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WMO Intercomparison of Thermometer Screens in Hot Desert Conditions

This report presents results of the WMO intercomparison study of thermometer screens and hygrometers in hot desert conditions. The study evaluates screen performance under high radiation, hygrometer performance in extreme temperatures, and impact of ancillary factors on sensors.

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WMO Intercomparison of Thermometer Screens in Hot Desert Conditions

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  1. RESULTS OF THE WMO INTERCOMPARISON OF THERMOMETER SCREENS/SHIELDS AND HYGROMETERS IN HOT DESERT CONDITIONS Muriel Lacombe WMO CIMO-TECO 2010, 31 August 2010

  2. Contents • Introduction • Background • Objectives • Methods • Site • Instruments • Calibration • QA/QC • Results • Conclusions

  3. Introduction • Background • Several intercomparisons of screens and hygrometers, all in temperate conditions • None in hot or cold desert conditions • Importance for climatology • Objectives • Performance of screens/shields for high radiations • Performance of hygrometers in high temperatures and very low humidity conditions • Evaluate the impact of ancillary factors on the sensors • Draft recommandations to CIMO

  4. Methods - 1 • Site • Ghardaïa, in central Algeria • Instruments • 29 screens/shields • artificially-ventilated: 7 different models • naturally-ventilated: 9 different models • 17 humidity sensors (8 different models) • 2 extra Thies ultrasonic wind sensors Most of sensors were installed in pairs. • Intercomparison period • From the 1st of Nov., 2008 to the 1st of Nov., 2009

  5. Methods - 2 • Calibration • Temperature measurements: calibrated Pt100 • Before the intercomparion, in the RIC of Trappes • On-site calibration • No calibration after the intercomparison for the moment • QA/QC • Daily check by local staff • Pictures taken once per month • Specific software to flag data according CBS recommendations

  6. Results • One full year of measurements • A large dataset although some gaps in the data • More than 500 000 minutes available for most of sensors • More than 95% available valid data • The analysis was made jointly by : • Météo-France : Muriel Lacombe and Michel Leroy • ONM: Djazia Bousri and Mohamed Mezred • Final report : published soon • A dedicated study for each model of instrument is available

  7. Temperature during the intercomparison period

  8. Relative humidity during the intercomparison period

  9. Choice of the references • Screens • Artificially-ventilated screen • The coldest during daytime • Choice: Eigenbrodt screen • Hygrometers • Thygan as first reference • Communication problems with Thygan, during the last 6 monthes • Another reference, the closest to Thygan • Choices: Thygan and Vaisala HMP45D

  10. Results for screens - 1 • General results • Nearly all small naturally-ventilated screens are warmer • Artificially-ventilated screens are not significantly colder • Large Stevenson screens are very close to thereference

  11. Results for screens - 2 • During cleardays and low wind speeds • Some small naturally-ventilated screens arecolder • Artificially-ventilated screens are generally warmer • Large Stevenson screens are very close to thereference

  12. Results for screens - 3 • Combined effect of wind and radiation

  13. Results for hygrometers - 1 • General results • 5 models gave very good results: 98% differences are within ± 3% • 2 models stayed within 4% of the reference • 1 model showed larger deviations

  14. Results for hygrometers – 2 • Periods with relative humidity lower than 20%

  15. Conclusions – What is next? • Final report : published soon • It is the first WMO intercomparison of screens/shields and hygrometers in hot desert conditions • Despite some gaps, a large dataset was collected during this intercomparison • A deep analysis was conducted: • To select references • To study each sensor’s behaviour • Experience and specific tools were developped • Database systems, QC softwares, macros/codes for plots… • A similar comparison is planned in Canada • for cold desert conditions

  16. Thank you !

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