Satellites are Essential for Climate Monitoring

1. On the Validation and Intercomparison of Global Water Vapor Climate Data Records (CDR’s) from Satellites John M. Forsythe and Thomas H. Vonder Haar Cooperative Institute for Research in the AtmosphereColorado State UniversityFort Collins, CO USAforsythe@cira.colostate.edu

2. Satellites are Essential for Climate Monitoring The NASA Water Vapor Project (NVAP) dataset, was originated at CIRA in the early 1990’s to employ satellites to study water vapor. Now covering 1988-2001, NVAP is a blended satellite dataset designed to provide daily, global views of layered and total water vapor. NVAP data available at http://eosweb.larc.nasa.gov/PRODOCS/nvap/table_nvap.html

3. Motivation NVAP Observations • Water vapor in the atmosphere is expected to increase with warming (~ 7 % / K; Trenberth et al). Water vapor is a key greenhouse gas. • Trends have been reported in surface and radiosonde observations (IPCC, 2001) and SSM/I over oceans (Wentz et al, 2000). • What is the error in our water vapor CDR’s? Can we detect trends? How can new sensors (Aqua, GPS) be used to refine the water vapor CDR? Wentz and Schabel (2000): + 2.1 % (~0.5 mm) / decade in Tropical Oceans (SSM/I) Ross and Elliott (2001) (radiosondes) : ~3 % / decade over N. America 1950 2000 2050 Year The NVAP (1988 – 1999) global average of TPW is 24.5 mm, with no significant trend

4. Blended satellite products are often used to measure climate variables: (e.g. ISCCP, GPCP, NVAP) Each sensor has strengths and weaknesses Multiple satellite products are blended to create the NVAP dataset. January 1, 2000 Total Precipitable Water (TPW) shown here. NVAP covers 1988 - 2001

5. Time series of NVAP Global Mean TPW, 1988 - 1999 (mm) Months from January, 1988 1988 1999 Annual Cycle ~ 10 % of Global Mean

6. Three Independent Satellite Measurements – Highly Coupled Mt. Pinatubo Eruption March 1991 Major El Nino begins May 1997 Multiple, unrelated CDR’s can reinforce each other

7. NOAA Operational TOVS seems to be biased low and to have larger amplitude than SSM/I or radiosondes

8. NVAP SSM/I Instruments Usage 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 | | | | | | | | | | | | | | | F8 11/15 F10 Every transition in this record represents a challenge for climate monitoring F11 4/99 1/00, 2/00 5/95 F13 F14 5/97 3/00 F15 1988, 1989, 1990, 1991, 1992 All months- F8 1993, 1994 All months - F10, F11 1995 All months - F10, F11, F13 starts 5/95, except: Jul - F10, F11 only 1996 All months - F10, F11, F13 1997 F10 ends 11/15, F11, F13, F14 starts May, except: Feb - F10, F11 only, Dec - F11, F14 only 1998 All months - F11, F13, F14, except: Apr - F11, F14 only, Aug - F11, F14 only 1999 F11 ends Apr, F13, F14 2000 F11 Jan and Feb, F13, F14, F15 starts March 2001 All months – F13, F14, F15

9. NVAP Mode (Most Common) Data Source Changes Through Time 8 = Radiosonde data only 7 = TOVS and SSM/I combination 6 = SSM/I only 5 = SSM/I interpolated / TOVS combination 4 = SSM/I interpolated 3 = TOVS only 2 = Space interpolated-filled 1 = Time interpolated-filled 0 = Missing data 1996 1992 A reanalysis of NVAP is needed to reduce time-dependent biases 1988 - 1999 Dominated by SSM/I over ocean More TOVS soundings produced by NESDIS after 1992

10. Ten GCOS Satellite Climate Monitoring Principles (from Tom Karl, NCDC) These have not been achieved for the water vapor CDR Minimize orbit drift Ensure sufficient overlap Replace prior to failure Rigorous pre-launch calibration Adequate on-board calibration Continue baseline instrument observations on decommissioned satellites Operational production of priority climate products Facilitate access to products, metadata, and raw data Need in situ baseline observations Real-time monitoring of observing system performance 1 7 2 8 3 9 4 10 5 The Unwritten Principle 6 Use of multiple observing systems and multiple analysis teams (for the same variable) 11

11. Conclusions • The blended NVAP global TPW anomalies have no significant trend from 1988 – 1999. • The operational TOVS record is discontinuous and has a downward trend. SSM/I and radiosonde show upwards global trends in TPW. • There are some significant regional trends. Whether these are real climate trends or instrument & algorithm effects requires further study and NVAP reanalysis. A climate dataset must be initially inspected for time-dependent biases, then it can be reanalyzed to sharpen it’s ability to detect trends.

12. Work in Progress at CIRA - • “Science Stewardship of Thematic Climate Data Records: • A Pilot Study with Global Water Vapor” • Supported by NESDIS/ORA • Problem: Aqua water vapor products became available in mid-2002, but NVAP only covers through 2001. Aqua is the most capable water vapor-sensing spacecraft flown. How does Aqua compare to NVAP? GPS? • Solution: Create ~ 6 months of NVAP from 2003-2004 using heritage data and algorithms and compare to Aqua (AIRS/HSB, AMSR) and GPS. • Expect to submit a journal paper with these results in December Goal: Demonstrate scientific stewardship (Bates, 2004, AMS Satellite Conf.) applied to the water vapor CDR.

13. Backup Slides

14. 1.9 % / decade Wentz and Schabel, Nature, Jan. 27, 2000 2.1 % / decade 1.0 % / decade

15. A similar trend analysis of NVAP is in progress at CIRA Figure 2.26: Trends in annual mean surface water vapour pressure, 1975 to 1995, expressed as a percentage of the 1975 to 1995 mean. Areas without dots have no data. Blue shaded areas have nominally significant increasing trends and brown shaded areas have significant decreasing trends, both at the 5% significance level. Biases in these data have been little studied so the level of significance may be overstated. From New et al. (2000). [IPCC 2001].