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
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
NCAR - Atmospheric Technology Division Vaisala Humicap Sensors A-type: older H-type: newer
NCAR - Atmospheric Technology Division WMO Radiosonde Reporting Sites
NCAR - Atmospheric Technology Division TOGA COARE Region
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
NCAR - Atmospheric Technology Division Expected moisture profile, tropical conditions ~1.25 g/kg
NCAR - Atmospheric Technology Division Surface vs. sonde mixing ratios. Kavieng, daytime soundings, Jan 93 1: g/kg
NCAR - Atmospheric Technology Division COARE - Surface to Mixed Layer Comparison VIZ & other sondes Vaisala A- type sondes Vaisala H- type sondes
NCAR - Atmospheric Technology Division VAISALA Test Program Sondes of various ages: NIST COARE 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 68 C Case materials Plastic parts Electronic parts Temperature dependence test
NCAR - Atmospheric Technology Division Relative Humidity Temperature Dependency 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
NCAR - Atmospheric Technology Division Correction applied to Kavieng Sounding January 5, Z Temperature Dew Point ‘Acceptable’ mixed layer Saturated cloud layer
NCAR - Atmospheric Technology Division Corrected (blue) vs. uncorrected (yellow) mixing ratios. Daytime soundings, Kavieng, Jan 93
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
NCAR - Atmospheric Technology Division Statistical Summary of COARE Corrections Note: ATD-operated sites only, H-type sondes only, ~4200 soundings.
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
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 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.
NCAR - Atmospheric Technology Division Humicap performance on the GPS Dropsonde Temperature Relative Humidity
NCAR - Atmospheric Technology Division H-Sondes in recent ATD Field Projects
NCAR - Atmospheric Technology Division US Upper Air Sites VIZ: +4 Stations in Alaska Vaisala: + 10 Stations in Alaska
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
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.