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

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60

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30 30

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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