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Z/R Relationships

 
WFO Last Reported Z/R Formula   Rate @ 30dBZ Name of Formula Bias Correction Factor
BUF 11am EDT 22-Oct-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 10-hr: 2.23 #15 under-estimation
CCX 11am EDT 22-Oct-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 5-hr: 1.94 #10 under-estimation
CLE 11am EDT 22-Oct-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 10-hr: 1.75 #51 under-estimation
DTX 11am EDT 22-Oct-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 168-hr: 2.13 #126 under-estimation
FCX 11am EDT 22-Oct-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 168-hr: 1.95 #1254 under-estimation
GRR 11am EDT 22-Oct-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 168-hr: 1.55 #146 under-estimation
HPX 11am EDT 22-Oct-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 168-hr: 1.29 #1266 under-estimation
HTX 11am EDT 22-Oct-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 168-hr: 1.14 #1094 under-estimation
ILN 11am EDT 22-Oct-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 10-hr: 1.37 #11 under-estimation
ILX 11am EDT 22-Oct-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 168-hr: 1.25 #467 under-estimation
IND 11am EDT 22-Oct-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 168-hr: 1.15 #960 under-estimation
IWX 8am EDT 22-Oct-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 168-hr: 1.57 #380 under-estimation
JKL 11am EDT 22-Oct-2014 Z=130R2 Cool Season (East) Stratiform Z/R icon 0.11 in/hr Cool Season (East) Stratiform Z/R 168-hr: 1.53 #2789 under-estimation
LOT 10am EDT 22-Oct-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 168-hr: 1.12 #379 under-estimation
LSX 10am EDT 22-Oct-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 168-hr: 1.12 #400 under-estimation
LVX 9am EDT 22-Oct-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 168-hr: 1.58 #1540 under-estimation
LWX 11am EDT 22-Oct-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 10-hr: 2.07 #14 under-estimation
MRX 11am EDT 22-Oct-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 168-hr: 1.65 #1931 under-estimation
OHX 11am EDT 22-Oct-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 168-hr: 1.18 #1605 under-estimation
PAH 11am EDT 22-Oct-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 168-hr: 0.91 #921 over-estimation
PBZ 11am EDT 22-Oct-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 5-hr: 1.60 #28 under-estimation
RLX 11am EDT 22-Oct-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 5-hr: 1.92 #17 under-estimation
VWX 11am EDT 22-Oct-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 168-hr: 1.10 #927 under-estimation

The table above shows WSR-88D Z/R relationships for sites used in the OHRFC radar mosaic. A Z/R relationship is a empirical formula that is used to estimate rainfall rates from reflectivity signal strength. Parameters that can be varied at each radar site are those shown in red.

Most stratiform rain events in the Ohio River Valley have a reflectivity signal strength of 20-40dBZ with occasional peaks to 45dBZ, but rarely anything higher (that isn't "brightbanding"). For more information and recommendations regarding Z/R relationships, see the Radar Operations Center's Handbook. Refer to Section 7.7.2 Z-R Coefficients. (This handbook is for agency use only, and requires a NOAA LDAP login.)

OHRFC recommends using the Marshall/Palmer Stratiform Z/R for all rain events that are mostly stratiform in nature, including events with embedded thunderstorms. Also, we recommend this Z/R for convective events where significant stratification occurs downwind of convection (such as lines of thunderstorms with a stratiform rain area behind the front).

OHRFC recommends only using the Summertime Deep Convection Z/R for typical events in July and August when isolated convection occurs with little or no blow-off (anvil) precip. Many forecasters have the notion that anytime thunderstorms might occur in stratiform rain, then the radar should be in this Deep Convection mode. OHRFC strongly disagrees and recommends the Marshall/Palmer Z/R for most rain events except for the isolated thunderstorms that occur without stratification -- which occurs most often in July and August..

OHRFC recommends using the Cool Season Stratiform Z/R for wintry events which might contain frozen/freezing precipitation, or where brightbanding is a possibility, and where lightning would occur only in rare "thunder-snow" situations.

OHRFC recommends using the Tropical Z/R or the Marshall/Palmer Z/R for remnants of hurricanes or tropical storms that move into our basin.

The bias correction factor can only be computed if there are at least 10 gage-radar pairs with both values 0.03 inches or more. Sometimes, data has to be searched back several hours to find at least 10 such pairs. (The actual number of pairs used in computing this bias factor is shown following the # sign.) The number of hours searched is shown preceding the bias factor. Bias factors are color-coded green if the radar estimates are within 15% of what the gages verify, or red otherwise. Values over 1.0 mean the radar is under-estimating. Values under 1.0 means the radar is over-estimating. This may seem counter-intuitive. To get the radar estimates to verify well with the gage values, the radar estimates would have to be multiplied by this bias factor shown for each radar.

Any cells with a yellow background under the "Last Reported" column indicate that the latest report is not up-to-date (perhaps the radar is down for maintenance).


National Weather Service
Ohio River Forecast Center
1901 South State Route 134
Wilmington, OH 45177-9708
Page last modified: Wednesday, June 19, 2013 11:32 AM
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