Atmospheric Temperature Measurement Biases on the Antarctic Plateau

1598 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 28

CHRISTOPHE GENTHON,DELPHINE SIX, AND VINCENT FAVIER Laboratoire de Glaciologie et Ge´ophysique de l’Environnement, CNRS/UJF, Saint Martin, d’He`res, France

MATTHEW LAZZARA AND LINDA KELLER Antarctic Meteorological Research Center, University of Wisconsin—Madison, Madison, Wisconsin

(Manuscript received 20 May 2011, in ﬁnal form 2 August 2011)

ABSTRACT

Observations of atmospheric temperature made on the Antarctic Plateau with thermistors housed in naturally (wind) ventilated radiation shields are shown to be significantly warm biased by solar radiation. High incoming solar flux and high surface albedo result in radiation biases in Gill (multiplate)-styled shields that can occasionally exceed 108C in summer in cases with low wind speed. Although stronger and more frequent when incoming solar radiation is high, biases exceeding 88C are found even when solar radiation is less than 200 W m22. Compared with sonic thermometers, which are not affected by radiation but are too complex to be routinely used for mean temperature monitoring, commercially available aspirated shields are shown to efficiently protect thermistor measurements from solar radiation biases. Most of the available in situ reports of atmospheric temperature on the Antarctic Plateau are from automatic weather stations that use passive shields and are thus likely warm biased in the summer. In spite of low power consumption, deploying aspirated shields at remote locations in such a difficult environment may be a challenge. Bias correction formulas are not easily derived and are obviously shield dependent. On the other hand, because of a strong dependence of bias to wind speed, filtering out temperature reports for wind speed less than a given threshold (about 4–6 m s21 for the shields tested here) may be an efficient way to quality control the data, albeit at the cost of significant data loss and records that are biased toward high wind speed cases.

1. Introduction shields (e.g., Gill) tend to offer less protection to reflected upwelling than to incoming downwelling radiation Surface meteorological measurements in the Antarctic (Richardson et al. 1999). Also, in the summer, incoming are conducted at manned stations as well as via automatic solar radiation may be very high and persistent. In fact, weather stations (AWSs). Other than a few exceptions, the largest daily mean is found at high latitudes in summer almost all of the manned stations are found along the because of the high solar angle and permanent daylight. coast of the continent. Hence, automatic weather stations Cold temperatures result in comparatively low thermal provide the bulk of the observations in the interior of the emission of temperature sensors, and thus a compara- continent. Cold temperatures and high incident solar ra- tively higher sensitivity to solar radiation. Sensors that diation make the measurement of atmospheric temper- directly measure the temperature of the air, for example, ature on the Antarctic Plateau in summer particularly sonic anemothermometers (SOs), rather than a device sensitive to radiation-induced biases. This is because the that is expected to be at the same temperature as the air, surface albedo of the Antarctic snow is very high (Grenfell are supposed to be little affected by solar radiation (e.g., et al. 1994). Thus, the temperature measurements may be Barnett and Suomi 1949). However, higher absolute ac- affected not only by the downward incoming but also by the curacy is obtained and lower maintenance and energy upward-reflected solar radiation. Most common radiation supply are required by thermistors that are available at a much lower cost. Thus, thermistors are the most generally employed temperature sensors for automatic log- Corresponding author address: Christophe Genthon, Laboratoire de Glaciologie et Geóphysique de l’Envrironnement, UJF – Grenoble ging in such an environment. 1/CNRS, LGGE UMR 5183, Grenoble, F-38041, France. Unshielded thermistors are known to be affected by ra- E-mail: [email protected] diation. The World Meteorological Organization (WMO)

DOI: 10.1175/JTECH-D-11-00095.1

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TABLE 1. Various temperature sensors at Dome C, shield type, location, and elevation above surface.

Sensor Shield Location Elevation above surface Generic PT100 thermistor Aspirated Young 43502 Tower Six levels, from 3.5 to 41.9 m PT1000 thermistor in Campbell Nonaspirated Campbell URS1 Tower Two levels, 18. and 25.3 m HMP45C thermohygrometer PT100 thermistor in Vaisala Nonaspirated Campbell URS1 Tower Four levels, 3.5, 10.6, 32.7, 41.9 m, HMP155 thermohygrometer to Jan 2010; all levels then on Weed Platinum Resistance Nonaspirated AWS 3 km from tower 1.7 m Thermometer 1000 V Applied Technology SAT-SX Irrelevant Tower Six levels, from 7 to 45 m sonic thermometer (only 7-m data used here)

has evaluated various kinds of mainly passively ventilated kinds of temperature sensors are available on the tower radiation shields for robust temperature measurements in 2009: Campbell Scientific HMP45C and Vaisala (e.g., Barnett et al. 1998). Passively ventilated shields are HMP155 thermohygrometers in passively ventilated naturally ventilated by the wind. In addition, forced Campbell URS1 Gill-styled radiation shields; PT100 ventilation shields have also been used to measure tem- thermistors in fan-aspirated Young 43502 radiation perature in cold environments over snow and ice surfaces shields; and Applied Technology SAT-SX sonic ane- (e.g., Georges and Kaser 2002). However, apparently mothermometers. A PT100 thermistor is a platinum little has been done in this respect in the Antarctic envi- resistor, the resistance of which varies around 100 V de- ronment, possibly because of limited energy resources pending on the temperature. The resistance is accurately and the logistical access required to operate and maintain measured using a Wheatstone bridge. Temperature mea- ventilation. Here we present results of a comparison of surements in the HMP155 sensor are actually also made temperatures recorded at Dome C on the Antarctic Pla- with an internal PT100 resistor (PT1000 in HMP45C), so teau by temperature sensors that are housed in various the main difference between the HMP and PT100 sensors kinds of shields and those that do not need shields. Large here, besides sensor conditioning, is the kind of radiation differences are recorded between mechanically aspirated shield that is used to house the sensors. Henceforth, they and naturally ventilated shields. will be referred to as the naturally ventilated (NV) and force ventilated (FV) sensors, respectively. The accuracy of a PT100/1000 thermistor is typically 6(0.28–0.58)C de- 2. Instruments pending on temperature, but that is the temperature of the Genthon et al. (2010) analyzed the meteorological ob- sensor itself, not necessarily that of the air around. SOs servations made in the summer of 2008 along a 45-m measure the speed of sound, which depends, among other toweratDomeC,Antarctica(758069S, 1238209E, and parameters, on the temperature of the air. Various other 3233 m ASL). The temperature sensors were Campbell factors are involved including air pressure, air moisture, HMP45Cs, housed in a Campbell Scientific URS1 pas- and wind speed. While the measurement of temperature sively ventilated shield. Genthon et al. (2010) reported fluctuations with the SO may be much more accurate, the occurrences of suspiciously warm events in low wind absolute accuracy is not better than 18. On the other hand, speed conditions, which were unlikely to reflect the real the SOs directly sense the temperature of the air, not that atmosphere and were most probably due to solar radia- of an intermediate device (e.g., a piece of platinum in tion. Corrections suggested by Huwald et al. (2009) were a PT100), the energy balance of which may be affected by tested but were obviously inadequate. Such corrections radiation. No radiation shield is thus required for SO in- are most probably shield dependent and possibly even struments. site dependent. Additional sensors and new shields were The NV and FV sensors are deployed side by side at the deployed in early 2009, which now allow a systematic same levels on the tower (Fig. 1), at 3.5, 10.6, 18.0, 25.3, comparison of different kinds of shields in a large range of 32.7, and 41.9 m above the surface. Young 05106 aero- wind, temperature, solar radiation, and solar angle con- vanes are also deployed at the same levels. Both NV and ditions over more than a full year. FV sensors are factory validated to 2808C for the HMP155 Table 1 summarizes the various temperature sensors and PT100, and 2408C for the HMP45C. The Young at Dome C used in the present study. For further general 05106 aerovane clearly fails below 2508C (Genthon et al. information about the overall setting at Dome C and the 2010). However, we found that by removing the grease on tower facility there, see Genthon et al. (2010). Three the bearings, at the risk of increased wear, the aerovanes

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FIG. 2. (top) Temperature 3.5 m above surface, and (bottom) wind FIG. 1. The (right) naturally ventilated and (left) mechanically speed 41.9 m above surface, in 2009. aspirated radiation shields on the Dome C tower. work fine at lower temperatures and provide a much (BSRN; online at http://www.bsrn.awi.de/). BSRN mon- more continuous record than the SO. However, we do not itoring at Dome C is operated by the Institute of At- expect accuracy on the wind measurements to be quite mospheric Sciences and Climate (ISAC) of the Italian the factory-stated nominal one (of 60.3 m s21). Mech- National Research Council, Bologna, Italy. Figure 3 anical aspiration in the FV is done by electric fans, with shows the calculated cosine of the solar zenith angle and the airflow around the sensors being 5–10 m s21.The incoming solar radiation on a horizontal surface at the top airflow is bottom up, with the fan at the top, avoiding risk of the atmosphere at Dome C, neglecting the weak con- that a heating fan motor may affect the air temperature tribution of the earth’s eccentricity. It may be expected around the thermometer. The SOs are not quite at the that radiation biases on temperature measurements de- same elevation as the other instruments (Table 1). In the pend on both the solar intensity and solar angle resulting case of a strong inversion, the fact that the sensors are not from the contribution of reflected radiation (Richardson exactly at the same elevation should be taken into ac- et al. 1999). Obviously, both are zero during the polar count when comparing the data; however, the strong in- night, when no radiation bias is expected on temperature versions occur when incoming solar radiation is either measurements. Variability in summer is characterized by low or nil, which is not in summer. a strong diurnal cycle resulting from projection on a hori- The NV and FV sensors are interrogated every 10 s, zontal surface. Incoming radiation at the top of the at- then averaged over 30 min. The SO sampling rate is mosphere reaches 876 W m22 at midday. The atmosphere 10 Hz, and all of the data are retained for turbulence at Dome C is very clean, and in cloud-free conditions the studies. Various problems have occasionally affected the transfer of solar radiation to the surface is little affected. operation of the instruments. However, the data used In 2009, according to BSRN data, solar incoming radiation here sample an ample record of cases of radiation-induced at the surface reached 827 W m22. warm biases for a large range of incoming radiation and There are several other atmospheric temperature wind conditions. Figure 2 displays the temperature re- sensors deployed at Dome C, none of which is in forced- corded in 2009 by the lowermost FV. Figure 2 also shows ventilated shields. The oldest meteorological station at wind at the upper level. They range from 2738 to 2238C, and from 0 to 17 m s21, respectively. The sensors have been variously affected by the extreme polar conditions at Dome C, and the records are not fully continuous. In particular, the SOs are very sensitive to frost deposition that frequently occurs at Dome C. Therefore, the SO records are highly discontinuous in spite of periodic heating. The SO data here can thus only be used to validate that the FV are little, if any, affected by solar radiation. FIG. 3. Calculated incoming solar irradiation on a horizontal Downward solar radiation at the surface is available at surface at (left) the top of the atmosphere and (right) the cosine of Dome C from the Baseline Surface Radiation Network solar angle. The red curve is the 24-h running mean.

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Dome C was deployed by the Antarctic Meteorological Research Center (AMRC) at the University of Wisconsin (UW; online at http://amrc.ssec.wisc.edu) in 1980. As further topographical studies revealed that the actual top of thedomewas;50 km away, the station was moved in 1995. The overall design of the station (http://amrc.ssec.wisc.edu/ aws/) did not change, and the UW Dome C AWS is one of the oldest automatic weather stations almost continu- ously operating on the Antarctic Plateau. It is located about 3 km from the tower, and the temperature sensor is currently about 1.7 m above the surface. It is a Weed platinum resistance thermometer of 1000 V shielded from radiation by a vertical piece of aluminum tube. The tube’s inner surface is black to avoid reflecting solar radiation toward the sensor. The outer surface is Mylar coated. This shield, designed by the UW AWS project, probably efficiently protects from lateral and some downward solar radiation and allows some natural ventilation. In addition to the Mylar covering, the sensor connections at the top of the shield are connected to the shield by a nylon-threaded coupling. The shield is mounted under the boom holding all of the sensors, so the shield is also shaded at the top by the boom. The open bottom of the shield does not protect from the upward-reflected solar radiation. This is a simple design used on many of the automatic meteorological stations in Antarctica. It is thus of interest to evaluate how temperature measurements in such conditions may be radiation biased. This design was an improvement over the original shield prototyped in the original Antarctic FIG. 4. The FV (black) and NV (red) (top) temperatures and AWSbyStanfordthatwasaverysmall(;8-cm diameter) (middle) difference FV 2 NV from the end of 2009 (negative days Gill-styled shield with only three vents. Errors were are before 1 January 2010) to mid-2010. Interruptions in January– possible in the temperature readings resulting from the February are due to datalogging failures. (bottom) The histogram sensor not being insulated from the shield, which resulted of number of bias events as a function of bias range. in the thermal conduction impacting measurements. In the following section, we compare and analyze the two sensors occasionally reach 28C or more. Radiation temperature reports by the various temperature sensors on biases are obviously minimal then. Thus, differences less the Dome C tower. We also correlate the biases with tem- than 28C are not necessarily explained by radiation and perature, wind speed, solar radiation, and solar zenith an- should be considered as noise in the present study, al- gle, and calculate linear multiple correlations to test for though this is conservative in summer because thermistor a simple empirical correction. Then, periods during which accuracy is better at warmer temperature. On the other vertical mixing allows for comparing sensors at somewhat hand, even in late fall when daylight is weak, albeit not different elevations are selected to evaluate the UW AWS zero, biases that reach more than 48C occur. It appears sensor radiation biases. that it does not take large amounts of incoming solar radiation to affect the temperature measurements on the Antarctic Plateau. 3. Observed radiation biases and regression Figure 5 compares NV, FV, and SO over a period of Figure 4 shows the temperatures reported by the NV large differences between NV and FV. The SO and FV and FV sensors on the lower tower level and the differ- reports agree well, with a moderate diurnal cycle in the ence, from the last few days of 2009 to mid-2010. Differ- difference probably reflecting the fact that the SO sensors ences occasionally reach more than 108C. Temperature are off by 3–4 m with respect to the height of the other differences of more than 28C, considered well above mea- sensors. Beyond this uncertainty, Fig. 5 demonstrates that surement accuracy, occur more than 6% of the time in the FV data are much more correct than the NV mea- summer. During the polar night, differences between the surements, which are prone to radiation-induced biases.

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late spring, summer, and early fall months when incoming solar radiation is higher and biases are frequent. Cases of temperature below 2398C are left out for HMP45C reports. Finally, we only consider temperature biases above 28C as discussed above. There are more bias cases at warmer temperature but bias amplitude is not significantly related to temperature (Fig. 6a). The relation to solar radiation is not straightforward (Fig. 6b). Although more frequent and stronger at higher irradiation, large biases occur for all values of solar radiation down to less than 200 W m22. The strongest dependence is found with respect to wind speed (Fig. 6c). The largest biases occur when there is no wind, and biases for winds above 6 m s21 are insigni- FIG. 5. (top) Temperature reports at the lowest tower level from ficantly different from noise. Obviously, a very simple FV (3.5 m, black), NV (3.5 m, red), and SO (8.4 m, green) at the and straightforward way to filter out radiation biases in end of 2009; and (bottom) the difference of NV and SO with re- temperature records produced with the kind of naturally spect to FV. ventilated radiation shields used here is to cut out all reports for winds less than 4–6 m s21, the latter value Henceforth, we will refer to the differences between the being a conservative threshold. In other windier places, FV and NV reports as radiation biases. for example, the coastal regions of Antarctica (Favier Figure 6 displays relationships between radiation bias et al. 2011), this may be a very efficient way to process the and temperature, wind, solar radiation, and solar zenith data. On the Antarctic Plateau, on the other hand, this angle. The observations are filtered to concentrate on the would leave aside the larger fraction of a dataset.

FIG. 6. Scatterplots of radiation bias (8C, y axis) against (a) air temperature, (b) incoming direct 1 diffuse solar radiation at the surface, (c) wind, and (d) cosine of solar zenith angle m when diffuse is less than 10% of total solar radiation.

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Solar angle may also be expected to modulate radiation biases in the case of direct exposure because radiation shields tend to protect more efficiently from downward than from upward radiation influx (Richardson et al. 1999). In fact, by construction (Fig. 1), one would expect protection to be minimal at a given upward angle, that is, in a range of solar angles considering the scattering properties and the directional reflectivity of the snow surfaces (Warren et al. 1998). Airborne ice particles and clouds also have scattering effects on radiation transfer in the atmosphere that result in the diffusive component of the incoming solar radiation. Figure 6d shows the temperature bias with respect to solar angle when the diffuse component is less than 10% of the total incoming solar (i.e., mostly clear sky), thus concentrating on the directional effect of the direct radiation. It shows that biases are more frequent and larger when solar zenith angles get smaller, that is, when the sun is higher on the horizon; yet significant biases are found for all zenith angles. 21 Figure 7 shows the multiple dependencies of the ra- FIG. 7. Radiation bias (8C) as a function of wind speed (m s ) and incoming (top) direct and (bottom) diffuse solar radiation diation biases with respect to wind speed and incoming 22 (W m ). solar radiation. The dependence on direct and diffuse solar radiation is shown separately because the two components are anticorrelated and cover relatively (but temperature observations made in naturally ventilation not fully) separate ranges of radiation values. As might shields is unlikely to be an easy task at a place like Dome be expected, the largest biases result from a combination C. Such correction is unlikely to be an alternative to using of strong radiation exposure and low wind speed. How- force-ventilated rather than passively ventilated shields ever, similar biases occur for lesser values of diffuse ra- when possible. diation, suggesting that scattered light more efficiently Finally, Fig. 8 compares temperature reports by the penetrates the shields to affect the sensor inside. UW AWS at Dome C with the FV observations at the Although separate linear regressions would be low lower level on the tower in January 2009. The tempera- (Fig. 6), multiple dependencies suggest that multiple re- ture sensors are not quite at the same elevation above the gressions may be tested to tentatively derive an empirical surface (1.7 m for the UW AWS sensor, 3.5 m for the FV bias correction. There are few places where direct and sensor). However, convection occurs in the early after- diffuse components of solar radiation are separately noon, which efficiently mixes the lower atmosphere available or may be accurately estimated, so we only (Georgiadis et al. 2002; Genthon et al. 2010). Therefore, keep total incoming solar radiation. Multiple linear re- although some of the differences between the sensor regression between the observed radiation bias and wind, ports may be related to the building of an inversion at downwelling solar radiation at the surface, and cosine of night, they mainly reflect the radiation bias in the after- solar angle yields the parameters and statistics reported in noon. Because the terrain is very flat and homogeneous, Table 2. Considering the number of available samples comparing two sensors 3 km apart is valid. It is clear that (;5000) and values of the t test, all of the correlations are the automatic station reports are seriously warm biased in significant. However, the multiple square correlation is summer. Figure 8 also shows a similar time sample for only 0.13. Thus, the fraction of variance reconstructed by comparison in late spring when solar radiation is very low. the multilinear model in Table 2 is very low. In addition, This occasionally shows very good agreement; otherwise, a counterintuitive negative coefficient for incoming solar the automatic weather station sensors report colder radiation, which probably reflects the complex contribu- temperature, consistent with the fact that the sensor is tions of solar angle and diffuse component, illustrates the closer to the surface in an environment characterized by limit of the approach. While radiation transfer and flow increasingly strong temperature inversions. modeling both inside and around the shields would ob- The different heights of the AWS and FV temperature viously be a much more defensible approach (Richardson sensors make quantitative direct comparison difficult. et al. 1999; Lin et al. 2001), a simple regression illustrates When the FV temperature is the same within 0.58C all that deriving a robust correction for radiation biases of along the tower, the assumption is made that the air is

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TABLE 2. Parameters of multiple linear regression of bias B against wind W, incoming solar radiation at surface S and cosine of zenith solar angle A. The model is B 5 CwW 1 CsS 1 CaA 1 I.

Estimate Std error t value Cw (coefficient for wind W) 20.38Cm21 s 0.018Cm21 s23 Cs (coefficient for solar S) 21.7 1023 8CW21 m2 2. 1024 8CW21 m2 8 Ca (coefficient for cosine of angle A) 11.98C 0.28C12 Intercept I 12.78C 0.18C28 well mixed by turbulence and/or convection, and the air effects of differences of sensors height above the surface, temperature should be the same all the way down to the it is found that the AWS data are significantly warm bi- level of the automatic weather station. Only then can the ased as much as 80% of the time in low-speed winds AWS reports be directly compared to the FV reports on during austral summer. AWSs are or have been operating the tower to evaluate the radiation bias of the AWS mea- at more than 100 sites in Antarctica. The reports are passed surements. A warm bias above 28C occurs 80% of the time on to the Global Telecommunication System (GTS) to be in such a situation. A bias above 58C occurs 47% of the potentially operationally used by weather analysis and time at the AWS station, but only 11% of the time on the forecasting services worldwide and to produce reanalyses. tower for the NV measurements. The naturally ventilated Obviously, there is no solar radiation bias when there is no Gill-styled shields on the tower, although clearly inefficient radiation, so such biases are expected to be nil in winter. In to avoid all of the radiation biases, thus do a better job than addition, Antarctica is a windy place. Winds are weak on the tubes that are used on the AWS. Other AWS stations theplateau,particularlyatthesummitofadomewhere in similar situations are likely to be similarly affected there is no locally produced katabatic flow. Thus, one may elsewhere. expect that many of the automatic weather stations and, more generally, many of the observations of atmospheric temperature in Antarctica are less affected by radiation 4. Final remarks and conclusions biases than shown here because they are made in winter Atmospheric temperature measurements with therm- and/or in windier places. However, one should be aware of istors housed in naturally ventilated radiation shields are potentially strong biases when using available temperature affected by solar radiation biases on the Antarctic Pla- reports in Antarctica. teau. The mean difference between naturally ventilated Obviously, existing and future meteorological systems and aspirated measurements over the period of analysis in Antarctica should be adapted and planned in ways here, excluding winter (with solar radiation less than that avoid radiation biases on temperature measure- 100 W m22) and cases of recorded differences less than ments. The use of forced ventilation is customary in other 28C (to retain only unambiguously significant biases), is regions of the world, but not in Antarctica, most obviously 3.58C. The difference occasionally reaches more than because the logistical and environmental constraints 108C. These are very significant biases that are bound to affect the use of such observations to analyze the local meteorology, build a climatology, or validate meteorological and climate models. It is thus strongly recom- mended that either sensors that are insensitive to solar radiation, for example, calibrated sonic thermometers, or sensors that are housed in radiation shields with forced ventilation are used in such environment. To our knowledge, very few of the meteorological systems that monitor the meteorology and climate of Antarctica use such devices. At Dome C, the only forced-ventilated systems are those cited in this work. The automatic weather stations, an invaluable source of meteorological and climate data over the last 30 yr in an otherwise largely data-void region, also use naturally ventilated shields. When comparing the reports from the FIG. 8. Samples of automatic weather station (black, 170 cm Dome C AWSs with the tower data when convective above surface) and FV (red, 3.5 m above surface) atmospheric instability mixes the lower atmosphere, thus avoiding the temperatures in (top) Jan and (bottom) May 2009.

Unauthenticated | Downloaded 09/27/21 10:13 PM UTC DECEMBER 2011 G E N T H O N E T A L . 1605 make it more difficult to implement ventilation. His- polar institutes is acknowledged. This is a contribution torically power available to an AWS prohibited the to the CNES/INSU CONCORDIASI IPY Project. Me- deployment of such devices. The results presented here teorological data are obtained as part of the OSUG- show that commercially aspirated radiation shields exist CENECLAM observatory. We thank ISAC for access to that do a good job at minimizing the radiation bias. Two the solar radiation data distributed by BSRN (http:// years of operation have shown that the Young 43502 can www.bsrn.awi.de/). Some of this material is based upon withstand extreme conditions, such as on the Antarctica work partially supported by the U.S. NSF-OPP under Plateau, reasonably well. However, this is next to a per- Grant ANT-0944018. manently manned Antarctic station, so power is not an issue. 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