![Sturmfront auf Doppler-Radar-Schirm](https://upload.wikimedia.org/wikipedia/commons/0/0c/Sturmfront_auf_Doppler-Radar-Schirm.jpg)
dBZ stands for ''
decibel relative to Z''. It is a
logarithmic dimensionless
A dimensionless quantity (also known as a bare quantity, pure quantity, or scalar quantity as well as quantity of dimension one) is a quantity to which no physical dimension is assigned, with a corresponding SI unit of measurement of one (or 1) ...
technical unit used in
radar
Radar is a detection system that uses radio waves to determine the distance ('' ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, we ...
, mostly in
weather radar
Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type (rain, snow, hail etc.). Modern weather radars are mostly pulse- ...
, to compare the equivalent
reflectivity
The reflectance of the surface of a material is its effectiveness in Reflection (physics), reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the respon ...
factor (Z) of a remote object (in mm
6 per
m3) to the return of a droplet of rain with a diameter of 1 mm (1 mm
6 per m
3).
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.
[ 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 dBZ usually indicate falling precipitation.
]
Principle
The radar reflectivity factor (Z) of precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
is dependent on the number (N0) and size (D) of reflectors (hydrometeor
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
s), which includes rain
Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...
, snow
Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes.
It consists of frozen crystalline water throughout ...
, graupel
Graupel (; ), also called soft hail, hominy snow, or snow pellets, is precipitation that forms when supercooled water droplets in air are collected and freeze on falling snowflakes, forming balls of crisp, opaque rime.
Graupel is distinct from ...
, and hail
Hail is a form of solid precipitation. It is distinct from ice pellets (American English "sleet"), though the two are often confused. It consists of balls or irregular lumps of ice, each of which is called a hailstone. Ice pellets generally fal ...
. Very sensitive radars can also measure the reflectivity of cloud drops and ice. For an exponential distribution of reflectors, Z is expressed by:
:
As rain droplets have a diameter on the order of 1 millimetre
330px, Different lengths as in respect to the electromagnetic spectrum, measured by the metre and its derived scales. The microwave is between 1 meter to 1 millimeter.
The millimetre (American and British English spelling differences#-re, -er, ...
, Z is in mm6m−3 ( μm3). By dividing Z with the equivalent return of a 1 mm drop in a volume of a meter cube (Z0) and using the logarithm of the result (because the values vary greatly from drizzle to hail), one obtains the logarithmic reflectivity LZ, in dBZ:
: dBZ
dBZ values can be converted to rainfall rates (R) in millimetres per hour using the Marshall-Palmer formula:
:
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
Radar is a detection system that uses radio waves to determine the distance ('' ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, we ...
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
The reflectance of the surface of a material is its effectiveness in Reflection (physics), reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the respon ...
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
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
and distinguish any artifacts affecting the radar return.
In combination with other information gathered by the radar during the same scan (dual polarization products and phase shifting
In physics and mathematics, the phase of a periodic function F of some real variable t (such as time) is an angle-like quantity representing the fraction of the cycle covered up to t. It is denoted \phi(t) and expressed in such a scale that it v ...
due to the Doppler effect
The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...
), meteorologists can distinguish between hail
Hail is a form of solid precipitation. It is distinct from ice pellets (American English "sleet"), though the two are often confused. It consists of balls or irregular lumps of ice, each of which is called a hailstone. Ice pellets generally fal ...
, rain
Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...
, snow
Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes.
It consists of frozen crystalline water throughout ...
, biologicals (birds, insects), and other atmospheric phenomena.
References
{{Meteorological equipment, state=uncollapsed
Radar meteorology
Units of meteorology measurement
Logarithmic scales of measurement