1. R. Van Malderen 1,5, E. Pottiaux 2,5, H. Brenot 3 and S. Beirle 4 R OYAL M ETEOROLOGICAL I NSTITUTE OF B ELGIUM 1 R OYAL O BSERVATORY OF B ELGIUM 2 B ELGIAN I NSTITUTE FOR S PACE A ERONOMY 3 M AX P LANCK I NSTITUTE FOR C HEMISTRY 4 S OLAR -T ERRESTRIAL C ENTRE OF E XCELLENCE 5

2. WMO CIMO TECO, Brussels, 16-18 Oct. 2012 What?inter-technique comparison between 4 different instruments measuring the same atmospheric variable Which variable?integrated water vapour (IWV) Where?world-wide, but first focus on Brussels, (50°48'N, 4°21'E, 100 m asl) as case study When? The different instruments cover different observation periods. Aims? assess the quality of the different measurements: the precision - accuracy - performance of the instruments obtain a better monitoring and understanding of the changing water vapour content in the atmosphere Introduction ROB

3. WMO CIMO TECO, Brussels, 16-18 Oct. 2012 1. Instruments and datasets 2. Case study: Brussels i.Data overview ii.Scatter plots iii.Impact of cloud cover 3. World-wide data exploitation i.Site selection ii.Scatter plot properties 4. Conclusions & Perspectives Outline ROB

4. WMO CIMO TECO, Brussels, 16-18 Oct. 2012 Radiosondes (RS) different types launched at all weather conditions CIMEL sun photometer direct sun measurements @ 940nm (and @ 675 and 870 nm for aerosol correction) clear sky only level 2 data from the AERONET website GNSS/GPS International GNSS Service (IGS) database (homog. reprocessing) at all weather conditions, always T surf and p surf are needed: ZTD  IWV GOME/SCIAMACHY/GOME-2 air mass corrected differential optical absorption spectroscopy method applied to nadir measurements around 700 nm. cloud cover is an issue Instruments & datasets World-wide exploitation Case study: Brussels Conclusions & perspectives ROB

5. WMO CIMO TECO, Brussels, 16-18 Oct. 2012  different instruments = different observation periods  scatter plots of simultaneous IWV measurements with respect to the GNSS device as reference Instruments & datasets World-wide exploitation Case study: Brussels Conclusions & perspectives ROB

6. GPS-CIMELGPS-RS9x GPS-GOME(2)/SCIA  bias with GPS ranges between -0.6 mm and +0.6 mm  best overall agreement between GPS and CIMEL sun photometer  regression slope closest to 1 for all- weather devices scatter plot  influence of cloud cover? WMO CIMO TECO, Brussels, 16-18 Oct. 2012 Instruments & datasets World-wide exploitation Case study: Brussels Conclusions & perspectives ROB

7.  cloud cover ↗  regression slopes ↘ and correlation coefficients ↘  GPS measurements incorporate contribution from clouds in directions towards satellites WMO CIMO TECO, Brussels, 16-18 Oct. 2012 GPS-CIMEL Instruments & datasets World-wide exploitation Case study: Brussels Conclusions & perspectives ROB

8.  selection of 28 sites world-wide (NH), with focus on CIMEL-GPS co-location and based on meteo data availability (GPS)! WMO CIMO TECO, Brussels, 16-18 Oct. 2012 Instruments & datasets World-wide exploitation Case study: Brussels Conclusions & perspectives ROB

9.  scatter plot properties for the 28 co-locations, ordered with increasing latitute from left to right  geographical dependency? WMO CIMO TECO, Brussels, 16-18 Oct. 2012 Instruments & datasets World-wide exploitation Case study: Brussels Conclusions & perspectives ROB

10.  similar inter-technique conclusions as for the Brussels case study! WMO CIMO TECO, Brussels, 16-18 Oct. 2012 Instruments & datasets World-wide exploitation Case study: Brussels Conclusions & perspectives ROB

11.  although originally tracing other slants/directions, very good agreement between the ground-based (2) and in-situ (1) devices  The IGS database of GPS measurements and the AERONET sun photometer measurements are very promising to be used for IWV trend analysis due to their homogeneous data reprocessing (IGS) and their regular instrument calibration (AERONET).  The weather observations bias (partly clear sky needed) in the sun photometer and GOME(2)/SCIAMACHY data series affects the comparison with all-weather devices, but what is the impact on the trends?  subject of a subsequent study WMO CIMO TECO, Brussels, 16-18 Oct. 2012 Instruments & datasets World-wide exploitation Case study: Brussels Conclusions & perspectives ROB

12. WMO CIMO TECO, Brussels, 16-18 Oct. 2012 Brussels (Belgium ) Calgary (Italy ) monthly means  although overall good agreement, small difference in trend slope between GPS and radiosonde time series (-0.15 vs. -0.45 mm/decade)  RS IWV < IGS IWV in early years: instrumentation change for RS?  Large difference in IWV trends (0.16 vs. 1.29 mm/decade) Instruments & datasets World-wide exploitation Case study: Brussels Conclusions & perspectives ROB

13. WMO CIMO TECO, Brussels, 16-18 Oct. 2012  rather consistent picture: IWV ↑, most significantly (> 0.5 mm/dec) in central Europe  trend difference in ZTD between 2 IGS stations (MADR, VILL) near Madrid (both use the same meteo station data)  Brussels? Instruments & datasets World-wide exploitation Case study: Brussels Conclusions & perspectives Summary: all European IGS stations starting in 1995/1996 ROB