperature calibration of the sensor thermistors. The black body radiative response for the pyrgeometers was carried out under the supervision of Ellsworth Dutton of ERL/NOAA in Boulder, Colorado following the procedure described in Dutton, 1993, (Dutton, E.G. 1993, "An extended comparison between LOWTRAN7 computed and observed broadband thermal irradiances: global extremes and surface conditions", J. Atmos. Ocean. Technol., 10, 326-336). This procedure provided both the calibration constant for the the thermopile, clw, and the coefficient used to make the dome-case temperature-difference correction, Adc.

The temperature calibration of the pyrgeometer thermistors was undertaken in the NCAR Sensor Calibration Laboratory. The pyrgeometers were calibrated in an associated Environmental Systems, Model SD 308 temperature calibration chamber. For each pair of pyrgeometers, a previously-calibrated platinum resistance thermometer was used as a reference and was attached to the top of the pyrgeometers near the domes. The instruments were then surrounded with a thin layer of insulation to ensure minimal temperature gradients. A temperature range of 10 °C to -40 °C was covered. From 10 °C to -30 °C, steps of 10 °C were imposed. Below -30 °C smaller steps were used. For the higher temperatures, a typical soak time of 2 hours was used to ensure stability and minimize any spatial gradients. For the lower temperatures, a 4 hour soak time at each point was used. The temperature accuracy is estimated to be ± 0.05 °C.

7.4 Radiometer Mounting

The radiometers were mounted in circular PVC ventilator housings. Different heights of the internal mounting bases ensured that the radiation-sensitive elements of the two dissimilar sensors, the Kipp and Zonen pyranometer CM21 and the Eppley pyrgeometers, were brought to the same level. Eppley radiation shields attached to these circular housings provided identical exposures, so that when mounted side-by-side neither radiometer impinged upon the other's field of view. Ventilation of the radiometers was provided by circulating air into the plenum of the housing and venting through the annular slot surrounding the radiometer's dome. A great deal of effort was expended, throughout the program, to improve the effectiveness of ventilation.

When the radiometer array was serviced, care was taken to avoid unnecessary disturbance of the snow surface in the field of view of the down-looking radiometers. Although some disturbance was caused, it was noted that the wind quickly restored the natural condition of the surface. The height of the radiometer array above the local surface varied throughout the program as snow built up or was eroded. Table 7.1 shows the heights of the downward-looking radiometers with dates, based on logbook entries. Note that some logbook entries referred to the height of the boom. These boom heights were adjusted to yield radiometer heights by subtracting 15 cm.

Table 7.4 Heights of Down-Looking Radiometers
Station 1   Station 2   Station 3   Station 4
Date Height   Date Height   Date Height   Date Height
98 04 07 1.58 m   98 04 16 1.83 m   98 04 14 1.40 m   98 04 21 1.75 m
98 06 12 1.37 m   98 06 12 2.08 m   98 06 19 1.56 m   98 06 08 1.75 m
98 06 22 1.58 m   98 06 23 2.12 m   98 06 30 1.64 m   98 06 13 1.72 m
98 07 06 1.66 m   98 07 08 1.65 m   98 07 09 1.79 m   98 07 29 1.96 m
98 07 22 1.87 m   98 07 25 2.29 m   98 07 31 1.76 m   98 08 26 2.05 m
98 08 04 1.97 m   98 07 31 2.42 m   98 08 10 1.83 m   98 09 08 2.00 m
98 08 29 1.93 m   98 08 08 2.27 m   98 08 25 1.85 m   98 09 28 1.90 m
98 09 28 1.90 m   98 08 10 2.39 m   98 09 28 2.13 m   98 09 30 1.88 m
98 09 30 1.91 m   98 08 26 2.29 m            
      98 09 28 1.82 m            

7.5 Radiometer Array Locations

Four SSSF radiometer arrays were deployed for the duration of SHEBA.

General descriptions of the four sites are given in Section 4. The down-looking radiometers at Atlanta and Baltimore viewed the local snow-covered sea ice throughout most of the duration of the field program. From late July/early August the situation became more complex as first localized, and then more wide-spread, surface melting occurred. Logbook entries after late August mention melt ponds surrounding the remote stations, sometimes covered in refrozen slush. A lead did open very near the Florida array in April, but the surface under the Florida array generally appears to have been subjected to less melt-water ponding and, even during the summer, the Florida down-looking radiometers viewed snow and ice.

The Cleveland station was involved with a pressure ridge in early February and the array was damaged by the ice. In April, the radiometer array was reinstalled on a three-legged stand independent of the PAM tripod and mast supporting the other sensors. This enabled the radiation array to be positioned anywhere in the general vicinity of the station. On 3 April this system was erected near Florida, and was then moved to the Seattle site on 16 April. At Seattle in late April a lead opened within 15m of the array. Subsequently, other leads opened and on 6 June there was open water within 6m of the array.
Radiometers over water
P. Guest, August 22, 1998
The station was moved on 8 June and June was re-established at the Maui site on 10. By this time melt-water ponds began to appear in the vicinity. On 25 July it was noted that although the radiometers were above white ice, the surrounding melt-water ponds would be within their field of view. The tripod was repositioned on 8 August so that the radiometers viewed the open water. From this date until Maui was decommissioned, on the 20 September, the radiometer array remained over a pond. The pond sometimes was open and sometimes partially covered with ice, described variously as brash, finger-rafting, white and new grey ice.

7.6 Level Sensors

The vertical orientation of the radiometer array was monitored continuously with an orthogonal pair of electronic level sensors. These sensors were nominally oriented at 45° to the axis of the boom supporting the radiometer array, in order to be aligned with the level adjustment axes of the array.

During the initial deployment of the SHEBA Flux-PAM stations, it was apparent that a null output of the electronic levels at Baltimore did not correspond with a level orientation of the radiometer array, and the radiometers could only be properly oriented with a bubble level. However, it appears that the radiometer arrays at the other stations were initially leveled using the electronic level sensors as a reference. Then in March or April 1998, NCAR staff discovered that the electronic level manufacturer had not informed them about known offsets in the sensors. Subsequently, this was not always fully appreciated by the field staff and a degree of confusion prevailed throughout the project about the proper reference for leveling the radiometers.

Fortunately each of the SHEBA radiometer arrays was leveled periodically with a bubble level, and the electronic level data immediately following those leveling events appear to be sufficiently consistent throughout the project to warrant using them to correct the data. The following tables list the offsets measured during those leveling events that appear to be valid. After removing these offsets from the data, it appears that the radiometers were generally maintained within 1°-1.5° of level, were often level to better than 1°, and were occasionally out of level by at most 2°-2.5°.

Table 7.6.1 Radiometer Level Offsets
Station 1   Station 2
Date Time lev.x
  Date Time lev.x
98 03 09 21:00 -0.98 0.12   98 04 08 22:45 0.24 0.42
98 03 31 23:00 -1.06 -0.06   98 04 18 04:30 0.04 0.52
98 04 08 00:30 -0.98 0.02   98 04 30 21:30 0.10 0.48
98 04 18 21:00 -1.12 -0.08   98 05 08 19:15 -0.08 0.24
98 04 21 20:00 -1.04 0.10   98 06 26 23:15 -0.10 0.88
98 05 08 00:30 -1.08 0.08   98 07 07 18:40 -0.02 0.44
98 07 06 18:20 -1.12 0.17   98 07 14 22:30 0.22  
98 07 21 19:30 -1.01 -0.06   98 07 23 23:15 0.18  
98 07 25 01:00 -0.86 0.10   98 07 30 22:15 0.22  
98 07 28 23:00 -0.97 0.06   98 08 07 23:45 0.10  
98 08 03 23:00 -0.92 -0.10   98 08 10 23:45 -0.08  
          98 09 06 22:50 -0.22  

7.6.1 Station 1 (Atlanta)

The radiometer array at Atlanta was not leveled with a bubble until March 1998. However the offsets measured from March through August are internally consistent and are assumed to apply for the entire project. The median values of the observed offsets are -1.01° in x and 0.06° in y; the maximum deviation of the measured offsets from the median values is 0.16°.

With the exception of leveling events, the radiometer levels remained fairly steady until mid-March and then again from mid-August through September. From mid-March until mid-August, settling of the PAM tripod and radiometer legs caused changes of the radiometer level on the order of 0.5°-0.6°. In late March and April this settling is inferred to have been caused by nearby lead activity, and from June until August it was caused by melting of the snow and ice. Note that the radiometers were erroneously `leveled' using the electronic levels at 00:20 GMT on June 27, causing a leveling error of 1.1° in x, and were correctly leveled again at 18:40 GMT on July 6. The maximum deviations of the radiometers from horizontal from October 1997 until March 1998 are inferred to be 1.0° in x and 0.3° in y. The maximum deviations of the radiometers from horizontal from March to September 1998 were 1.1° in x and 0.6° in y.

7.6.2 Station 2 (Cleveland, Seattle, Maui)

Campbell data logger #3 and radiation array #3, originally installed at Cleveland, had a cold temperature problem with the data logger multiplexer. Consequently logger and radiation array #3 were swapped with units #4 from Florida on November 1. Radiation array #4 was not leveled with a bubble until April 8, after Station 2 was resurrected at the Seattle site. Station 2 was moved to the Maui site on June 10 and decommissioned on September 20. On July 8, the y axis of the radiometer levels failed. The levels were replaced on August 22, but the y axis data appear to remain defective. The level offsets are moderately consistent from April through August 22, after ignoring the data from a few anomalous leveling events. The medians of the offsets during this period are 0.1° in x and 0.46° in y; the maximum deviations of the measured offsets from the median values are 0.2° in x and 0.4° in y. These values are presumed to also apply during October 1997 at the Florida site and from November through February at the Cleveland site. The offset of the x axis of the levels installed on August 22 is assumed to be -0.22°, the only measurement available.

The radiometer levels remained fairly steady while Station 2 was at the Cleveland site (November through February) The maximum deviations of the radiometers from horizontal at the Cleveland site are inferred to be 0.2° in x and 0.6° in y. The radiometer levels were also fairly steady at the Seattle site until the end of May when surface melting and nearby active leads caused larger movements of the radiometers. The maximum deviations of the radiometers from horizontal in early June were 0.4° in x and 2.1° in y.

There was considerable movement of the radiometers from June through August 22, while station 2 was at the Maui site, associated with melting, cracking and ridging of the floes. The maximum deviations of the radiometers from horizontal during this period were 1.5° in x and, prior to July 8, 2.2° in y. The extreme in the y axis occurred following a spontaneous jump of 2° in the level output on June 22, and it is possible that this was an early symptom of the more obvious failure of the y axis on July 8.

Table 7.6.2 Radiometer Level Offsets
Station 3   Station 4
Date Time lev.x
  Date Time lev.x
97 10 12 22:00 1.12 2.84   98 04 02 01:30 0.44 0.0
98 04 15 01:30 0.40 -0.28   98 04 09 05:15 0.48 0.10
98 05 02 22:00 0.42 -0.40   98 04 20 23:30 0.38 -0.02
98 05 12 21:30 0.42 -0.34   98 05 08 18:10 0.38 -0.06
98 06 29 18:30 0.24 -0.14   98 06 06 23:30 0.46 -0.24
98 07 09 18:00 0.38 -0.32   98 08 01 19:15 0.40 0.24
98 07 20 23:30 0.44 -0.56   98 08 03 00:00 0.40 0.00
98 07 30 18:30 0.58 -0.32   98 08 07 20:00 0.44 0.08
98 08 10 23:00 0.62 -0.64   98 08 11 22:00 0.28 0.00

7.6.3 Station 3 (Baltimore)

The radiation array at Baltimore was leveled with a bubble during the station installation in October 1997 and the offsets, 1.12° for x and 2.84° for y, are presumed to apply until March 18, when a new level sensor was installed at Baltimore. For March through September, the medians of the observed offsets are 0.42° in x and -0.33° in y; the maximum deviation of the measured offsets from the median values is 0.3°. On August 24, the radiometer level was obviously adjusted during a site visit, but the adjustment was not mentioned in the logbook and the implied y axis offet is not consistent with previous values. Then the logbook records a site visit on September 6 and notes that the radiometers were adjusted using a bubble level, but no adjustment is apparent in the data. These last two events have been ignored in estimating the level offsets.

From October through March, the radiometer levels at Baltimore were fairly steady; the maximum deviations of the radiometers from horizontal were 0.4° in x and 0.3° in y. From April through September, the maximum deviations of the radiometers from horizontal were 0.7° in x and 1.7° in y. These extremes occurred because of tripod settling during surface melting from mid-June through mid-August.

7.6.4 Station 4 (Florida)

The radiation array at Florida, #3, was originally installed at Cleveland, but those two arrays were swapped in early November because of the multiplexer problem at Cleveland. Radiation array #3 appears not to have been properly leveled until April 2, 1998, following a move of the station to a new location. The level offsets are moderately consistent from April through August, after ignoring the data from a few anomalous leveling events. The medians of the offsets during this period are 0.4° in x and 0.0° in y; the maximum deviations of the measured offsets from the median values are 0.1° in x and 0.24° in y. These offsets are presumed to also apply during October 1997 at the Cleveland site and from November through March at the Florida site.

With the exception of leveling events, Florida radiometer levels were fairly steady from October 1997 through the end of May. The deviations of the radiometers from horizontal at the end of October are inferred to be 0.4° in both x and y. The maximum deviations of the radiometers from horizontal from November through May are 0.9° in x and 0.6° in y. In early June, the radiometers were at times out of level by 5° or more due to rapid melting of the surface. This was fixed on June 5 and 6 by moving the tripod and installing plywood ablation shields; however surface melting and settling of the tripod continued until the end of August. The maximum deviations of the radiometers from horizontal after June 6 were 2.1° in x and 1.5° in y.

7.7 Riming/Ventilation

7.7.1 Introduction

Ventilation of the radiometer domes is an important consideration in the Arctic sea ice environment. When leads of open water are formed, warm sea-water is exposed to the cold atmosphere. Water evaporates and quickly condenses to yield a supercooled water fog, termed `smoke'. This supercooled water fog freezes on exposed surfaces to yield rime, a fluffy low-density deposit. In addition to the riming mechanism, there is also the possibility of the direct deposition of hoar from the vapor phase, when locally supersaturated air encounters the cold surfaces. Irrespective of the specific mechanism of production and whether the deposit was either rime or hoar, the effect of the growth of the low-density frost deposit was to degrade the response of the radiometers.

For the short-wave radiometer the effect can range from a partial reduction of light intensity due to obstruction when the dome is covered with frost, to an apparent enhancement of light intensity when low angle direct sunlight illuminates a partial frost covering and scatters light directly into the radiometer. For the long-wave radiometer the effect can be more subtle. A covering of frost results in the radiometer sensing infrared radiation associated with the frost itself, which is at the ambient air temperature, rather than the true incident radiation. The contribution of this component of the radiation depends on the extent of coverage. Any significant frost coverage of the radiometers renders the radiation data suspect. During the passage of supercooled fogs it would be very difficult to avoid the deposition of rime/hoar. However, there was the hope that, after the condition had abated, ventilation of the domes with under-saturated ambient air would result in the removal of the frost. Heating of the air, even by a fraction of a degree, would enhance this effect. A serious limitation which constrained the efficiency of the ventilation of the Flux-PAM radiometer array was the total amount of power available at the remote stations. Only 20-30 Watts was generated by the thermo-electric generators for the operation of all functions of the remote station. Of this, only ~8 Watts was available for ventilation and/or heating, ~2 Watts per radiometer.

7.7.2 Ventilation/Heating of Radiometers

A succession of different ventilation/heating systems were devised for the radiometer arrays over the first several months of the SHEBA deployment.
Single fan ventilator
E. Andreas, October 12, 1997
The original system, installed when the stations were initially deployed, employed a single 0.7 Watt, 4.5 cm square Comair Rotron muffin fan mounted in an enclosure with flexible ducts to deliver air to the four separate ventilator housings. This was ineffective and successive changes were made to attempt to improve the ability to remove frost. On October 22, resistance heaters were installed in the outlets of the fan enclosures at Cleveland and Florida, Baltimore was modified on November 11, and Atlanta was also modified in the latter time period. Although shipboard measurements definitely indicated that this modification increased the temperature of the outflow, in the arctic environment this heat appeared to be dissipated before it could affect the radiometers. Prior to their replacement by subsequent ventilator systems, the single-fan, four-duct configurations were modified in the field by restricting the ventilation to two ducts at Atlanta and only one at Baltimore. In no configuration was the system effective and, on December 13, a new design was introduced for testing at Florida. Cylindrical 3-cm-diameter, 0.4 Watt, Micronel ducted axial fans were fitted to each of the four radiometer ventilator housings in small insulated holders which contained 1 Watt resistance heaters. By December 17 it was observed that these new `turbo' ventilators were also not successful in keeping frost from the domes, but they were retained until February.

The next variant, tested at Florida during late February, removed the resistance heater from the axial fan holder and utilized six 0.2 Watt resistance heaters, installed in the ventilation plenum distributed around the ventilator slot. Again the ventilator/heater combination failed to prevent the accumulation of rime.

Juice can ventilator intakes
E. Andreas, May 17, 1998
A final configuration involved milling a rectangular hole in the side of the ventilator housings and mounting the original 0.7 Watt Comair Rotron fans within cowls fitted directly on each ventilator housing. These rectangular ventilators were first installed at Florida (4 March) and Atlanta (9 March), utilizing materials available from the ship. The cowls for these ventilators were fabricated from the brightly colored fruit juice containers shown here.

Machined ventilator intakes
E. Andreas, May 16, 1998
Later, custom-made cowls were manufactured at NCAR and shipped to the SHEBA ice camp. The Cleveland and Baltimore radiometer arrays were fitted with this form of ventilator on April 4 and 14, respectively. The air-flow produced by these individual, cowl-mounted external Comair Rotron fans was comparable to that for the line-powered ventilators of the ship-based NOAA Environmental Technology Laboratory (ETL) radiometer array and the NOAA Atmospheric Radiation Measurement group (ARM) radiometer array, but was not as efficient in defrosting the radiometers. The line-powered ventilators consumed ~20 Watts to produce the air-flow, and apparently the additional heating due to this greater power dissipation made the critical difference that kept the ETL and ARM radiometers generally rime-free.

In addition to the improvements in the ventilation to reduce the riming of all radiometers, the Kipp and Zonen incoming and outgoing short-wave radiometers were specifically modified using electrical resistance heater elements. Starting with the Florida radiometers on 10 April, 2.5 Watt, 30 ohm Minco heating elements were introduced into the short-wave radiometer inter-dome space. Addition of the Minco heating elements was subsequently carried out for Baltimore, Seattle, and Atlanta on April 14, 19 and 22, respectively.

7.7.3 Investigation of effects of removal of Kipp and Zonen outer dome.

When Station 2 was initially resurrected adjacent to Florida in April 1998, an experiment was undertaken to determine the effect of the removal of the pyranometer outer dome on both the calibration and the riming characteristics of the radiometer. This was motivated by the observation in the field that the double-dome pyranometers generally became rimed before the adjacent single-dome pyrgeometers. On April 4, the outer domes of the Station 2 pyranometers were removed, and for the following week the radiometers were operated with only the single inner dome. The result of the week-long study was that the riming of the Station 2 single-dome pyranometers was slightly less than that of the Florida double-dome pyranometers. However, inspection of the short-wave radiation flux data indicated that the flux measured by the Station 2 single-dome pyranometer was ~5-8% greater than the flux measured by the adjacent Florida array. With such a minimal improvement in the riming characteristics and a noticeable change in the response, it was decided to abandon the idea of removing the outer domes.

7.7.4 Manual cleaning

During the course of station visits, the radiometers were often manually cleaned to remove frost and ice deposits, although in some cases the record indicates that the deposits were resistant to these attempts. Tables 7.7.1-7.7.4 list the recorded manual cleaning incidents, including references to the corresponding logbook entries. It is certainly likely that additional manual cleaning events occurred which were not recorded.

7.7.1 Site 1, Atlanta
Date Time Event Log   Date Time Event Log
GMT       GMT    
97 10 16 ? Ice on domes 41   98 03 20 12:00 Cleaned domes 328
97 10 25 21:00? Cleaned all sensors 97   98 03 31 22:00 Cleaned SW domes 346
97 12 23 20:00 Cleaned domes 167   98 04 06 23:00 Cleaned SW domes 365
97 12 23 20:30 Cleaned domes 187   98 04 11 23:30 Cleaned domes 384
98 01 15 20:00 Cleaned domes 217   98 04 18 20:00 Cleaned domes 404
98 02 0 2 08:00 Frost on domes 242   98 04 21 18:30 Cleaned domes 414
98 02 08 23:45 Could not clean domes 249   98 04 28 23:00 Cleaned domes 434
98 02 11 19:00 Cleaned domes 274   98 07 21 9:30 Cleaned domes 592
98 03 05 20:00 Removed array 299   98 08 03 15:00 Cleaned domes 614
98 03 09 20:00 Replaced array 308   98 08 14 20:00? Ice on domes 635

Table 7.7.2 Site 2, Cleveland/Seattle/Maui
Date Time Event Log   Date Time Event Log
GMT       GMT    
97 10 12 19:00? Set-up 37   98 04 04 04:00 Restarted station 353
97 10 19 22:30 Cleaned domes 52   98 04 10 15:00 Reinstalled outer domes 375
97 10 22 22:00 Removed array 56   98 04 12 02:15 Cleaned domes 382
97 10 21 22:00 Replaced array 59   98 04 19 22:45 Cleaned SW 409
97 12 18 19:30 Cleaned domes 155   98 04 22 23:00 Replaced SW dome 418
97 12 27 20: 00? Cleaned domes 173   98 05 28 02:00? Ice on domes 489
98 01 05 20:00? Frost on domes 191   98 07 06 19:00 Cleaned SW domes 571
98 01 17 20:00 Cleaned domes 220   98 07 23 23:00 Cleaned domes 595
98 02 06 06:00 Cleveland damaged 258   98 08 10 23:30 Cleaned domes 625

Table 7.7.3 Site 3, Baltimore
Date Time Event Log   Date Time Event Log
GMT       GMT    
97 10 12 21:00 Set-up 33   98 03 04 20:30 Cleaned domes 296
97 10 16 00:00? Ice on SW domes 42   98 03 18 20:00 Cleaned domes 324
97 12 24 20:30 Cleaned domes 170   98 03 31 01:00 Removed array 344
98 01 01 21:00 Cleaned domes 183   98 04 14 17:00 Replaced array 395
98 01 13 21:00 Cleaned domes 213   98 05 01 23:00 Cleaned SW domes 443
98 02 18 00:00? Cleaned domes 264   98 08 10 23:00 Cleaned domes 626

Table 7.7.4 Site 4, Florida
Date Time Event Log   Date Time Event Log
GMT       GMT    
97 12 14 00:00 Removed/
reinstalled array
142   98 03 05 00:00 Replaced array 297
97 12 17 01:00 Cleaned domes 148   98 03 07 22:00 Cleaned domes 304
97 12 20 08:30 SW domes frosted 161   98 03 08 2000? Cleaned SW domes 306
97 12 23 23:00 Cleaned domes 168   98 03 13 20:00 Cleaned domes 315
97 12 29 22:45 Cleaned domes 175   98 03 16 18:30 Cleaned domes 320
98 01 08 20:00 Removed array 201   98 03 22 18:00 Cleaned domes 331
98 01 09 18:00 Replaced array 205   98 03 24 22:30 Cleaned domes 336
98 01 10 20:30 Cleaned domes 208   98 04 03 18:00 Cleaned domes 351
98 01 11 23:15 Frost on domes 209   98 04 05 18:00 Cleaned domes? 359
98 01 31 ??:?? Cleaned domes 239   98 04 09 00:00-
reinstalled array
98 02 03 20:00? Frost on domes 243   98 04 21 19:15 Cleaned domes 413
98 02 21 22:00 Removed array 270   98 05 08 21:00 Cleaned domes 455
98 02 24 20:00 Replaced array 277   98 08 07 19:00 Cleaned domes 619
98 02 28 00:00 Removed/
reinstalled array
286   98 09 18 20:00? Cleaned domes 696
98 03 02 01:00 Removed array 290          

7.7.5 Riming Index

It was observed in the field, that whenever any one radiometer of the array became rimed, then generally all the radiometers also became rimed. Advantage can be taken of this by examining the response of the up-looking pyrgeometer. Any significant riming of the dome of the up-looking pyrgeometer results in the pyrgeometer viewing this frost coating rather than the sky. As this frost coating is at the ambient near-surface temperature there is a significant difference in the apparent energy flux. Examination of pyrgeometer data, when manual cleaning removed the frost coating the dome, allowed the magnitude of this difference to be seen. Between January 18-25, 1998, the radiation array at Florida was cleaned six times, and each time the rime re-established itself. As can be seen from Figure 7.1, step changes of 30 - 40 W/m², corresponding to a temperature difference of ~12 °C, were observed each time the rime was removed and the radiometer viewed the sky.

Manual cleaning of frost from R<sub></sub> Figure 7.1 Manual cleaning of frost from the up-looking pyrgeometer at the Florida station.

Although the ETL and ARM down-welling long-wave radiation data did show a systematic difference of a few W/m², examination of their year-long record indicated that both normally remained rime-free. Assuming a spatially-homogeneous sky cover, the differences between the down-welling long-wave radiation data from the remote stations and the comparable data from the well-maintained ETL and ARM radiometers can provide a clue as to the riming conditions. Whenever the difference exceeds some threshold, then riming can be suspected on the remote station radiation array.

The down-welling radiation differences for each Flux-PAM station, referenced to both the ETL and ARM radiometers, are included in the hourly SHEBA data as and Individual plots for Stations 1, 2, 3, and 4 show the index based upon the ETL data. It is suggested that whenever the value exceeds ~20 W/m², the radiation data from the remote station should be regarded with suspicion, as riming is likely to be present. It may be seen that riming conditions persisted throughout November, December, January and February, and then diminished, occurring much less regularly in the months of March, April and May, and were practically absent later in the year.

Table of Contents Previous Top Next