2.2.4. 气象目标雷达截面

2.2.4. Radar Cross Section for Meteorological Targets

The field of radar meteorology expressesthe reflectivity of weather targets such as rain or snow in terms of anormalized factor called the reflectivity (here called the volume reflectivity)and usually represented with the symbol Z (Sauvageot, 1992; Doviak and Zrnic,1993).

Weather targets are an example of volumeclutter.

The actual observed echo is the compositebackscatter of many raindrops, suspended water particles, hailstones, orsnowflakes in the radar's resolution cell.

Suppose the RCS of the ith individualscatterer is σi and assume noncoherent addition. The index of refraction is a function ofboth the temperature and wavelength.

However, for wavelengths between 3 and 10 cm [radar frequencies between X band (10GHz) and C band (3 GHz)] and temperatures between 0 and 20°C, the value of |K| is approximately arelatively constant 0.93 for scatterers composed of water and 0.197 for ice. Z被称为反射率因子，通常以mm6/m3为单位表示。

Z is called the reflectivity factor and isusually expressed in units of mm6/m3.

Due to the large range of values observedfor Z, it is commonly expressed on a decibel scale and denoted as dBZ.

Using this definition in Eq. (2.42) givesthe following expression for the observed RCS Thus, given a measured echo power, theradar range equation can be used to estimate η, and then Eq. (2.44) can be usedto convert η to Z.

Because it is related only to the volumedensity and size of scatterers, meteorologists prefer to express radar echostrength in terms of the reflectivity Z rather than the RCS η.

Z值与空气中的含水量或降水率有关。

The value of Z canthen be related to the amount of water in the air or the precipitation rate.

——本文译自Mark A. Richards所著的《Fundamentals of Radar Signal Processing（Second edition）》