1. NCAR - Atmospheric Technology Division Reporting work by: Erik Miller (ATD) Junhong Wong (ATD) Ari Paukkunen et al. (Vaisala, OY) Funded by: ATD, Vaisala; NSF/ATM, NOAA/OGP Presented by: David Carlson and Hal Cole (NCAR/ATD) Dry Bias in Global Radiosonde Humidity Data: Cause, Correction, and Consequences

2. NCAR - Atmospheric Technology Division Outline The Humicap Sensor Problems with TOGA COARE data Developing a correction algorithm Corrected TOGA COARE data Consequences Vaisala changes ATD data sets US data Global data Summary A very good humidity sensor if clean Global data sets probably have a dry bias

3. NCAR - Atmospheric Technology Division Vaisala Humicap Sensors A-type: older H-type: newer

4. NCAR - Atmospheric Technology Division WMO Radiosonde Reporting Sites

5. NCAR - Atmospheric Technology Division TOGA COARE Region

6. NCAR - Atmospheric Technology Division Uncorrected COARE Sounding SURFACE CONDITIONS: TEMPERATURE = 28.4 C SURFACE DEW POINT = 24.4 C SONDE DEW POINT = 22.5 C SURFACE RH = 78.8% SONDE RH = 69.6% DELTA RH = 9.2% Temperature Dew Point Dry mixed layer Dry cloud layers

7. NCAR - Atmospheric Technology Division Expected moisture profile, tropical conditions ~1.25 g/kg

8. NCAR - Atmospheric Technology Division Surface vs. sonde mixing ratios. Kavieng, daytime soundings, Jan 93 1:1 -1.25 g/kg

9. NCAR - Atmospheric Technology Division COARE - Surface to Mixed Layer Comparison VIZ & other sondes Vaisala A- type sondes Vaisala H- type sondes

10. NCAR - Atmospheric Technology Division VAISALA Test Program Sondes of various ages: 1990 - NIST 1992 - COARE 1995 - 1998 production sondes Saturation tests at cold temperatures 10-day rechecks Baseline checks Calibration checks Decontamination tests New sondesExposure tests Basic calibration Hysteresis test Short-term drift Bake @ 68 C Case materials Plastic parts Electronic parts Temperature dependence test

11. NCAR - Atmospheric Technology Division Relative Humidity Temperature Dependency Model @ 20 deg C Temperature Dependency Correction Sensor Arm Heating (Daytime) Correction Age ? ???Age Correction Ambient Temp Adjustment (Temp Dependency Model) Corrected relative humidity No Yes No Yes Prototype ATD/Vaisala Correction Algorithm Calibration Correction Surface Reference Adjustment Contamination Correction Any Surface or Pre- launch Data? Correction at Calibration Temp

12. NCAR - Atmospheric Technology Division Correction applied to Kavieng Sounding January 5, 1993 2300Z Temperature Dew Point ‘Acceptable’ mixed layer Saturated cloud layer

13. NCAR - Atmospheric Technology Division Corrected (blue) vs. uncorrected (yellow) mixing ratios. Daytime soundings, Kavieng, Jan 93

14. NCAR - Atmospheric Technology Division SURFACE CONDITIONS: TEMPERATURE = 13.3 C DEW POINT = 13.3 C OBS: Fog, drizzle Another correction: Denver, 1997 Temperature Dew Point

15. NCAR - Atmospheric Technology Division Statistical Summary of COARE Corrections Note: ATD-operated sites only, H-type sondes only, ~4200 soundings.

16. NCAR - Atmospheric Technology Division Long-term Issues Vaisala will fix the contamination problem The H-type sonde has very good performance characteristics Many ATD data sets may need correction COARE: 4200 done, 8000 to go US data may have a geographic bias Reanalysis efforts need an automated reliable correction algorithm All users need an improved error analysis of radiosonde humidity data, corrected and uncorrected ATD should build a sonde sensor test system

17. NCAR - Atmospheric Technology Division Vaisala Corrective Actions 1. New desiccant Clay-type drying agent in foil bag replaced with a mixture of silica gel and activated carbon in August 1998. Vaisala tests show contamination effects reduced by > 50%. 2. Protective sensor arm cover Protective sensor arm cover under development should completely isolate sensor from contaminants. Cover with desiccant should eliminate contamination. Available in September 1999.

18. NCAR - Atmospheric Technology Division Humicap performance on the GPS Dropsonde Temperature Relative Humidity

19. NCAR - Atmospheric Technology Division H-Sondes in recent ATD Field Projects

20. NCAR - Atmospheric Technology Division US Upper Air Sites VIZ: +4 Stations in Alaska Vaisala: + 10 Stations in Alaska

21. NCAR - Atmospheric Technology Division ATD Dropsonde + NSSL Glidersonde = Reference Radiosonde? GPS Receiver MC68HC12 MICRO GPS Antenna Refsonde Interface 400 MHz Xmitter Cont Mod A/D & 16 Channel Mux Serial Interface Control 400 MHz Antenna Press Reference Temp Reference Dew Pt Reference Test Sensors 13 Extra Channels REFERENCE SONDE ELECTRONICS RS232

22. NCAR - Atmospheric Technology Division Summary & Conclusions A small but pervasive dry bias occurs in most Vaisala radiosonde data. ATD-VAISALA correction algorithms can fix the bias. Vaisala will make changes to correct the problem. If clean, the Humicap sensor provides very good data.