1598 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 28 Atmospheric Temperature Measurement Biases on the Antarctic Plateau 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 re- flected 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 gen- erally employed temperature sensors for automatic log- Corresponding author address: Christophe Genthon, Laboratoire de Glaciologie et Ge´ophysique 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 Ó 2011 American Meteorological Society Unauthenticated | Downloaded 09/27/21 10:13 PM UTC DECEMBER 2011 G E N T H O N E T A L . 1599 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 Unauthenticated | Downloaded 09/27/21 10:13 PM UTC 1600 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 28 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).