Aerospace and Electronic Systems Magazine July 2017 Tutorial XI - 50

Weather Radar: Operation and Phenomenology
including the WSR-88D, attenuation is low enough that the radar
is able to see into and through significant precipitation. However,
it is substantial enough that it cannot be neglected [11]. Higherfrequency weather radar systems (see Section III) exist but suffer
greater attenuation and thus tend to only operate at shorter ranges
than their S-band counterparts. The total attenuation loss because
of rain in a unit volume is
∞

k p =  σ a ( D ) + σ ts ( D) N ( D )dD
0
∞

=  σ e ( D) N ( D)dD,

(19)

where σa is the effective absorption cross section, σts is the effective
scattering cross section, σe = σa + σts is the extinction cross section,
and N(D) is the drop size distribution (DSD)5. The total attenuation
over the path of propagation is a function of range. In the case of
small raindrop diameter-to-wavelength ratios [11], the absorption
effect is more dominant than the scattering effect and can be approximated as

σa =

π 2 D3
ℑ( K ),
λ

(20)

π 2 D3
ℑ( denotes
σa =
K ),
where D is the raindrop diameter in millimeters,
the
λ
imaginary component of a complex number, and K is
K=

m2 − 1
.
m2 + 2

(21)

Here, m = n − jnκ is the complex refractive index of water, where
the real part n is often called simply the refractive index and κ is
the attenuation index. The rate of attenuation, usually expressed
in decibels per kilometer, has been fairly well characterized for a
range of rainfall rates, DSDs, and frequencies [2]. Traditionally,
attenuation because of rain is estimated based on the reflectivity factor or, equivalently, the scattering RCS of rain (discussed
in Section V.A) as observed and estimated by the radar [10]. The
assumed reflectivity-attenuation relationship for the WSR-88D is
shown in Fig. 2.

I. CLUTTER AND ANOMALOUS PROPAGATION
The signal received by a weather radar is sometimes contaminated
with clutter that interferes with the signal the user would like to observe [7]. In meteorological applications, clutter typically includes
objects such as terrain, buildings, and trees.6 Typically, clutter only
contaminates the signal when the antenna is at a low elevation
angle [11]. However, on rare occasions, abnormally significant
5

The DSD is defined as the probability distribution of raindrop
diameters scaled by the raindrop concentration, i.e., the total
number of raindrops in a unit volume.
Technically, clutter also includes signals from birds and insects.
However, in application, echoes from birds and insects are distinguished from stationary clutter and are given a separate classification of biological scatterers.

Figure 2.

Specific two-way attenuation versus reflectivity as assumed in WSR88D calculations.

departures from the atmosphere's typical refractive index gradient result in a phenomenon called anomalous propagation (AP).
AP occurs when significant temperature inversions or large moisture gradients cause abnormally large refractive gradients, which
in turn bend the path of the transmitted wave back toward Earth.
Because clutter consists of returns from nominally stationary objects, the clutter slow-time spectrum is centered at zero velocity
with a narrow spectral width. These two attributes are often used
to discriminate clutter from meteorological echoes [10]. In the
case of AP, it may not be possible to recover usable meteorological
signals from the contaminated resolution cells. However, in other
cases, clutter mitigation is generally achieved by first applying a
notch filter to the slow-time data sequence. Rather than applying
the notch filter indiscriminately, a site-specific clutter map may
be used to determine not only which portions of the data require a
clutter filter but also what the parameters of the clutter filter should
be (i.e., null width and depth) [17].

III. WEATHER RADAR HISTORY
The use of radar technology by local television meteorologists is
a familiar part of modern life. Originally designed to detect aircraft during World War II, radar was quickly adapted to observe
precipitation and provide insights into local weather events [18],
[19]. The U.S. Weather Bureau used modified AN/APS-2F S-band
radars starting in 1947. These radars became the Weather Surveillance Radar (WSR) 1, WSR-1A, WSR-3, and WSR-4 systems. In
the 1950s, the first operational weather radars, the WSR-57s, were
installed at Weather Bureau sites across the United States. Some
of these radars were operational well into the 1990s. The WSR74C and the WSR-74S weather radars were deployed in the 1970s.
The current national network of weather radars, operated by the
NWS, consists of 160 S-band, dual-pol, Doppler-capable WSR88D (NEXRAD) radar systems [20], [21]. Many basic specifications for these radars are given in Table 1.

IEEE A&E SYSTEMS MAGAZINE

JULY 2017, Part II of II

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine July 2017 Tutorial XI