Evaluation of radiative effect on the measurement of the surface air temperature by thermometers using the ground-based microwave radiometer

Transcription

1 WMO Technical Conference on Meteorological and Environmental Instruments and Methods of Observation, TECO-2016 Madrid, Spain, September 2016 P3(74) Evaluation of radiative effect on the measurement of the surface air temperature by thermometers using the ground-based microwave radiometer YAMAMOTO Akira Meteorological Research Institute Japan Meteorological Agency

2 2 Summary Brightness temperature of the atmospheric radiation is examined for absolute evaluation of radiative effect on the measurement of the surface air temperature by thermometers. The data of the strongest atmospheric absorption channel of Multi-channel ground-based microwave radiometer (MWR) at low elevation angles are extrapolated to zero elevation angle. They are compared to surface air temperature by two thermometers. Estimated value fall within the range of variation by surface thermometers.

3 Air temperature has been measured by thermometers over 300 years, but a thermometer measures only a temperature of a thermometer Heat budget of a thermometer (Kondo, 1982) T s : Temperature of a thermometer T: Air Temperature 3 energy from outside sources radiation emitted by a thermometer heat exchange between air and a thermometer Even though a temperature of a thermometer is equal to that of air, a time change of temperature of a thermometer is not zero generally. R includes solar / longwave radiation, heat conduction, latent heat and artificial heat. Numerous types of thermometer screens/shields has been developed to minimize the impact of radiation. Hooke, 1665 Japan Meteorological Agency

4 Numerous types of thermometer screens/shields has been developed, but their characteristics are different for each type of them. This is a relative evaluation. 4 WMO field intercomparison of thermometer screens/shields and humidity measuring instruments: Ghardaia, Algeria, November 2008-October 2009 (Lacombe et al. 2011) There is no recognised reference system for measuring the true air temperature. not all designs of artificially ventilated screens that are available on the market are satisfactory for use as a reference. (ISO 17714:2007)

5 Brightness temperature of the atmospheric radiation is examined in this study as a candidate for measuring the reference air temperature. 5 The atmosphere is almost transparent to electromagnetic waves but they were absorbed and emitted at some absorption bands of atmosphere, and brightness (power of electromagnetic waves) is proportional to the temperature of atmosphere. The method has already been put to practical use in microwave radiometer to measure a profile of atmosphere temperature but surface air temperature. A horizontal brightness at strong absorption bands is proportional to mean air temperature by some distance, however, it is difficult to measure by a radiometer directly, because of aperture angles and antenna siderobes. B 2kT λ = B 2 B: Brightness T B : Brightness temperature k: Boltzmann constant λ: wavelength f: band width An example of weighting function. f

6 6 Target data of analysis in this study An existing observational data meet following conditions: Using an existing instrument: a ground-based microwave radiometer, Measurements at low elevation angles, Simultaneous measurement by surface thermometer. The observational data at Kochi, Japan in May 2010 operated by Japanese Cloud Seeding Experiments for Precipitation Augmentation (JCSEPA) study, which meets above conditions, are analyzed.

7 An observational site at Kochi, Japan in 2010 operated by Japanese Cloud Seeding Experiments for Precipitation Augmentation (JCSEPA) study Platinum resistance thermometer in an artificial ventilated screen (METIC TD- 500). 1.5 meters high. Kagami observational site Multi-channel ground-based microwave radiometer MP-3000A (Radiometrics Corp.). 3 meters high. 7 Rotronic S3 Temperature and Relative Humidity Sensor Meteorological Research Institute Profiler Operator s Manual The data from 11 to 31 May 2010 were analyzed. Data with one hour precipitation more than 0.5mm were excluded. (Radiometrics Corp.)

8 Horizontal transparency of from 51 to 59GHz and a weighting function at 59GHz channel calculated by LBLRTM (Clough et al. 2005) MP-3000A observational channels A Weighting function for 59GHz channel is of the order of 100 meters with a larger weight at a shorter distance meter horizontal transparency at surface A Weighting function for 59GHz channel of MP-3000A

9 9 The hourly variation of the differences of the two types of surface thermometer C - Japan Standard Time Not only the difference of properties of shields but also the difference of installed height might be cause of the variation.

10 10 Elevation scan measurements were performed at this observation for nine elevation angles It takes almost one minute to measure at these angles.

11 11 The boxplot variation of the difference between the temperature by the platinum resistance thermometer and the 59GHz brightness temperature by MWR C JST the lowest angle height 164m - The data at , , were extrapolated to angle zero.

12 12 Brightness temperature at the lowest angle represents the air temperature of the layer by 10 meters order height, so it make quite large difference especially in daytime. Brightness temperature extrapolated to angle zero is not much difference from temperature by surface thermometer within the range of variation by surface thermometers.

13 13 Extrapolation to angle zero T τ θ BB oooooo TT, θθ = BB(TT)(1 ee ττ sin θθ) Considering virtual layer with a temperature TT optical thickness τ At Elevation angle θθ = 0, Brightness temperature BB oooooo = BB(TT) is measured τ is determined to statistically ensure that BB oooooo varies linearly with ee ττ sin θθ at lower three elevation angles. τ=0.33 was obtained. BB oooooo at lower three elevation angles for each measurement were linear regressed by ee ττ sin θθ, then intercept was obtained.

14 The boxplot variation of the difference between the temperature by two types of thermometer and the 59GHz brightness temperature by MWR 14 - 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. -

15 Another candidate for measurement of reference surface temperature: Very thin wire 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 also expense problem to be solved. We introduce a very thin handmade thermocouples after Moriwaki et al. (2003) and Kurzeja (2010) that is a less expensive way, and started experimental measurement. 15

16 G Preliminary Result of temperature measurement by very thin thermocouples 16 T 200 -T 13 1cm 13μm wire T 25 -T 13 G Temperature differences between by 13, 25 and 200 μm radius thermocouples (T 13, T 25 and T 200 ) with global solar radiation G from to 1739 Japan Standard Time (JST) 25 to 1557JST 27 June 2016 at Tsukuba, Japan.

17 17 Acknowledgments The observation at Kagami observation site was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan under the program of Special Coordination Funds for Promoting Science and Technology, Japanese Cloud Seeding Experiments for Precipitation Augmentation (JCSEPA) and the data provided by Dr. TAJIRI Takuya. The calculation using LBLRTM was supported by Dr. ISHIMOTO Hiroshi.

18 References Clough, S. A., M. W. Shephard, E. J. Mlawer, J. S. Delamere, M. J. Iacono, K. Cady- Pereira, S. Boukabara, and P. D. Brown, 2005: Atmospheric radiative transfer modeling: A summary of the AER codes. J. Quant. Spectrosc. Radiat. Transf., 91, Hooke R. 1665: Micrographia. 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. 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 World Meteorological Organization, 106 pp. Kondo J. 1982: Taiki kyokaiso no Kagaku (Science of Atmospheric Boundary Layer) (in Japanese), Tokyo do shuppan, 290 pp. Kurzeja, R., 2010: Accurate temperature measurements in a naturally-aspirated radiation shield. Boundary-Layer Meteorol., 134, Moriwaki, R., M. KANDA, and Y. Kimoto, 2003: Dependencies of Profiles of Turbulent Fluxes on Atmospheric Stability in an Urban Surface Layer (in Japanese). Proc. Hydraul. Eng., 47,