TECO-2006, WMO, Geneva, 05.12.2006

1. Global Criteria for Tracing the Improvements of Radiosondes over the Last Decades P. Jeannet1), C. A. Bower2), B. Calpini1) 1) MeteoSwiss, Payerne 2) US NWS, Silver Spring TECO-2006, WMO, Geneva, 05.12.2006

2. Task, action, deliverables 2004: WMO-CIMO ET on UASI-1 • Task: develop performance measures to demonstrate the continuous improvement in the quality of upper-air observations. • Action: elaborate global criteria for tracing the improvements, based on previous intercomparisons and recent radiosonde development, and including remote sensing • Deliverables: IOM report on global criteria for tracing the improvements of radiosondes

3. Preliminary analysis • Following methods were considered: • (1) using the previous IOM reports on the WMO international radiosonde comparisons, • (2) comparing radiosonde measurements with ECMWF model values, • (3) elaborating first a general CIMO questionnaire to the NMHSs, or • (4) extracting numbers from the scientific literature.

4. First WMO radiosonde comparisons:WMO World Comparison of Radiosondes at Payerne, Switzerland: 1950 and 1956

5. WMO Radiosonde Comparison (Phase I) at Beaufort Park, U.K., 1984.

6. WMO Radiosonde Comparison (Phase VI) at Vacaos, Mauritius, 2005

7. using the previous IOM reports Candidate criteria: priority to a short list • Temperature: • Mean difference @10hPa or 30 hPa DAY/NIGHT time + standard deviation • Geopotential height • Mean difference @10hPa or 30 hPa DAY/NIGHT time + standard deviation • Mean difference @100hPa DAY/NIGHT time + standard deviation • Humidity • Mean difference in the temperature range -35 to -45C,… (tropospheric values only) + standard deviation • (Wind)

8. Criteria (temperature and geopotential)

9. using the previous IOM reports • Candidate criteria are based on differences between simultaneous measurements obtained with different types of radiosondes launched under the same balloon (50-100 launches during an intercomparison) • The first WMO radiosonde comparisons defined 15 pressure categories in the comparison of simultaneous measurements. The 10 hPa category considered all measurements between 8.4 and 11.9 hPa, as defined by the link sondes. The 30 hPa category was more exactly centred at 32 hPa (24.5 – 41.5). The 100 hPa category range was 84 – 119 hPa. This ensured that the statistics were relying on a sufficient number of time-paired measurements. In the more recent radiosonde comparisons, 2 km wide altitude categories were introduced instead of the previous ones. • This method represents a valuable tool for comparison over the last two decades. • Examples (Excel file and graph): systematic temperature differences @10 hPa, @ day time

10. using the previous IOM reports Systematic temperature differences at 10 hPa, day time, in degree Celsius. All individual values extracted from the IOM reports without any modification Value Fig. Number of IOM report

11. using the previous IOM reports All individual values of the previous slide, without any additional information Graph is „anonymous“

12. using the previous IOM reports • Results presented in somewhat different forms • final reports Brazil and Mauritius ! • Some intercomparisons addressing a “given” class of parameters and thus…not presenting all the necessary results. • Brazil 2001: relative humidity measurements in the tropics and performance of the GPS sondes. • No true reference sonde, but “link radiosondes”: thus only relative numbers can be extracted, but they are still somewhat related to absolute accuracy • Different sondes’ types…and additionally different data post processing (correction of the radiation error on temperature, etc.)

13. Results for geopotential altitude Bias of the geopotential altitude around 10 hPa (simultaneous measurements)

14. Results for geopotential altitude Estimated random errors of the geopotential altitude around 10 hPa (simultaneous measurements)

15. Results for temperature Bias of the temperature around 10 hPa (simultaneous measurements ) night/day time

16. Results for temperature Standard deviation of the temperature around 10 hPa (simultaneous measurements)

17. Results for temperature • Results of process analyses would bring explanations related to these improvements (Fig. from J. Nash)

18. Results for pressure Bias of the pressure around 100 hPa (simultaneous measurements)

19. Results for pressure Estimated random errors of the pressure measurements around 100 hPa (simultaneous measurements)

20. Results for humidity (see Mauritius report)

21. Conclusions: radiosonde improvements over the last 20 years • Geopotential height around 31 km (10 hPa): the largest improvements (one order of magnitude) due to GPS • Temperature: an improvement by a factor of ~3 around 31 km. • Pressure: large improvements, GPS technology is a way of improving the pressure measurement accuracy in the stratosphere • Humidity: most challenging parameter, strong deficiencies in the past, the Mauritius intercomparison documents a large improvement over any hygristor in the past. • Wind: not studied, but large improvement due to GPS.

22. Final remarks • The proposed criteria can be extracted from the IOM reports, as well as from other/further radiosondes intercomparisons. • They rely on comparisons of simultaneous (time-paired) measurements. • This method provides valuable results, but also suffers from some limitations despite the fact that the WMO intercomparisons are very carefully organized. • Remote sensing was not introduced in this study.