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Z/R Relationships

 
WFO Last Reported Z/R Formula   Rate @ 30dBZ Name of Formula Bias Correction Factor
BUF 6am EDT 28-Aug-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 168-hr: 1.03 #269 unbiased estimation
CCX 6am EDT 28-Aug-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 10-hr: 0.99 #19 unbiased estimation
CLE 6am EDT 28-Aug-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 10-hr: 0.95 #14 over-estimation
DTX 6am EDT 28-Aug-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 168-hr: 1.13 #140 under-estimation
FCX 6am EDT 28-Aug-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 5-hr: 0.94 #13 over-estimation
GRR 6am EDT 28-Aug-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 168-hr: 0.64 #244 over-estimation
HPX 6am EDT 28-Aug-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 10-hr: 0.56 #12 over-estimation
HTX 6am EDT 28-Aug-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 168-hr: 0.70 #628 over-estimation
ILN 6am EDT 28-Aug-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 5-hr: 1.17 #11 under-estimation
ILX 6am EDT 28-Aug-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 5-hr: 0.35 #19 over-estimation
IND 6am EDT 28-Aug-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 10-hr: 0.54 #18 over-estimation
IWX 6am EDT 28-Aug-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 168-hr: 0.80 #696 over-estimation
JKL 6am EDT 28-Aug-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 5-hr: 0.90 #13 over-estimation
LOT 6am EDT 28-Aug-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 168-hr: 0.80 #561 over-estimation
LSX 6am EDT 28-Aug-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 5-hr: 0.74 #22 over-estimation
LVX 6pm EDT 27-Aug-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 0.001-hr: 0.80 #11 over-estimation
LWX 6am EDT 28-Aug-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 10-hr: 1.57 #18 under-estimation
MRX 6am EDT 28-Aug-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 168-hr: 1.20 #1400 under-estimation
OHX 6am EDT 28-Aug-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 168-hr: 0.97 #889 unbiased estimation
PAH 6am EDT 28-Aug-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 5-hr: 0.37 #18 over-estimation
PBZ 6am EDT 28-Aug-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 5-hr: 1.38 #16 under-estimation
RLX 6am EDT 28-Aug-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 5-hr: 1.43 #23 under-estimation
VWX 6am EDT 28-Aug-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 5-hr: 0.51 #13 over-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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