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{{other uses|DBZ (disambiguation)}}
{{short description|Unit of measure used in weather radar}}
{{More citations needed|date=May 2023}}
{{lowercase|title=dBZ (meteorology)}}
[[File:Sturmfront auf Doppler-Radar-Schirm.jpg|thumb|right|The scale of dBZ values can be seen along the bottom of the image.]]
'''dBZ''' stands for ''[[decibel]]Decibel relative to Z''.', Itor '''dBZ''', is a [[logarithmlogarithmic unit|logarithmic]]ic [[Dimensionless quantity|dimensionless]] technical unit used in [[radar]], mostly in [[weather radar]], to compare the equivalent [[reflectivity]] factor (Z) of a radar signal reflected off a remote object (in mm<sup>6</sup> per [[Cubic meter|m]]<sup>3</sup>]]) to the return of a droplet of rain with a diameter of 1&nbsp;mm (1&nbsp;mm<sup>6</sup> per [[meter|m]]<sup>3</sup>).<ref name="WeatherGlossary">{{cite web |title=Weather Glossary: D's |url= httphttps://wwww1.srh.noaaweather.gov/jetstreamglossary/append/glossary_dindex.htmphp?letter=d | work =[[National NWSWeather JetStreamService]] |accessdate=January title9, =2019}}</ref><!---<ref>{{cite web|title=Weather Glossary: D's |url=http://www.srh.noaa.gov/jetstream/append/glossary_d.htm accessdate|work=NWS JetStream |accessdate=2014-02-21}}</ref>--> It is proportional to the number of drops per unit volume and the sixth power of drops' diameter and is thus used to estimate the rain or snow intensity.<ref name=Yau/> With other variables analyzed from the radar returns it helps to determine the type of precipitation. Both the radar reflectivity factor and its logarithmic version are commonly referred to as ''reflectivity'' when the context is clear. In short, the higher the dBZ value, the more likely it is for severe weather to occur in the form of precipitation.
 
Values above 20&nbsp;dBZ usually indicate falling precipitation.<ref>{{cite web |url=https://www.weather.gov/jetstream/radarfaq#reflcolor |title=RIDGE Radar Frequently Asked Questions |accessdate=2019-08-08 |archiveurl=https://web.archive.org/web/20190331123302/https://www.weather.gov/jetstream/radarfaq#reflcolor |archivedate=2019-03-31 |url-status=live}}</ref>
== Principle ==▼
 
▲== Principle ==
The radar reflectivity factor ('''Z''') of [[precipitation]] is dependent on the number ('''N<sub>0</sub>''') and size ('''D''') of reflectors ([[hydrometeor]]s), which includes [[rain]], [[snow]], [[graupel]], and [[hail]]. Very sensitive radars can also measure the reflectivity of cloud drops and ice. For an exponential distribution of reflectors, Z is expressed by:<ref name=Yau>{{Cite book
|title=Short Course in Cloud Physics, Third Edition
Line 13 ⟶ 16:
|pages=190
|isbn=0750632151}}<!--|accessdate=2014-02-21--></ref>
: <math>Z = \int_{0}^{Dmax} N_0 e^{-\Lambda D} D^6dD6\mathrm{d}D </math>▼
 
As rain droplets have a diameter ofon the order of 1 [[millimetre]], Z is in mm<sup>6</sup>m<sup>−3</sup> ([[μm]]<sup>3</sup>). By dividing Z with the equivalent return of a 1&nbsp;mm drop in a volume of a meter cube (Z<sub>0</sub>) and using the logarithm of the result (because the values vary greatly from drizzle to hail), one obtains the logarithmic reflectivity XL<sub>Z</sub>, in dBZ:▼
▲:<math>Z = \int_{0}^{Dmax} N_0 e^{-\Lambda D} D^6dD </math>
: <math>XL_Z = 10 \log_{10} \frac {Z}{Z_0}</math> dBZ▼
 
dBZ values can be converted to rainfall rates (R) in millimetres per hour using the Marshall-Palmer formula:<ref>{{cite web |url=http://www.desktopdoppler.com/help/nws-nexrad.htm#rainfall%20rates |archive-url=https://web.archive.org/web/20160113151652/http://www.desktopdoppler.com/help/nws-nexrad.htm#rainfall%20rates |url-status=dead |archive-date=January 13, 2016 |title=NWS NEXRAD |accessdate=January 13, 2016}}</ref>▼
▲As rain droplets have a diameter of the order of 1 [[millimetre]], Z is in mm<sup>6</sup>m<sup>−3</sup> ([[μm]]<sup>3</sup>). By dividing Z with the equivalent return of a 1&nbsp;mm drop in a volume of a meter cube (Z<sub>0</sub>) and using the logarithm of the result (because the values vary greatly from drizzle to hail), one obtains the logarithmic reflectivity X, in dBZ:
: <math>\frac{\mathrm{R}}{\mathrm{mm/h}} = \left ( \frac{10^{(XdBZ/(10 dBZ))}}{200} \right )^{5 \over 8}</math>▼
 
▲:<math>X = \log_{10} \frac {Z}{Z_0}</math> dBZ
 
▲dBZ values can be converted to rainfall rates in millimetres per hour using the Marshall-Palmer formula:<ref>{{cite web |url=https://web.archive.org/web/20160113151652/http://www.desktopdoppler.com/help/nws-nexrad.htm#rainfall%20rates |title=NWS NEXRAD |accessdate=January 13, 2016}}</ref>
▲:<math>\frac{\mathrm{R}}{\mathrm{mm/h}} = \left ( \frac{10^{(X/(10 dBZ))}}{200} \right )^{5 \over 8}</math>
 
[[File:NOAA Doppler DBZ scale.jpg|thumb|left|NOAA dBZ scale for weather radar]]
{| class="wikitable"
|+Reflectivity in dBZ versusand Rainrate
! 10logL<sub>10Z</sub> (Z/Z<sub>0</sub>dBZ)
! ''R'' (mm/h)
! ''R'' (in/h)
Line 32 ⟶ 33:
|-
| 5
| 0.1907
| < 0.01
| Trace accumulation or mist
| Hardly noticeable
|-
| 10
| 0.3115
| < 0.01
| LightTrace accumulation or mist
|-
| 15
| 0.543
| 0.0201
| Trace accumulation
| Mist
|-
| 20
| 0.916
| 0.0402
| VeryLight lightrain
|-
| 25
| 1.573
| 0.0605
| Light rain
|-
| 30
| 32.347
| 0.1310
| Light to moderate rain
|-
| 35
| 5.886
| 0.2322
| Moderate rain
|-
| 40
| 11.1553
| 0.4445
| Moderate to heavy rain
|-
| 45
| 1823.637
| 0.7392
| Heavy rain
| Moderate to heavy
|-
| 50
| 3448.386
| 1.3590
| Heavy rain, small hail possible
|-
| 55
| 58.72100
| 2.314
| Very heavy/small rain, hail possible.
|-
| 60
| 141205
| 5.578
| Extreme/moderateVery heavy rain, hail likely.
|-
| 65
| 287421
| 1116.316
| Extreme/Very heavy rain, hail very likely, large hail possible.
|}
{{clear}}
 
== Other quantities ==
The definition of Z above shows that a large number of small hydrometeors will reflect as one large hydrometeor. The signal returned to the [[radar]] will be equivalent in both situations, so a group of small hydrometeors is virtually indistinguishable from one large hydrometeor on the resulting radar image. The [[reflectivity]] image is just one type of image produced by a radar. Using it alone, a meteorologist could not tell with certainty the type of [[precipitation]] and distinguish any [[Artifact (error)|artifacts]] affecting the radar return.▼
 
▲The definition of Z above shows that a large number of small hydrometeors will reflect as one large hydrometeor. The signal returned to the [[radar]] will be equivalent in both situations, so a group of small hydrometeors is virtually indistinguishable from one large hydrometeor on the resulting radar image. The [[reflectivity]] image is just one type of image produced by a radar. Using it alone a meteorologist could not tell with certainty the type of [[precipitation]] and distinguish any [[Artifact (error)|artifacts]] affecting the radar return.
 
In combination with other information gathered by the radar during the same scan (dual [[Polarization (waves)|polarization]] products and [[phase shifting]] due to the [[Doppler effect]]), meteorologists can distinguish between [[hail]], [[rain]], [[snow]], biologicals (birds, insects), and other atmospheric phenomena.
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{{Meteorological equipment|state=uncollapsed}}
 
[[Category:Logarithmic scales of measurement]]▼
[[Category:Meteorological quantities]]
[[Category:Radar meteorology]]
[[Category:Units of meteorology measurement]]
▲[[Category:Logarithmic scales of measurement]]
 
 
{{Climate-stub}}
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