1 Review of WMO test results on the accuracy of radiosonde relative humidity sensors John Nash Observing Methods Technology Centre, Development, Met Office Expert Team on Upper Air System Comparisons, Agenda Item 3.1

2 Outline Introduction to radiosonde relative humidity sensors Test methods Statistical results Summary

3 Sensors in use Goldbeaters skin [ Russia, China] Carbon hygristor[USA (part), India, China?] RS80 Humicap A or H [50 per cent of global network] RS90 Humicap [ Finland +??] Meteolabor “Snow white” [working reference?]

4 Challenge of radiosonde water vapour measurements (1) Very large range of saturation vapour pressure with temperature: 20 deg C 23 hPa -20 deg C 1.2 hPa -50 deg C 0.04 hPa -80 deg C hPa

5 Challenge of radiosonde water vapour measurements (2) If temperature of relative humidity sensor is 1 deg C higher than ambient temperature, the following errors in true relative humidity for ambient temperature result at 20 deg C -6 per cent w.r.t.water at -20 deg C -7 per cent w.r.t water at -50 deg C -7 per cent w.r.t. water

6 Simultaneous comparison between GBS (green), Hygristor (purple) and A- Humicap(blue), PREFRS, Crawley 1992 Height [km] Relative Humidity

7 Height [km] Relative Humidity

8 Height [km] Relative Humidity

9 Height [km] RS80 H-Humicap

10 Estimates of radiosonde systematic bias from Kazkhstan, 1989 and Wallops Island, Va. 1995

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13 Summary(I) Goldbeater’s skin Random error in measurements about 8-12 per cent, Systematic errors 10 to 15 per cent at high and low humidity Poor response at temperatures lower than -30 °C Sippican Hygristor Random error in measurements can be as good as 3 to 5 per cent at high humidity and early in flight. Sensors unstable when exposed to high humidity especially in low cloud Measurements are of poor reproducibility below 25 per cent R.H. Response unreliable at temperatures lower than about -40°C

14 Summary (2) Vaisala Humicap -several variants with different sensor properties In UK, with H-Humicap systematic errors and reproducibility at high and low humidity in the lower and middle troposphere are between 3 and 5 per cent. Significant dry bias is rare, because radiosondes are used before chemical contamination becomes an issue. This is not true in many areas of the world where chemical contamination continues to produce a dry bias of up to 10 per cent at high humidity.

15 Summary (3) RS80-H and RS90 have calibrations that are probably within 10 per cent at temperatures lower than -40 °C at night, but A- Humicap has much larger low bias in upper cloud. RS90 sensors respond down to pressures of at least 100 hPa and temperatures lower than -70 °C Day time heating of the sensors needs to be compensated to get the full benefit of improved manufacturing calibration facility

16 New generation radiosondes should have sensors capable of meeting the stated user requirement of 5 per cent R.H. accuracy for the first time in most of the troposphere