2.2.4. 气象目标雷达截面

2.2.4. Radar Cross Section for Meteorological Targets

雷达气象学领域用一个称为反射率(这里称为体反射率)的标准化因子表示雨雪等天气目标的反射率,通常用符号Z表示(Sauvageot,1992; Doviak and Zrnic, 1993)。

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).

气象目标是体杂波的一个例子。

Weather targets are an example of volumeclutter.

实际观测到的气象回波是雷达分辨率单元中许多雨滴、悬浮水颗粒、冰雹或雪花的复合后向散射。

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

假设第i个独立散射体的RCS为σi,采用非相干叠加。

Suppose the RCS of the ith individualscatterer is σi and assume noncoherent addition.

折射率是温度和波长的函数。

The index of refraction is a function ofboth the temperature and wavelength.

但是,对于3到10 cm之间的波长[X波段(10 GHz)和C波段(3 GHz)之间的雷达频率]和0到20°C之间的温度,由水组成的散射体的|K|值大约是一个相对恒定的常数0.93,对于冰组成的散射体则是0.197。

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.

由于观测到的Z值范围很大,所以通常用分贝表示,并表示为dBZ。

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

利用式(2.42)中的定义给出了观察到的RCS的表达式为

Using this definition in Eq. (2.42) givesthe following expression for the observed RCS

因此,在给定实测回波功率的情况下,可利用雷达距离方程估算η,然后用式(2.44)将η转换为Z。

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.

由于只与散射体的体密度和大小有关,气象学家倾向于用反射率Z来表示雷达回波强度,而不是用雷达散射截面η来表示。

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)》