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Z/R Relationships

 
WFO Last Reported Z/R Formula   Rate @ 30dBZ Name of Formula Bias Correction Factor
BUF 8am EDT 26-Nov-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 168-hr: 1.45 #406 under-estimation
CCX 8am EDT 26-Nov-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 10-hr: 0.84 #13 over-estimation
CLE 8am EDT 26-Nov-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 168-hr: 1.40 #1345 under-estimation
DTX 8am EDT 26-Nov-2014 Z=130R2 Cool Season (East) Stratiform Z/R icon 0.11 in/hr Cool Season (East) Stratiform Z/R 168-hr: 2.03 #279 under-estimation
FCX 8am EDT 26-Nov-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 0.001-hr: 1.08 #58 under-estimation
GRR 8am EDT 26-Nov-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 168-hr: 1.96 #337 under-estimation
HPX 8am EDT 26-Nov-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 168-hr: 1.62 #1345 under-estimation
HTX 8am EDT 26-Nov-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 168-hr: 1.38 #870 under-estimation
ILN 8am EDT 26-Nov-2014 Z=130R2 Cool Season (East) Stratiform Z/R icon 0.11 in/hr Cool Season (East) Stratiform Z/R 168-hr: 1.33 #2157 under-estimation
ILX 8am EDT 26-Nov-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 168-hr: 1.74 #919 under-estimation
IND 8am EDT 26-Nov-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 168-hr: 1.51 #1321 under-estimation
IWX 8am EDT 26-Nov-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 168-hr: 1.75 #915 under-estimation
JKL 8am EDT 26-Nov-2014 Z=130R2 Cool Season (East) Stratiform Z/R icon 0.11 in/hr Cool Season (East) Stratiform Z/R 0.001-hr: 1.50 #27 under-estimation
LOT 8am EDT 26-Nov-2014 Z=130R2 Cool Season (East) Stratiform Z/R icon 0.11 in/hr Cool Season (East) Stratiform Z/R 168-hr: 1.16 #998 under-estimation
LSX 8am EDT 26-Nov-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 168-hr: 1.41 #768 under-estimation
LVX 8am EDT 26-Nov-2014 Z=130R2 Cool Season (East) Stratiform Z/R icon 0.11 in/hr Cool Season (East) Stratiform Z/R 168-hr: 1.65 #1806 under-estimation
LWX 8am EDT 26-Nov-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 2-hr: 0.54 #14 over-estimation
MRX 7am EDT 26-Nov-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 0.001-hr: 1.78 #73 under-estimation
OHX 8am EDT 26-Nov-2014 Z=300R1.4 Summer Deep Convection Z/R icon 0.09 in/hr Summer Deep Convection Z/R 168-hr: 1.47 #1458 under-estimation
PAH 8am EDT 26-Nov-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 168-hr: 1.26 #1272 under-estimation
PBZ 8am EDT 26-Nov-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 3-hr: 0.49 #11 over-estimation
RLX 8am EDT 26-Nov-2014 Z=130R2 Cool Season (East) Stratiform Z/R icon 0.11 in/hr Cool Season (East) Stratiform Z/R 0.001-hr: 2.14 #39 under-estimation
VWX 8am EDT 26-Nov-2014 Z=200R1.6 Marshall/Palmer Stratiform Z/R icon 0.11 in/hr Marshall/Palmer Stratiform Z/R 168-hr: 1.53 #1255 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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