Adjustment of Global Gridded Adjustment of Global Gridded Precipitation for Systematic Precipitation for Systematic
BiasBias
Jennifer AdamJennifer Adam
Department of Civil and Department of Civil and Environmental EngineeringEnvironmental EngineeringUniversity of WashingtonUniversity of Washington
MotivationMotivation
• Systematic bias results in a net Systematic bias results in a net underestimation of precipitationunderestimation of precipitation
• Most global precipitation products Most global precipitation products are not adjusted for systematic biasare not adjusted for systematic bias
• Model runs forced with unadjusted Model runs forced with unadjusted precipitation estimates will not precipitation estimates will not accurately perform a water balanceaccurately perform a water balance
Wind-Induced Wind-Induced Undercatch Undercatch
Snow: 10 to >50%Snow: 10 to >50%
Rain: 2 to 10%Rain: 2 to 10%
Wetting LossesWetting Losses 2 to 10%2 to 10%
Evaporation LossesEvaporation Losses 0 to 4%0 to 4%
Treatment of Trace Treatment of Trace Precipitation as ZeroPrecipitation as Zero
Significant in Cold Significant in Cold Arid RegionsArid Regions
Splash-out and Splash-out and splash-insplash-in 1 to 2%1 to 2%
Blowing and Drifting Blowing and Drifting SnowSnow ????
Sevruk, 1982
Wind-Induced UndercatchWind-Induced Undercatch
• Influencing Influencing Factors:Factors:– Wind speedWind speed– TemperatureTemperature– Gauge typeGauge type– Gauge heightGauge height– WindshieldWindshield– ExposureExposure
Nespor and Sevruk, 1999
Precipitation Gauges of the Precipitation Gauges of the WorldWorld
• ~50 types of ~50 types of National National Standard Standard gauges gauges
Sevruk et al., 1989Sevruk et al., 1989
1998 World Meteorological 1998 World Meteorological Organization (WMO) Solid Precipitation Organization (WMO) Solid Precipitation Measurement Intercomparison Measurement Intercomparison (Goodison et al. 1998)(Goodison et al. 1998)
• Goals:Goals:– Introduce reference method for gauge Introduce reference method for gauge
calibrationcalibration– Derive standard method to adjust for wind-Derive standard method to adjust for wind-
induced solid precipitation undercatchinduced solid precipitation undercatch
CATCH RATIO (CR) = Measured CATCH RATIO (CR) = Measured PrecipitationPrecipitation
True PrecipitationTrue Precipitation
WMO Intercomparison ResultsWMO Intercomparison Results
ObjectiveObjective
• To improve gridded precipitation data To improve gridded precipitation data used to force large-scale hydrology modelsused to force large-scale hydrology models
Methodology OverviewMethodology Overview
• Create mean monthly “catch ratios” Create mean monthly “catch ratios” gridded ½gridded ½˚̊ by ½ by ½˚̊ globally globally
• Apply to existing gridded precipitation Apply to existing gridded precipitation products (time-series or climatologies) products (time-series or climatologies) during the period of 1979 through 1998during the period of 1979 through 1998
Step 1:Step 1: Selection of Correction Selection of Correction DomainDomain
• Wind-Induced Solid Precipitation Undercatch:Wind-Induced Solid Precipitation Undercatch:
– Countries that experience >½ of precipitation as snow during Countries that experience >½ of precipitation as snow during the coldest month of the year.the coldest month of the year.
– 30 countries in the Northern Hemisphere were selected30 countries in the Northern Hemisphere were selected
• Wind-Induced Liquid Precipitation Undercatch: Wind-Induced Liquid Precipitation Undercatch: WorldwideWorldwide
• Wetting Losses: WorldwideWetting Losses: Worldwide
• NOAA CPC Summary of day Stations (NCAR)NOAA CPC Summary of day Stations (NCAR)• 1994 through 1998 daily data1994 through 1998 daily data• Coincident P, TCoincident P, Tmaxmax, T, Tminmin, Wind Speed measurements, 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:Step 2: Choose Meteorological Choose Meteorological StationsStations
+
Step 3:Step 3: Wind-Induced Solid Wind-Induced Solid Precipitation UndercatchPrecipitation Undercatch
• Apply on a daily basisApply on a daily basis• Assume gauge type and height per Assume gauge type and height per
countrycountry
+
Step 4:Step 4: Wind-Induced Liquid Wind-Induced Liquid Precipitation Undercatch Precipitation Undercatch (Legates, (Legates, 1987)1987)
• e.g. e.g. κκr r = 1.0 + 0.011 μ= 1.0 + 0.011 μ2 2 wwhphp22
• Apply on a monthly basisApply on a monthly basis
Step 5:Step 5: Wetting Losses Wetting Losses (Legates, 1987)(Legates, 1987)
• Assume one measurement per day at Assume one measurement per day at each stationeach station
• 0.02 < 0.02 < ΔPΔPwrwr < 0.30 mm/day < 0.30 mm/day
• ΔPΔPwsws = = ½ ½ ΔPΔPwrwr
)()1( wrgra PPRP
)( wsgs
PPCR
R
+
Liquid
Solid
Step 6:Step 6: Apply Bias Adjustment Apply Bias Adjustment ModelModel
a
gall
P
PCR
Step 7:Step 7: Determine Mean Monthly Determine Mean Monthly Catch Ratios for each stationCatch Ratios for each station
Step 8:Step 8: Interpolate Catch Ratios Interpolate Catch Ratios to to ½ ° x ½ ° globally½ ° x ½ ° globally
Step 9:Step 9: Apply to an existing Apply to an existing Gridded Precipitation ProductGridded Precipitation Product
Mean Monthly Observed
Mean Monthly Adjusted
CanadaCanada
• Unique Precipitation Gauge NetworkUnique Precipitation Gauge Network– Liquid Precipitation: AES Type BLiquid Precipitation: AES Type B– Solid Precipitation:Solid Precipitation:
~125 Nipher Gauges~125 Nipher Gauges
~2500 Snow Ruler Stations~2500 Snow Ruler Stations
• Previous Bias Adjustment Efforts over Previous Bias Adjustment Efforts over CanadaCanada– Groisman (1998)Groisman (1998)– Mekis and Hogg (1999)Mekis and Hogg (1999)
• 6,692 stations 6,692 stations
• Monthly analysisMonthly analysis
• Assumed CR = Assumed CR = 90%90%
• 495 stations495 stations
• Daily analysisDaily analysis
• Utilized WMO Utilized WMO ResultsResults
Groisman ÷ Mekis and Hogg (1979 Groisman ÷ Mekis and Hogg (1979 – 1990)– 1990)
• Ratios applied to Groisman station dataRatios applied to Groisman station data
• Mean Monthly Catch Ratios calculatedMean Monthly Catch Ratios calculated
ResultsResults
Gridded Catch RatiosGridded Catch Ratios
Catch Ratio (%)
Adjusted Gridded PrecipitationAdjusted Gridded Precipitation
• Catch Ratios Applied to Willmott and Matsuura Catch Ratios Applied to Willmott and Matsuura (2001) Monthly Time-Series from 1979 through 1998(2001) Monthly Time-Series from 1979 through 1998
Precipitation (mm/month)
Adjustment EffectsAdjustment Effects
• Global Global Mean Mean Annual Annual IncreasIncrease of e of 11.2%11.2%
All Adjustments
Wind-Induced SnowUndercatch
Wetting LossesWind-Induced Rain Undercatch
Global Dataset ComparisonsGlobal Dataset Comparisons
SummarySummary
• Adjusts existing gridded precipitation Adjusts existing gridded precipitation products for wind-induced undercatch and products for wind-induced undercatch and wetting losses on a mean monthly basiswetting losses on a mean monthly basis
• Effort focused on snow-dominated regions Effort focused on snow-dominated regions and solid precipitation undercatch and solid precipitation undercatch
• Utilizes the recent WMO Solid Precipitation Utilizes the recent WMO Solid Precipitation Measurement Intercomparison resultsMeasurement Intercomparison results
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.
Questions?Questions?
Limitations in MethodologyLimitations in Methodology
Wind-Induced UndercatchWind-Induced Undercatch
• Gauge RepresentationGauge Representation– Gauge type or shield uniform over Gauge type or shield uniform over
countrycountry– Gauge height uniform, wind sensor Gauge height uniform, wind sensor
height at 10 mheight at 10 m
• Regression Equation ApplicationRegression Equation Application– N and rN and r22
– Equation developed for what gauge?Equation developed for what gauge?
60 60 60 55 55 55 55 55 45
0
20
40
60
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rland
Fo
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To
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40 40 35 35 35
45 45
30 30
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20
40
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Ice
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Scoring System – Solid Scoring System – Solid PrecipitationPrecipitation
Data Set ComparisonsData Set Comparisons
Comparison Against Yang et al.Comparison Against Yang et al.
• Greenland: Yang Greenland: Yang 2.5% lower (wind 2.5% lower (wind sensor height, rain sensor height, rain undercatch eqn.)undercatch eqn.)
• Siberia: Yang 1.6% Siberia: Yang 1.6% lower (rain lower (rain undercatch eqn.)undercatch eqn.)
• Alaska: Yang 3.5% Alaska: Yang 3.5% lower lower (shielding,gauge (shielding,gauge height, wind height, wind sensor height, rain sensor height, rain undercatch eqn.)undercatch eqn.)
Gridded Global Dataset Gridded Global Dataset ComparisonsComparisons
AdjusteAdjusted d WillmottWillmott
20012001
LegatesLegates
19871987Original Original Willmott Willmott
20012001
CRU 0.5CRU 0.5
20002000GPCC GPCC 19941994
SeriesSeries
1979-1979-19981998
Climatol.Climatol.
1920-1920-19801980
SeriesSeries
1950-1950-19991999
SeriesSeries
1901-1901-19981998
Climatol.Climatol.
1961-1961-19901990
Bias-Bias-AdjustedAdjusted
Bias-Bias-AdjustedAdjusted
NoNo
AdjustmeAdjustmentnt
AttempteAttemptedd
NoNo
AdjustmeAdjustmentnt
AttempteAttemptedd
NoNo
AdjustmeAdjustmentnt
AttempteAttemptedd
Legates (1987) Global Legates (1987) Global Precipitation ProductPrecipitation Product
• ½½°° by ½ by ½°° monthly precipitation 1920-1980 monthly precipitation 1920-1980 climatology (global land areas)climatology (global land areas)
• Accounts for:Accounts for:– Wind-Induced Undercatch (Liquid and Solid)Wind-Induced Undercatch (Liquid and Solid)– Wetting LossesWetting Losses– Evaporation LossesEvaporation Losses
• Adjustments determined from mean Adjustments determined from mean monthly meteorological datamonthly meteorological data
Mean Annual Precipitation Vs. Mean Annual Precipitation Vs. LatitudeLatitude
• Determined Catch Ratio (CR) Regression Determined Catch Ratio (CR) Regression Equations for the most common National Equations for the most common National Standard Precipitation GaugesStandard Precipitation Gauges– Hellmann, US NWS 8”, Tretyakov, Nipher, othersHellmann, US NWS 8”, Tretyakov, Nipher, others
CATCH RATIO (CR) = Measured PrecipitationCATCH RATIO (CR) = Measured Precipitation True PrecipitationTrue Precipitation
• Accounts for Wind-Induced Undercatch of Accounts for Wind-Induced Undercatch of Soliid PrecipitationSoliid Precipitation
WMO Intercomparison ResultsWMO Intercomparison Results
Double-Fenced International Double-Fenced International Reference (DFIR)Reference (DFIR)
• Encloses Encloses the the Shielded Shielded TretyakoTretyakov Gaugev Gauge
UCAR
Wetting LossesWetting Losses
• Influencing Influencing Factors:Factors:– Gauge typeGauge type– ClimateClimate– MeasuremenMeasuremen
t t MethodologyMethodology
Evaporation LossesEvaporation Losses
• Influencing Influencing Factors:Factors:– Gauge typeGauge type– ClimateClimate– MeasuremeMeasureme
nt nt MethodologMethodologyy
)( ewga PPPP
Wind-Induced Undercatch
Wetting Losses
Evaporation Losses
Adjusted Precipitation
Gauge-Measured Precipitation
Sevruk, 1982
)()1( erwrgra PPPRP
)eswsgs PPPR +
Liquid
SolidLegates, 1987
)()1( erwrgra PPPRP
)( eswsgs PPPR +
• Evaporation Losses IgnoredEvaporation Losses Ignored
)()1( wrgra PPRP
)( wsgs PPR +
1
CRs
• Use “Catch Ratio” for Solid PrecipitationUse “Catch Ratio” for Solid Precipitation
Overview of ProjectOverview of Project
• Create mean monthly “catch ratios” Create mean monthly “catch ratios” gridded ½gridded ½˚̊ by ½ by ½˚̊ globally globally
• Apply to existing gridded precipitation Apply to existing gridded precipitation products (time-series or climatologies) products (time-series or climatologies) during the period of 1979 through 1998during the period of 1979 through 1998