Evaluation of Some Candidates for Measuring the Reference Air Temperature Including Very Thin Thermocouples and Ground-Based Microwave Radiometer

Effect due to solar radiation is unavoidable in surface air temperature measurement by thermometer. Although thermometer screen/shield is used to reduce this effect, impacts due to radiation from screen/shield itself and heat conduction from it remain. The characteristics of screen/shield are different for each type of them. It is considered that reference temperature measurements have little effect due to radiation in principle are effective to evaluate these characteristics. Very thin resistive wire (ISO, 2007) and ultrasonic anemometer and thermometer (Lacombe et al. 2011) have been proposed as candidates for it
ISO 17714:2007 suggests a very thin resistive wire as a potential candidate for measuring the reference air temperature. We have many technical issues to be solved to achieve the practical use of this technique, and cost must be reduced. We introduce a very thin handmade thermocouple after Moriwaki et al. (2003) and Kurzeja (2010). It is able to test thermocouples in several diameters saving expenses. Thermocouples with 0.013mm, 0.025mm, and 0.2mm diameter have been examined in this study. Comparing the difference of the data from the three diameter thermocouples and referring the theoretical analysis by Kurzeja (2010), if wind speed is over 1 ms^-1, radiation effect of 0.013mm diameter thermocouples might be less than 0.1K.
Brightness temperature of the atmospheric radiation as another candidate is examined. The observational data of multi-channel ground-based microwave radiometer MP-3000A (Radiometrics) are analyzed. The brightness temperature of the strongest atmospheric absorption channel (58.8GHz) at low elevation angles are extrapolated to zero elevation angle. They are compared to surface air temperature measured by two thermometers: the platinum resistance thermometer in the artificial ventilated screen and temperature and relative humidity sensor in the air-inlet of MP-3000A with an artificial ventilation. The distribution of differences shows the least change in the night time and quite large change in daytime with the maximum median around noon. It is plausible explanation that radiative characteristics of the instruments is possible cause of the diurnal change of differences.
The other candidate such as ultrasonic anemometer will be evaluated.
Moreover, the intercomparison between candidate methods whose effect due to radiation is minimal in principle is necessary.

References
Kurzeja, R., 2010: Accurate temperature measurements in a naturally-aspirated radiation shield. Boundary-Layer Meteorol., 134, 181–193.
Lacombe, M., D. Bousri, M. Leroy, and M. Mezred, 2011: WMO field intercomparison of thermometer screens/shields and humidity measuring instruments: Ghardaia, Algeria, November 2008-October 2009. World Meteorological Organization, 106 pp.
International Organization for Standardization, 2007: ISO 17714:2007 Meteorology - Air temperature measurements - Test methods for comparing the performance of thermometer shields/screens and defining important characteristics. 19 pp.
Moriwaki, R., M. Kanda, and Y. Kimoto, 2003: Dependencies of Profiles of Turbulent Fluxes on Atmospheric Stability in an Urban Surface Layer. Proc. Hydraul. Eng., 47, 1–6 (in Japanese).