1. Adjustment of Global Gridded Precipitation for Systematic Bias Jennifer Adam Department of Civil and Environmental Engineering University of Washington

3. Motivation Systematic bias results in a net underestimation of precipitation Systematic bias results in a net underestimation of precipitation Most global precipitation products are not adjusted for systematic bias Most global precipitation products are not adjusted for systematic bias Model runs forced with unadjusted precipitation estimates will not accurately perform a water balance Model runs forced with unadjusted precipitation estimates will not accurately perform a water balance

4. Wind-Induced Undercatch Snow: 10 to >50% Rain: 2 to 10% Wetting Losses 2 to 10% Evaporation Losses 0 to 4% Treatment of Trace Precipitation as Zero Significant in Cold Arid Regions Splash-out and splash-in 1 to 2% Blowing and Drifting Snow ?? Sevruk, 1982

5. Wind-Induced Undercatch Influencing Factors: Influencing Factors: –Wind speed –Temperature –Gauge type –Gauge height –Windshield –Exposure Nespor and Sevruk, 1999

6. Precipitation Gauges of the World ~50 types of National Standard gauges ~50 types of National Standard gauges Sevruk et al., 1989 Sevruk et al., 1989

7. 1998 World Meteorological Organization (WMO) Solid Precipitation Measurement Intercomparison (Goodison et al. 1998) Goals: Goals: –Introduce reference method for gauge calibration –Derive standard method to adjust for wind-induced solid precipitation undercatch CATCH RATIO (CR) = Measured Precipitation True Precipitation True Precipitation

9. WMO Intercomparison Results

10. Objective To improve gridded precipitation data used to force large-scale hydrology models To improve gridded precipitation data used to force large-scale hydrology models

11. Methodology Overview Create mean monthly “catch ratios” gridded ½˚ by ½˚ globally Create mean monthly “catch ratios” gridded ½˚ by ½˚ globally Apply to existing gridded precipitation products (time-series or climatologies) during the period of 1979 through 1998 Apply to existing gridded precipitation products (time-series or climatologies) during the period of 1979 through 1998

12. Step 1: Selection of Correction Domain Wind-Induced Solid Precipitation Undercatch: Wind-Induced Solid Precipitation Undercatch: –Countries that experience >½ of precipitation as snow during the coldest month of the year. –30 countries in the Northern Hemisphere were selected Wind-Induced Liquid Precipitation Undercatch: Worldwide Wind-Induced Liquid Precipitation Undercatch: Worldwide Wetting Losses: Worldwide Wetting Losses: Worldwide

13. NOAA CPC Summary of day Stations (NCAR) NOAA CPC Summary of day Stations (NCAR) 1994 through 1998 daily data 1994 through 1998 daily data Coincident P, T max, T min, Wind Speed measurements Coincident P, T max, T min, Wind Speed measurements 7,878 stations were used (4,647 for snow analysis) 7,878 stations were used (4,647 for snow analysis) Step 2: Choose Meteorological Stations

14. + Step 3: Wind-Induced Solid Precipitation Undercatch Apply on a daily basis Apply on a daily basis Assume gauge type and height per country Assume gauge type and height per country

15. + Step 4: Wind-Induced Liquid Precipitation Undercatch (Legates, 1987) e.g. κ r = 1.0 + 0.011 μ 2 w hp 2 e.g. κ r = 1.0 + 0.011 μ 2 w hp 2 Apply on a monthly basis Apply on a monthly basis Step 5: Wetting Losses (Legates, 1987) Assume one measurement per day at each station Assume one measurement per day at each station 0.02 < ΔP wr < 0.30 mm/day 0.02 < ΔP wr < 0.30 mm/day ΔP ws = ½ ΔP wr ΔP ws = ½ ΔP wr

16. + Liquid Solid Step 6: Apply Bias Adjustment Model

17. Step 7: Determine Mean Monthly Catch Ratios for each station Step 8: Interpolate Catch Ratios to ½ ° x ½ ° globally to ½ ° x ½ ° globally Step 9: Apply to an existing Gridded Precipitation Product Mean Monthly Observed Mean Monthly Adjusted

18. Canada Unique Precipitation Gauge Network Unique Precipitation Gauge Network –Liquid Precipitation: AES Type B –Solid Precipitation: ~125 Nipher Gauges ~2500 Snow Ruler Stations Previous Bias Adjustment Efforts over Canada Previous Bias Adjustment Efforts over Canada –Groisman (1998) –Mekis and Hogg (1999)

19. 6,692 stations 6,692 stations Monthly analysis Monthly analysis Assumed CR = 90% Assumed CR = 90% 495 stations 495 stations Daily analysis Daily analysis Utilized WMO Results Utilized WMO Results

20. Groisman ÷ Mekis and Hogg (1979 – 1990) Ratios applied to Groisman station data Ratios applied to Groisman station data Mean Monthly Catch Ratios calculated Mean Monthly Catch Ratios calculated

21. Results

22. Gridded Catch Ratios Catch Ratio (%)

23. Adjusted Gridded Precipitation Catch Ratios Applied to Willmott and Matsuura (2001) Monthly Time-Series from 1979 through 1998 Catch Ratios Applied to Willmott and Matsuura (2001) Monthly Time-Series from 1979 through 1998 Precipitation (mm/month)

24. Adjustment Effects Global Mean Annual Increase of 11.2% Global Mean Annual Increase of 11.2% All Adjustments Wind-Induced Snow Undercatch Wetting LossesWind-Induced Rain Undercatch

25. Global Dataset Comparisons

26. Summary Adjusts existing gridded precipitation products for wind-induced undercatch and wetting losses on a mean monthly basis Adjusts existing gridded precipitation products for wind-induced undercatch and wetting losses on a mean monthly basis Effort focused on snow-dominated regions and solid precipitation undercatch Effort focused on snow-dominated regions and solid precipitation undercatch Utilizes the recent WMO Solid Precipitation Measurement Intercomparison results Utilizes the recent WMO Solid Precipitation Measurement Intercomparison results

27. Acknowledgements: Dennis Lettenmaier, Steve Burges, Bart Nijssen and the Land Surface Hydrology Research Group Supported by NASA grant NAG5-9416 to the University of Washington.

28. Questions?

29. Limitations in Methodology

30. Wind-Induced Undercatch Gauge Representation Gauge Representation –Gauge type or shield uniform over country –Gauge height uniform, wind sensor height at 10 m Regression Equation Application Regression Equation Application –N and r 2 –Equation developed for what gauge?

31. Scoring System – Solid Precipitation

32. Data Set Comparisons

33. Comparison Against Yang et al. Greenland: Yang 2.5% lower (wind sensor height, rain undercatch eqn.) Greenland: Yang 2.5% lower (wind sensor height, rain undercatch eqn.) Siberia: Yang 1.6% lower (rain undercatch eqn.) Siberia: Yang 1.6% lower (rain undercatch eqn.) Alaska: Yang 3.5% lower (shielding,gauge height, wind sensor height, rain undercatch eqn.) Alaska: Yang 3.5% lower (shielding,gauge height, wind sensor height, rain undercatch eqn.)

34. Gridded Global Dataset Comparisons Adjusted Willmott 2001Legates1987 Original Willmott 2001 CRU 0.5 2000 GPCC 1994 Series1979-1998Climatol.1920-1980Series1950-1999Series1901-1998Climatol.1961-1990 Bias- Adjusted NoAdjustmentAttemptedNoAdjustmentAttemptedNoAdjustmentAttempted

35. Legates (1987) Global Precipitation Product ½° by ½° monthly precipitation 1920-1980 climatology (global land areas) ½° by ½° monthly precipitation 1920-1980 climatology (global land areas) Accounts for: Accounts for: –Wind-Induced Undercatch (Liquid and Solid) –Wetting Losses –Evaporation Losses Adjustments determined from mean monthly meteorological data Adjustments determined from mean monthly meteorological data

36. Mean Annual Precipitation Vs. Latitude

37. Determined Catch Ratio (CR) Regression Equations for the most common National Standard Precipitation Gauges Determined Catch Ratio (CR) Regression Equations for the most common National Standard Precipitation Gauges –Hellmann, US NWS 8”, Tretyakov, Nipher, others CATCH RATIO (CR) = Measured Precipitation True Precipitation True Precipitation Accounts for Wind-Induced Undercatch of Soliid Precipitation Accounts for Wind-Induced Undercatch of Soliid Precipitation WMO Intercomparison Results

38. Double-Fenced International Reference (DFIR) Encloses the Shielded Tretyakov Gauge Encloses the Shielded Tretyakov Gauge UCAR

39. Wetting Losses Influencing Factors: Influencing Factors: –Gauge type –Climate –Measurement Methodology

40. Evaporation Losses Influencing Factors: Influencing Factors: –Gauge type –Climate –Measurement Methodology

41. Wind-Induced Undercatch Wetting Losses Evaporation Losses Adjusted Precipitation Gauge-Measured Precipitation Sevruk, 1982

42. + Liquid Solid Legates, 1987

43. + Evaporation Losses Ignored Evaporation Losses Ignored

44. + 1 CR s Use “Catch Ratio” for Solid Precipitation Use “Catch Ratio” for Solid Precipitation

45. Overview of Project Create mean monthly “catch ratios” gridded ½˚ by ½˚ globally Create mean monthly “catch ratios” gridded ½˚ by ½˚ globally Apply to existing gridded precipitation products (time-series or climatologies) during the period of 1979 through 1998 Apply to existing gridded precipitation products (time-series or climatologies) during the period of 1979 through 1998