THE USE OF DUAL-POLARIMETRIC RADAR THE USE OF DUAL-POLARIMETRIC RADAR DATA TO IMPROVE RAINFALLDATA TO IMPROVE RAINFALL

ESTIMATION ACROSS THE ESTIMATION ACROSS THE TENNESSEE RIVER VALLEYTENNESSEE RIVER VALLEY

W.A. PetersenW.A. PetersenNASA – Marshall Space Flight Center, Huntsville, ALNASA – Marshall Space Flight Center, Huntsville, AL

P. N. Gatlin, L. D. CareyP. N. Gatlin, L. D. CareyUniversity of Alabama in Huntsville – University of Alabama in Huntsville –

Earth Systems Science Center, Huntsville, ALEarth Systems Science Center, Huntsville, AL

S. R. JacksS. R. JacksTennessee Valley Authority, Knoxville, TNTennessee Valley Authority, Knoxville, TN

MotivationMotivation

Reduction of costs associated with Reduction of costs associated with maintenance of large rain gauge networkmaintenance of large rain gauge network

Provide a custom-tailored rainfall product Provide a custom-tailored rainfall product specific to the end-user’s needsspecific to the end-user’s needs

Independent validation of ARMOR rain Independent validation of ARMOR rain rate algorithmsrate algorithms

Ground-validation for TRMM satellite Ground-validation for TRMM satellite measurementsmeasurements

Tennessee River WatershedTennessee River Watershed

AL

MS

TN

GA

KY

SC

NC

112 sub-basins 112 sub-basins 1840 km1840 km22

189 rain gauges 189 rain gauges maintained by maintained by TVATVA

11 sub-basins within 100 km of the ARMOR dual-pol. radar

AAdvanced dvanced RRadar for adar for MMeteorological & eteorological & OOperational perational RResearchesearch

Location: Location: Huntsville International Airport, Huntsville, AL Huntsville International Airport, Huntsville, AL

(Altitude 206m)(Altitude 206m) C-band dual-polarimetric Doppler radarC-band dual-polarimetric Doppler radar Simultaneous transmit and receive of H, VSimultaneous transmit and receive of H, V Variables: Z, V, W, ZDR, Variables: Z, V, W, ZDR, ΦΦDPDP, , ρρhvhv Operations:Operations:

24-hrs a day / 7 days24-hrs a day / 7 days Rain volumetric scans at least every 5-min Rain volumetric scans at least every 5-min

(tilts: 0.7°,1.5 °,2.0 °)(tilts: 0.7°,1.5 °,2.0 °) Also used in research mode (e.g., RHIs, full Also used in research mode (e.g., RHIs, full

volumes, vertically pointing scans)volumes, vertically pointing scans) Routine calibration:Routine calibration:

Receiver calibrationsReceiver calibrations Solar scansSolar scans Self-consistency amongst variablesSelf-consistency amongst variables Comparisons with TRMM and rain gaugesComparisons with TRMM and rain gauges

ARMOR Rainfall Estimation ARMOR Rainfall Estimation Processing System (AREPS)Processing System (AREPS)

Grid rain rates (1 km2 spacing)

T1-line

ARMORNSSTC

End-user

Summation of rain rates

Compute point and areal N-hr rainfall

estimates

Raw Iris Files

ICE PRESENT?

NO

YES

KDP 0.3 and ZH 35? R = R(KDP)YES

NO

ZH BAD? YE

S

R = R(ZHRAIN)

R=BADNO

KDP 0.3, ZH 35.0 dBZZDR 0.5 dB?

YES

R > 50 mm/hr, dBZ > 50 ,or Z, ZDR corr. too large ?

ZH > 30 dBZ, ZDR 0.5 dB?

R = R(ZH,ZDR)

R = R(ZH)

ARMORARMORRAIN RATERAIN RATE

ALGORITHMALGORITHM

(1) R(KDP,ZDR)(2) R(KDP)(3) R(ZH,ZDR)

R = R(ZH)GOOD DATA? YES

NO

R=BAD

KDP ≥ 0.5?

KDP< 0.5?

YES

R = R(KDP)

YES

R =R(KDP,ZDR)

YES

R =R(ZH,ZDR)

no

no

NO

YES

NO 1-hr

Accumulation

6-hr (N-hr)Accumulation

AREPS CoverageAREPS Coverage

100 km from 100 km from ARMORARMOR

11 sub-basins11 sub-basins 42 rain gauges42 rain gauges

AREPS Distributed Rainfall ProductsAREPS Distributed Rainfall Products Rainfall products created every 5-min:Rainfall products created every 5-min:

1-hr and 6-hr basin/sub-basin rainfall 1-hr and 6-hr basin/sub-basin rainfall statistics (mean, max, min, etc)statistics (mean, max, min, etc)

Rainfall at critical locations (e.g., dams)Rainfall at critical locations (e.g., dams) rainfall accumulation images (1-hr, 6-hr)rainfall accumulation images (1-hr, 6-hr)

Text files transmitted every hour to TVAText files transmitted every hour to TVA Contain previous hour’s rainfallContain previous hour’s rainfall used as input by inflow model inputused as input by inflow model input

6-hour accumulation statistics

6-hr Basin Mosaic

1-hr rainfall (also create 6-hr rainfall)

Verification: Point Comparisons Verification: Point Comparisons ARMOR vs. TVA rain gaugesARMOR vs. TVA rain gauges(October 2007 – June 2008)(October 2007 – June 2008)

Original bias and error targets achieved (+/-20%, +/-25% respectively) Constant monitoring of calibration maintains precision and accuracy

of product

Before Calibration Correction

Bias = -10% (-0.99 mm)

Error = 12%

Bias = -17% (-1.80 mm)

Error = 18%

After Correction

Radar Rainfall Estimate Improved

Verification: Sub-basinsVerification: Sub-basinsARMOR vs. rain gauge-derived areal meanARMOR vs. rain gauge-derived areal mean

(January 2008 – July 2008)(January 2008 – July 2008)

Radar rainfall estimates Radar rainfall estimates averaged over each sub-basinaveraged over each sub-basin

rain-gauge network used by rain-gauge network used by TVA to compute Theissen TVA to compute Theissen polygon values to represent polygon values to represent each sub-basineach sub-basin

Radar underestimates sub-Radar underestimates sub-basin rainfall by only basin rainfall by only 8%8%

Random error = 20%Random error = 20% Largely attributed to Theissen Largely attributed to Theissen

polygons (i.e, density of rain polygons (i.e, density of rain gauge network with respect to gauge network with respect to sub-basin boundaries)sub-basin boundaries)

Gauge derived accum. (mm)

Rad

ar d

eriv

ed a

ccum

. (m

m)

Gauge-Estimated Basin Means vs. Radar

BASIN GAUGE (in) ARMOR (in)

Decatur-Wheeler 0.79 0.25

Guntersville-Decatur 0.43 0.46

Upper Bear Creek 0.00 0.06

Town Creek 0.00 0.12

Why are their gauge-radar differences?

• Case 1 (no gauge rain when there is rain)

Rain narrowly missed gauge, but radar captured

• Case 2 (isolated gauge “deluge”)

Single gauge located in heavy rain maximum- single point translated to entire basin- results in overestimate of basin mean

• Case 3 (Gauge and radar match)

More gauges, broader rain distribution

Result: Distributed Radar Rainfall Measurement Benefits TVA

•Water management impacts?

•How might the application of distributed rainfall measurements be extended?

6-Hour Rain Accumulation (in): 12 – 6 PM, 7/9/2008

1

1

2

3

What’s next?What’s next?

Employ NCAR hydrometeor identification Employ NCAR hydrometeor identification algorithm to remove clutter and improve algorithm to remove clutter and improve precipitation calculationsprecipitation calculations

Correct for partial beam blockageCorrect for partial beam blockage Use ARMOR to polarimetrically “tune” Use ARMOR to polarimetrically “tune”

nearby NEXRAD until upgradednearby NEXRAD until upgraded Examine radar dominated rainfall Examine radar dominated rainfall

estimates in a distributed model vs gauge estimates in a distributed model vs gauge only estimatesonly estimates