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Comparison of PPS and QPE Rainfall Estimates for Summer 2013 Heavy Precipitation Events. Mitchell Gaines National Weather Service Mount Holly, NJ. Purpose .
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Comparison of PPS and QPE Rainfall Estimates for Summer 2013 Heavy Precipitation Events Mitchell Gaines National Weather Service 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 28,2013 Source: NJ.com
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
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 blockage • QPE • 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
KDIX/KDOX Coverage to 150 km KDIX KDOX
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.
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)
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
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.
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)
Questions • Special thanks to the • -NROW Conference Steering Committee • -The University of Albany • National Weather Service Mount Holly, NJ • SOO Al Cope