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. 2 Contents 1.Introduction Background Objectives 2.Methods Site Instruments Calibration QA/QC 3.Results 4.Conclusions

3. 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. 4 Methods - 1 Site  Ghardaïa, in central Algeria Instruments  29 screens/shields oartificially-ventilated: 7 different models onaturally-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 1 st of Nov., 2008 to the 1 st of Nov., 2009

5. 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. 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 oMore than 95% available valid data  The analysis was made jointly by : oMétéo-France : Muriel Lacombe and Michel Leroy oONM: Djazia Bousri and Mohamed Mezred  Final report : published soon  A dedicated study for each model of instrument is available

7. 7 Temperature during the intercomparison period

8. 8 Relative humidity during the intercomparison period

9. 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. 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 the reference

11. 11 Results for screens - 2 During clear days and low wind speeds  Some small naturally-ventilated screens are colder  Artificially-ventilated screens are generally warmer  Large Stevenson screens are very close to the reference

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

13. 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. 14 Results for hygrometers – 2 Periods with relative humidity lower than 20%

15. 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: oTo select references oTo study each sensor’s behaviour  Experience and specific tools were developped oDatabase systems, QC softwares, macros/codes for plots…  A similar comparison is planned in Canada ofor cold desert conditions

16. Thank you !