Z/R Relationships

WFO	Last Reported	Z/R Formula		Rate @ 30dBZ	Name of Formula	Bias Correction Factor
BUF	12am EDT 08-Nov-2009	Z=200R1.6		0.11 in/hr	Marshall/Palmer Stratiform Z/R	10-hr: 1.39 #184 under-estimation
CCX	12am EDT 08-Nov-2009	Z=200R1.6		0.11 in/hr	Marshall/Palmer Stratiform Z/R	10-hr: 0.91 #683 over-estimation
CLE	12am EDT 08-Nov-2009	Z=200R1.6		0.11 in/hr	Marshall/Palmer Stratiform Z/R	10-hr: 1.45 #594 under-estimation
DTX	12am EDT 08-Nov-2009	Z=200R1.6		0.11 in/hr	Marshall/Palmer Stratiform Z/R	10-hr: 1.86 #73 under-estimation
FCX	12am EDT 08-Nov-2009	Z=300R1.4		0.09 in/hr	Summer Deep Convection Z/R	10-hr: 1.73 #938 under-estimation
GRR	12am EDT 08-Nov-2009	Z=200R1.6		0.11 in/hr	Marshall/Palmer Stratiform Z/R	10-hr: 0.92 #70 over-estimation
HPX	12am EDT 08-Nov-2009	Z=200R1.6		0.11 in/hr	Marshall/Palmer Stratiform Z/R	10-hr: 1.20 #627 under-estimation
HTX	9pm EDT 07-Nov-2009	Z=300R1.4		0.09 in/hr	Summer Deep Convection Z/R	10-hr: 1.76 #466 under-estimation
ILN	12am EDT 08-Nov-2009	Z=200R1.6		0.11 in/hr	Marshall/Palmer Stratiform Z/R	10-hr: 1.13 #1146 under-estimation
ILX	12am EDT 08-Nov-2009	Z=200R1.6		0.11 in/hr	Marshall/Palmer Stratiform Z/R	10-hr: 1.07 #425 under-estimation
IND	12am EDT 08-Nov-2009	Z=200R1.6		0.11 in/hr	Marshall/Palmer Stratiform Z/R	10-hr: 0.95 #576 unbiased estimation
IWX	12am EDT 08-Nov-2009	Z=200R1.6		0.11 in/hr	Marshall/Palmer Stratiform Z/R	10-hr: 0.89 #304 over-estimation
JKL	12am EDT 08-Nov-2009	Z=130R2		0.11 in/hr	Cool Season (East) Stratiform Z/R	10-hr: 1.12 #1442 under-estimation
LOT	12am EDT 08-Nov-2009	Z=200R1.6		0.11 in/hr	Marshall/Palmer Stratiform Z/R	10-hr: 1.05 #292 under-estimation
LSX	12am EDT 08-Nov-2009	Z=200R1.6		0.11 in/hr	Marshall/Palmer Stratiform Z/R	10-hr: 0.93 #383 over-estimation
LVX	12am EDT 08-Nov-2009	Z=300R1.4		0.09 in/hr	Summer Deep Convection Z/R	10-hr: 1.38 #738 under-estimation
LWX	12am EDT 08-Nov-2009	Z=200R1.6		0.11 in/hr	Marshall/Palmer Stratiform Z/R	10-hr: 1.13 #153 under-estimation
MRX	4pm EDT 07-Nov-2009	Z=300R1.4		0.09 in/hr	Summer Deep Convection Z/R	10-hr: 1.82 #751 under-estimation
OHX	12am EDT 08-Nov-2009	Z=300R1.4		0.09 in/hr	Summer Deep Convection Z/R	10-hr: 1.63 #640 under-estimation
PAH	12am EDT 08-Nov-2009	Z=200R1.6		0.11 in/hr	Marshall/Palmer Stratiform Z/R	10-hr: 1.10 #519 under-estimation
PBZ	12am EDT 08-Nov-2009	Z=130R2		0.11 in/hr	Cool Season (East) Stratiform Z/R	10-hr: 1.38 #963 under-estimation
RLX	12am EDT 08-Nov-2009	Z=130R2		0.11 in/hr	Cool Season (East) Stratiform Z/R	10-hr: 1.02 #1268 unbiased estimation
VWX	12am EDT 08-Nov-2009	Z=200R1.6		0.11 in/hr	Marshall/Palmer Stratiform Z/R	10-hr: 1.22 #635 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, authorization, instructions, and recommendations regarding Z/R relationships, see the Radar Operations Center's page.

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 following table lists the radars that OHRFC uses in the precipitation mosaic along with the Z/R relationship each radar is set to.

The bias correction factor can only be computed if there are at least 10 gage-radar pairs with values 0.01 inches or more. Sometimes, data has to be searched back several hours to find at least 10 pairs. (The actual number of pairs used in computing this bias factor is shown following the # sign.) This 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.