Comparison of PPS and QPE Rainfall Estimates for Summer 2013

Heavy Precipitation Events

Mitchell Gaines National Weather Service

Mount Holly, NJ

WFO Philadelphia/Mount Holly NJ

Purpose

To evaluate the PPS and QPE Algorithms and processes of rainfall estimation with relation to ground truth precipitation measurements for several heavy rainfall events from this past Summer over the Philadelphia region.

Photo: Pennsville, NJ Jul 28,2013Source: NJ.com

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Rainfall Records Broken

- Wettest June and July on record at Philadelphia International Airport.

- 8.03 inches in about six hours (daily rainfall record) at Philadelphia on July 28th

- Wettest June on record in Wilmington, DE, and Atlantic City, NJ

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Legacy PPS vs. Dual-Pol QPE

PPS- From base reflectivity (Z) Digital Hybrid Scan- Single Z-R relation used at any time (choice of coefficients)- Gage bias adjustment available- Suffers from bright band, hail contamination, beam blockageQPE- From Z, ZDR, CC, KDP, MLDA, HCA Digital Precip Rate- Three relations: R(Z), R(Z,ZDR), R(KDP) plus adjustments for

frozen hydro-meteors- No gage bias adjustment- Dual-pol input should mitigate many PPS limitations

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KDIX

KDOX

KDIX/KDOX Coverage to 150 km

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Methodology

Identify heavy rainfall events over the Philadelphia region this summer which led to flash flooding. (6/3/13, 6/7/13, 6/10/13, 6/18/13, 7/1/13, 7/13/13, 7/23/13, 7/28/13)

Use Multi-Radar Multi-Sensor System (MMRS) http://nmq.ou.edu/legacy2012.html- Compare rain gauge data to radar estimates from both the PPS

and QPE for that particular point. - Exclude data less than 20km or more than 150 km from the

radar.- Excluded zero or trace values, gauge or radar.

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Methodology (Cont’d)

Use of DIX and DOX WSR 88D radars to apply estimation methods.

Variables measured to determine preference- RMSE, Correlation Coefficient, Mean Bias - Graphical scatter plots

- Focus on July 28th event ( 03z to 03z timeframe with scatterplots)

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July 28th Heavy Rainfall

- Localized heavy rainfall event

- 8.03 inches of rain in about six hours at Philadelphia International, less than one inch at Northeast Philadelphia. Dual-pol (QPE) rainfall estimate image

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Synoptic Overview, July 28

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PPS DIX Radar 7/28

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QPE DIX Radar 7/28

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PPS DOX 7/28

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QPE DOX 7/28

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Overall Results

6/7 (Andrea) DIX Mean Bias G/R Correlation Coeff RMSE (in)

QPE  1.31  .57  .89

PPS  1.61  .57  1.10

6/10 DIX Mean Bias G/R Correlation Coeff RMSE (in)

QPE  1.00  .77  .48

PPS  1.45  .76  .58

6/18 DIX Mean Bias G/R Correlation Coeff RMSE (in)

QPE  .74  .91  .35

PPS  .98  .86  .34

6/3 DIX Mean Bias G/R Correlation Coeff RMSE (in)

QPE  .91  .80  .2

PPS  1.06  .78  .21

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Overall Results

7/23-24 DIX Mean Bias G/R Correlation Coeff RMSE (in)

QPE  .73  .80  .52

PPS  1.13  .79  .35

7/28-29 DIX Mean Bias G/R Correlation Coeff RMSE (in)

QPE  .98  .94  .35

PPS  1.37  .93  .66

7/12 Mean Bias G/R Correlation Coeff RMSE (in)

QPE  .89  .88  .34

PPS  1.17  .87  .34

7/1 (12Z) Mean Bias G/R Correlation Coeff RMSE (in)

QPE .82 .87 .36

PPS 1.27 .87 .32

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Average Results

AVG DIX Mean Bias G/R Correlation Coeff RMSE(in)

QPE .93 .81 .44

PPS 1.26 .80 .49

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Conclusions

- Lower mean bias with QPE

- Similar CC values

- Lower RMSE values with QPE, all but 7/23.

- DOX results featured higher Dual-Pol totals while Legacy was a better fit in several cases.

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Conclusions

QPE is preferred for rainfall amounts over 2 inches in heavy rainfall events. - Much better at “capturing” higher amounts

of QPF. - Seen in “curve” throughout many of the

scatterplots - Use of melting layer more of a role in cold

season - Zimmerman Presentation (NWS, Wakefield)

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Special thanks to the

-NROW Conference Steering Committee

-The University of Albany

- National Weather Service Mount Holly, NJ

- SOO Al Cope

Questions