Reflectivity (radar)

In radar technology, reflectivity is a measure of the backscatter cross-section of diffuse targets and is used in precipitation radar to determine the rain rate. The measure is comparable to the effective reflecting area of the reconnaissance radar . The reflectivity depends on the size and number of raindrops per volume and has the unit of measurement mm ⁶ / m ³ . It is proportional to the energy that is scattered back to the antenna by any scattering particles in the radar beam.

Formally, the reflectivity Z is defined as:

${\displaystyle Z=\sum _{i=1}^{n}\,D_{i}^{6}}$

with as the sphere-equivalent particle diameter and as the number of raindrops per square meter ${\displaystyle D}$ ${\displaystyle i}$

Typical reflectivity values ​​for different types of precipitation

	Reflectivity Z
heavy rain ( stratiform )	${\displaystyle 200\cdot R^{1,6}}$
heavy rain shower ( orographic )	${\displaystyle 31\cdot R^{1,71}}$
Thunderstorm ( convective )	${\displaystyle 286\cdot R^{1,37}}$
snow	${\displaystyle 2000\cdot R^{2}}$

However, since the energy received by the antenna depends on many factors, the reflectivity was specially defined for meteorological targets in which all other influences were combined into just one factor and one exponent in order to establish an empirical relationship to the rain rate R with two variables . The reflectivity therefore only depends on the properties of the targets. When it rains , the greater the number and size of raindrops in the air, the greater it is. ${\displaystyle (a,b)}$

The reflectivity is usually given in the logarithmic unit dBZ , where dB is the abbreviation for decibels and Z stands for reflectivity:

${\displaystyle Z\,[dBZ]=10\cdot \log \left({a\cdot R^{b}}\right)}$

In some representations, the reflectivity is also represented by the lowercase letter z , while the uppercase letter Z is understood as a logarithmic size specification.

Differential reflectivity

By comparing the echo signals received with different orientations of the linear polarization , polarimetric radar devices can determine a differential reflectivity, with the aid of which the size of the hydrometeors can be determined and a distinction can be made between rain and hail. The differential reflectivity is calculated according to:

${\displaystyle Z_{\mathrm {DR} }=10\log \left({\frac {z_{\mathrm {h} }}{z_{\mathrm {v} }}}\right)=Z_{\mathrm {h} }-Z_{\mathrm {v} }}$

The measured value of the differential reflectivity can the set of values (fall "on edge" indicator for hailstones, only these can) of slightly negative, zero for hail or rain drops with a diameter of less than 0.3 mm, and relatively high positive values of up Assume 5–6 dB for larger raindrops flattened by air resistance.

literature

• Andreas Bott: Synoptic Meteorology: Methods of Weather Analysis and Forecast . Springer, 2016, ISBN 978-3-662-48195-0 .