<s>
Weather	B-Application
radar	I-Application
,	O
also	O
called	O
weather	B-Application
surveillance	I-Application
radar	I-Application
(	O
WSR	O
)	O
and	O
Doppler	B-Application
weather	I-Application
radar	I-Application
,	O
is	O
a	O
type	O
of	O
radar	B-Application
used	O
to	O
locate	O
precipitation	O
,	O
calculate	O
its	O
motion	O
,	O
and	O
estimate	O
its	O
type	O
(	O
rain	O
,	O
snow	O
,	O
hail	O
etc	O
.	O
)	O
.	O
</s>
<s>
Modern	O
weather	B-Application
radars	I-Application
are	O
mostly	O
pulse-Doppler	B-Algorithm
radars	I-Algorithm
,	O
capable	O
of	O
detecting	O
the	O
motion	O
of	O
rain	O
droplets	O
in	O
addition	O
to	O
the	O
intensity	O
of	O
the	O
precipitation	O
.	O
</s>
<s>
During	O
World	O
War	O
II	O
,	O
radar	B-Application
operators	O
discovered	O
that	O
weather	O
was	O
causing	O
echoes	O
on	O
their	O
screen	O
,	O
masking	O
potential	O
enemy	O
targets	O
.	O
</s>
<s>
Soon	O
after	O
the	O
war	O
,	O
surplus	O
radars	B-Application
were	O
used	O
to	O
detect	O
precipitation	O
.	O
</s>
<s>
Since	O
then	O
,	O
weather	B-Application
radar	I-Application
has	O
evolved	O
and	O
is	O
used	O
by	O
national	O
weather	O
services	O
,	O
research	O
departments	O
in	O
universities	O
,	O
and	O
in	O
television	O
stations	O
 '	O
weather	O
departments	O
.	O
</s>
<s>
Radar	B-Application
output	O
is	O
even	O
incorporated	O
into	O
numerical	B-General_Concept
weather	I-General_Concept
prediction	I-General_Concept
models	O
to	O
improve	O
analyses	O
and	O
forecasts	O
.	O
</s>
<s>
During	O
World	O
War	O
II	O
,	O
military	O
radar	B-Application
operators	O
noticed	O
noise	O
in	O
returned	O
echoes	O
due	O
to	O
rain	O
,	O
snow	O
,	O
and	O
sleet	O
.	O
</s>
<s>
In	O
the	O
United	O
States	O
,	O
David	O
Atlas	O
at	O
first	O
working	O
for	O
the	O
Air	O
Force	O
and	O
later	O
for	O
MIT	O
,	O
developed	O
the	O
first	O
operational	O
weather	B-Application
radars	I-Application
.	O
</s>
<s>
Marshall	O
and	O
his	O
doctoral	O
student	O
Walter	O
Palmer	O
are	O
well	O
known	O
for	O
their	O
work	O
on	O
the	O
drop	B-Algorithm
size	I-Algorithm
distribution	I-Algorithm
in	O
mid-latitude	O
rain	O
that	O
led	O
to	O
understanding	O
of	O
the	O
Z-R	O
relation	O
,	O
which	O
correlates	O
a	O
given	O
radar	B-Application
reflectivity	O
with	O
the	O
rate	O
at	O
which	O
rainwater	O
is	O
falling	O
.	O
</s>
<s>
In	O
the	O
United	O
Kingdom	O
,	O
research	O
continued	O
to	O
study	O
the	O
radar	B-Application
echo	O
patterns	O
and	O
weather	O
elements	O
such	O
as	O
stratiform	O
rain	O
and	O
convective	O
clouds	O
,	O
and	O
experiments	O
were	O
done	O
to	O
evaluate	O
the	O
potential	O
of	O
different	O
wavelengths	O
from	O
1	O
to	O
10	O
centimeters	O
.	O
</s>
<s>
By	O
1950	O
the	O
UK	O
company	O
EKCO	O
was	O
demonstrating	O
its	O
airborne	O
'	O
cloud	O
and	O
collision	O
warning	O
search	O
radar	B-Application
equipment	O
 '	O
.	O
</s>
<s>
Between	O
1950	O
and	O
1980	O
,	O
reflectivity	O
radars	B-Application
,	O
which	O
measure	O
position	O
and	O
intensity	O
of	O
precipitation	O
,	O
were	O
incorporated	O
by	O
weather	O
services	O
around	O
the	O
world	O
.	O
</s>
<s>
The	O
early	O
meteorologists	O
had	O
to	O
watch	O
a	O
cathode	B-Device
ray	I-Device
tube	I-Device
.	O
</s>
<s>
In	O
1953	O
Donald	O
Staggs	O
,	O
an	O
electrical	O
engineer	O
working	O
for	O
the	O
Illinois	O
State	O
Water	O
Survey	O
,	O
made	O
the	O
first	O
recorded	O
radar	B-Application
observation	O
of	O
a	O
"	O
hook	O
echo	O
"	O
associated	O
with	O
a	O
tornadic	O
thunderstorm	O
.	O
</s>
<s>
The	O
first	O
use	O
of	O
weather	B-Application
radar	I-Application
on	O
television	O
in	O
the	O
United	O
States	O
was	O
in	O
September	O
1961	O
.	O
</s>
<s>
Weather	O
Bureau	O
WSR-57	O
radar	B-Application
site	O
in	O
Galveston	O
in	O
order	O
to	O
get	O
an	O
idea	O
of	O
the	O
size	O
of	O
the	O
storm	O
.	O
</s>
<s>
During	O
the	O
broadcast	O
,	O
he	O
held	O
that	O
transparent	O
overlay	O
over	O
the	O
computer	O
's	O
black-and-white	O
radar	B-Application
display	O
to	O
give	O
his	O
audience	O
a	O
sense	O
both	O
of	O
Carla	O
's	O
size	O
and	O
of	O
the	O
location	O
of	O
the	O
storm	O
's	O
eye	O
.	O
</s>
<s>
During	O
the	O
1970s	O
,	O
radars	B-Application
began	O
to	O
be	O
standardized	O
and	O
organized	O
into	O
networks	O
.	O
</s>
<s>
The	O
first	O
devices	O
to	O
capture	O
radar	B-Application
images	O
were	O
developed	O
.	O
</s>
<s>
The	O
number	O
of	O
scanned	O
angles	O
was	O
increased	O
to	O
get	O
a	O
three-dimensional	O
view	O
of	O
the	O
precipitation	O
,	O
so	O
that	O
horizontal	O
cross-sections	O
(	O
CAPPI	B-Algorithm
)	O
and	O
vertical	O
cross-sections	O
could	O
be	O
performed	O
.	O
</s>
<s>
In	O
May	O
1973	O
,	O
a	O
tornado	B-Language
devastated	O
Union	O
City	O
,	O
Oklahoma	O
,	O
just	O
west	O
of	O
Oklahoma	O
City	O
.	O
</s>
<s>
For	O
the	O
first	O
time	O
,	O
a	O
Dopplerized	O
10cm	O
wavelength	O
radar	B-Application
from	O
NSSL	O
documented	O
the	O
entire	O
life	O
cycle	O
of	O
the	O
tornado	B-Language
.	O
</s>
<s>
The	O
researchers	O
discovered	O
a	O
mesoscale	O
rotation	O
in	O
the	O
cloud	O
aloft	O
before	O
the	O
tornado	B-Language
touched	O
the	O
ground	O
–	O
the	O
tornadic	B-Algorithm
vortex	I-Algorithm
signature	I-Algorithm
.	O
</s>
<s>
NSSL	O
's	O
research	O
helped	O
convince	O
the	O
National	O
Weather	O
Service	O
that	O
Doppler	O
radar	B-Application
was	O
a	O
crucial	O
forecasting	O
tool	O
.	O
</s>
<s>
The	O
Super	O
Outbreak	O
of	O
tornadoes	B-Language
on	O
3	O
–	O
4	O
April	O
1974	O
and	O
their	O
devastating	O
destruction	O
might	O
have	O
helped	O
to	O
get	O
funding	O
for	O
further	O
developments	O
.	O
</s>
<s>
Between	O
1980	O
and	O
2000	O
,	O
weather	B-Application
radar	I-Application
networks	O
became	O
the	O
norm	O
in	O
North	O
America	O
,	O
Europe	O
,	O
Japan	O
and	O
other	O
developed	O
countries	O
.	O
</s>
<s>
Conventional	O
radars	B-Application
were	O
replaced	O
by	O
Doppler	O
radars	B-Application
,	O
which	O
in	O
addition	O
to	O
position	O
and	O
intensity	O
could	O
track	O
the	O
relative	O
velocity	O
of	O
the	O
particles	O
in	O
the	O
air	O
.	O
</s>
<s>
In	O
the	O
United	O
States	O
,	O
the	O
construction	O
of	O
a	O
network	O
consisting	O
of	O
10cm	O
radars	B-Application
,	O
called	O
NEXRAD	B-Algorithm
or	O
WSR-88D	B-Algorithm
(	O
Weather	B-Application
Surveillance	I-Application
Radar	I-Application
1988	O
Doppler	O
)	O
,	O
was	O
started	O
in	O
1988	O
following	O
NSSL	O
's	O
research	O
.	O
</s>
<s>
In	O
Canada	O
,	O
Environment	O
Canada	O
constructed	O
the	O
King	O
City	O
station	O
,	O
with	O
a	O
5cm	O
research	O
Doppler	O
radar	B-Application
,	O
by	O
1985	O
;	O
McGill	O
University	O
dopplerized	O
its	O
radar	B-Application
(	O
J	O
.	O
S	O
.	O
Marshall	O
Radar	B-Application
Observatory	O
)	O
in	O
1993	O
.	O
</s>
<s>
This	O
led	O
to	O
a	O
complete	O
Canadian	B-Algorithm
Doppler	I-Algorithm
network	I-Algorithm
between	O
1998	O
and	O
2004	O
.	O
</s>
<s>
In	O
April	O
2013	O
,	O
all	O
United	O
States	O
National	O
Weather	O
Service	O
NEXRADs	B-Algorithm
were	O
completely	O
dual-polarized	O
.	O
</s>
<s>
Since	O
2003	O
,	O
the	O
U.S.	O
National	O
Oceanic	O
and	O
Atmospheric	O
Administration	O
has	O
been	O
experimenting	O
with	O
phased-array	O
radar	B-Application
as	O
a	O
replacement	O
for	O
conventional	O
parabolic	O
antenna	O
to	O
provide	O
more	O
time	O
resolution	O
in	O
atmospheric	O
sounding	O
.	O
</s>
<s>
Also	O
in	O
2003	O
,	O
the	O
National	O
Science	O
Foundation	O
established	O
the	O
Engineering	O
Research	O
Center	O
for	O
Collaborative	O
Adaptive	O
Sensing	O
of	O
the	O
Atmosphere	O
(	O
CASA	O
)	O
,	O
a	O
multidisciplinary	O
,	O
multi-university	O
collaboration	O
of	O
engineers	O
,	O
computer	O
scientists	O
,	O
meteorologists	O
,	O
and	O
sociologists	O
to	O
conduct	O
fundamental	O
research	O
,	O
develop	O
enabling	O
technology	O
,	O
and	O
deploy	O
prototype	O
engineering	O
systems	O
designed	O
to	O
augment	O
existing	O
radar	B-Application
systems	I-Application
by	O
sampling	O
the	O
generally	O
undersampled	O
lower	O
troposphere	O
with	O
inexpensive	O
,	O
fast	O
scanning	O
,	O
dual	O
polarization	O
,	O
mechanically	O
scanned	O
and	O
phased	O
array	O
radars	B-Application
.	O
</s>
<s>
Weather	B-Application
radars	I-Application
send	O
directional	O
pulses	O
of	O
microwave	O
radiation	O
,	O
on	O
the	O
order	O
of	O
one	O
microsecond	O
long	O
,	O
using	O
a	O
cavity	B-Architecture
magnetron	I-Architecture
or	O
klystron	O
tube	O
connected	O
by	O
a	O
waveguide	O
to	O
a	O
parabolic	O
antenna	O
.	O
</s>
<s>
This	O
means	O
that	O
part	O
of	O
the	O
energy	O
of	O
each	O
pulse	O
will	O
bounce	O
off	O
these	O
small	O
particles	O
,	O
back	O
towards	O
the	O
radar	B-Application
station	I-Application
.	O
</s>
<s>
Thus	O
10cm	O
(	O
S-band	B-Algorithm
)	O
radar	B-Application
is	O
preferred	O
but	O
is	O
more	O
expensive	O
than	O
a	O
5cm	O
C-band	B-Algorithm
system	O
.	O
</s>
<s>
3cm	O
X-band	B-Algorithm
radar	I-Algorithm
is	O
used	O
only	O
for	O
short-range	O
units	O
,	O
and	O
1cm	O
Ka-band	O
weather	B-Application
radar	I-Application
is	O
used	O
only	O
for	O
research	O
on	O
small-particle	O
phenomena	O
such	O
as	O
drizzle	O
and	O
fog	O
.	O
</s>
<s>
W	B-Algorithm
band	I-Algorithm
(	O
3mm	O
)	O
weather	B-Application
radar	I-Application
systems	O
have	O
seen	O
limited	O
university	O
use	O
,	O
but	O
due	O
to	O
quicker	O
attenuation	O
,	O
most	O
data	O
are	O
not	O
operational	O
.	O
</s>
<s>
Radar	B-Application
pulses	O
diverge	O
as	O
they	O
move	O
away	O
from	O
the	O
radar	B-Application
station	I-Application
.	O
</s>
<s>
Thus	O
the	O
volume	O
of	O
air	O
that	O
a	O
radar	B-Application
pulse	O
is	O
traversing	O
is	O
larger	O
for	O
areas	O
farther	O
away	O
from	O
the	O
station	O
,	O
and	O
smaller	O
for	O
nearby	O
areas	O
,	O
decreasing	O
resolution	O
at	O
farther	O
distances	O
.	O
</s>
<s>
meters	O
,	O
calculated	O
from	O
the	O
duration	O
in	O
seconds	O
of	O
the	O
pulse	O
times	O
the	O
speed	O
of	O
light	O
)	O
,	O
r	O
is	O
the	O
distance	O
from	O
the	O
radar	B-Application
that	O
the	O
pulse	O
has	O
already	O
traveled	O
(	O
in	O
e.g.	O
</s>
<s>
Between	O
each	O
pulse	O
,	O
the	O
radar	B-Application
station	I-Application
serves	O
as	O
a	O
receiver	O
as	O
it	O
listens	O
for	O
return	O
signals	O
from	O
particles	O
in	O
the	O
air	O
.	O
</s>
<s>
Since	O
the	O
Earth	O
is	O
round	O
,	O
the	O
radar	B-Application
beam	O
in	O
vacuum	O
would	O
rise	O
according	O
to	O
the	O
reverse	O
curvature	O
of	O
the	O
Earth	O
.	O
</s>
<s>
This	O
bends	O
the	O
radar	B-Application
beam	O
slightly	O
toward	O
the	O
ground	O
and	O
with	O
a	O
standard	O
atmosphere	O
this	O
is	O
equivalent	O
to	O
considering	O
that	O
the	O
curvature	O
of	O
the	O
beam	O
is	O
4/3	O
the	O
actual	O
curvature	O
of	O
the	O
Earth	O
.	O
</s>
<s>
r	O
=	O
distance	O
radar	B-Application
–	O
target	O
,	O
</s>
<s>
ae	O
=	O
Earth	O
radius	B-Protocol
,	O
</s>
<s>
θe	O
=	O
elevation	O
angle	O
above	O
the	O
radar	B-Algorithm
horizon	I-Algorithm
,	O
</s>
<s>
A	O
weather	B-Application
radar	I-Application
network	O
uses	O
a	O
series	O
of	O
typical	O
angles	O
that	O
are	O
set	O
according	O
to	O
its	O
needs	O
.	O
</s>
<s>
This	O
scenario	O
will	O
be	O
repeated	O
on	O
many	O
angles	O
to	O
scan	O
the	O
entire	O
volume	O
of	O
air	O
around	O
the	O
radar	B-Application
within	O
the	O
maximum	O
range	O
.	O
</s>
<s>
Usually	O
,	O
the	O
scanning	O
strategy	O
is	O
completed	O
within	O
5	O
to	O
10	O
minutes	O
to	O
have	O
data	O
within	O
15km	O
above	O
ground	O
and	O
250km	O
distance	O
of	O
the	O
radar	B-Application
.	O
</s>
<s>
For	O
instance	O
in	O
Canada	O
,	O
the	O
5cm	O
weather	B-Application
radars	I-Application
use	O
angles	O
ranging	O
from	O
0.3	O
to	O
25	O
degrees	O
.	O
</s>
<s>
Due	O
to	O
the	O
Earth	O
's	O
curvature	O
and	O
change	O
of	O
index	O
of	O
refraction	O
with	O
height	O
,	O
the	O
radar	B-Application
cannot	O
"	O
see	O
"	O
below	O
the	O
height	O
above	O
ground	O
of	O
the	O
minimal	O
angle	O
(	O
shown	O
in	O
green	O
)	O
or	O
closer	O
to	O
the	O
radar	B-Application
than	O
the	O
maximal	O
one	O
(	O
shown	O
as	O
a	O
red	O
cone	O
in	O
the	O
center	O
)	O
.	O
</s>
<s>
Because	O
the	O
targets	O
are	O
not	O
unique	O
in	O
each	O
volume	O
,	O
the	O
radar	B-Application
equation	O
has	O
to	O
be	O
developed	O
beyond	O
the	O
basic	O
one	O
.	O
</s>
<s>
Assuming	O
a	O
monostatic	B-Algorithm
radar	I-Algorithm
where	O
:	O
</s>
<s>
where	O
is	O
received	O
power	O
,	O
is	O
transmitted	O
power	O
,	O
is	O
the	O
gain	O
of	O
the	O
transmitting/receiving	O
antenna	O
,	O
is	O
radar	B-Application
wavelength	O
,	O
is	O
the	O
radar	B-Application
cross	O
section	O
of	O
the	O
target	O
and	O
is	O
the	O
distance	O
from	O
transmitter	O
to	O
target	O
.	O
</s>
<s>
In	O
order	O
to	O
compare	O
the	O
data	O
coming	O
from	O
different	O
distances	O
from	O
the	O
radar	B-Application
,	O
one	O
has	O
to	O
normalize	O
them	O
with	O
this	O
ratio	O
.	O
</s>
<s>
Reflectivity	O
perceived	O
by	O
the	O
radar	B-Application
(	O
Ze	O
)	O
varies	O
by	O
the	O
sixth	O
power	O
of	O
the	O
rain	O
droplets	O
 '	O
diameter	O
(	O
D	O
)	O
,	O
the	O
square	O
of	O
the	O
dielectric	O
constant	O
(	O
K	O
)	O
of	O
the	O
targets	O
and	O
the	O
drop	B-Algorithm
size	I-Algorithm
distribution	I-Algorithm
(	O
e.g.	O
</s>
<s>
Since	O
variation	O
in	O
diameter	O
and	O
dielectric	O
constant	O
of	O
the	O
targets	O
can	O
lead	O
to	O
large	O
variability	O
in	O
power	O
return	O
to	O
the	O
radar	B-Application
,	O
reflectivity	O
is	O
expressed	O
in	O
dBZ	O
(	O
10	O
times	O
the	O
logarithm	O
of	O
the	O
ratio	O
of	O
the	O
echo	O
to	O
a	O
standard	O
1mm	O
diameter	O
drop	O
filling	O
the	O
same	O
scanned	O
volume	O
)	O
.	O
</s>
<s>
Radar	B-Application
returns	O
are	O
usually	O
described	O
by	O
colour	O
or	O
level	O
.	O
</s>
<s>
The	O
colours	O
in	O
a	O
radar	B-Application
image	O
normally	O
range	O
from	O
blue	O
or	O
green	O
for	O
weak	O
returns	O
,	O
to	O
red	O
or	O
magenta	O
for	O
very	O
strong	O
returns	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
U.S.	O
National	O
NEXRAD	B-Algorithm
radar	B-Application
sites	O
use	O
the	O
following	O
scale	O
for	O
different	O
levels	O
of	O
reflectivity	O
:	O
</s>
<s>
Strong	O
returns	O
(	O
red	O
or	O
magenta	O
)	O
may	O
indicate	O
not	O
only	O
heavy	O
rain	O
but	O
also	O
thunderstorms	O
,	O
hail	O
,	O
strong	O
winds	O
,	O
or	O
tornadoes	B-Language
,	O
but	O
they	O
need	O
to	O
be	O
interpreted	O
carefully	O
,	O
for	O
reasons	O
described	O
below	O
.	O
</s>
<s>
When	O
describing	O
weather	B-Application
radar	I-Application
returns	O
,	O
pilots	O
,	O
dispatchers	O
,	O
and	O
air	O
traffic	O
controllers	O
will	O
typically	O
refer	O
to	O
three	O
return	O
levels	O
:	O
</s>
<s>
level	O
1	O
corresponds	O
to	O
a	O
green	O
radar	B-Application
return	O
,	O
indicating	O
usually	O
light	O
precipitation	O
and	O
little	O
to	O
no	O
turbulence	O
,	O
leading	O
to	O
a	O
possibility	O
of	O
reduced	O
visibility	O
.	O
</s>
<s>
level	O
2	O
corresponds	O
to	O
a	O
yellow	O
radar	B-Application
return	O
,	O
indicating	O
moderate	O
precipitation	O
,	O
leading	O
to	O
the	O
possibility	O
of	O
very	O
low	O
visibility	O
,	O
moderate	O
turbulence	O
and	O
an	O
uncomfortable	O
ride	O
for	O
aircraft	O
passengers	O
.	O
</s>
<s>
level	O
3	O
corresponds	O
to	O
a	O
red	O
radar	B-Application
return	O
,	O
indicating	O
heavy	O
precipitation	O
,	O
leading	O
to	O
the	O
possibility	O
of	O
thunderstorms	O
and	O
severe	O
turbulence	O
and	O
structural	O
damage	O
to	O
the	O
aircraft	O
.	O
</s>
<s>
This	O
is	O
not	O
an	O
analysis	O
of	O
the	O
radar	B-Application
data	O
itself	O
but	O
a	O
post-treatment	O
done	O
with	O
other	O
data	O
sources	O
,	O
the	O
primary	O
being	O
surface	O
reports	O
(	O
METAR	O
)	O
.	O
</s>
<s>
Over	O
the	O
area	O
covered	O
by	O
radar	B-Application
echoes	O
,	O
a	O
program	O
assigns	O
a	O
precipitation	O
type	O
according	O
to	O
the	O
surface	O
temperature	O
and	O
dew	O
point	O
reported	O
at	O
the	O
underlying	O
weather	O
stations	O
.	O
</s>
<s>
Then	O
the	O
program	O
does	O
interpolations	B-Algorithm
to	O
produce	O
an	O
image	O
with	O
defined	O
zones	O
.	O
</s>
<s>
These	O
will	O
include	O
interpolation	B-Algorithm
errors	O
due	O
to	O
the	O
calculation	O
.	O
</s>
<s>
More	O
sophisticated	O
programs	O
use	O
the	O
numerical	B-General_Concept
weather	I-General_Concept
prediction	I-General_Concept
output	O
from	O
models	O
,	O
such	O
as	O
NAM	B-Application
and	O
WRF	B-Application
,	O
for	O
the	O
precipitation	O
types	O
and	O
apply	O
it	O
as	O
a	O
first	O
guess	O
to	O
the	O
radar	B-Application
echoes	O
,	O
then	O
use	O
the	O
surface	O
data	O
for	O
final	O
output	O
.	O
</s>
<s>
Until	O
dual-polarization	O
(	O
section	O
Polarization	O
below	O
)	O
data	O
are	O
widely	O
available	O
,	O
any	O
precipitation	O
types	O
on	O
radar	B-Application
images	O
are	O
only	O
indirect	O
information	O
and	O
must	O
be	O
taken	O
with	O
care	O
.	O
</s>
<s>
Light	O
precipitation	O
such	O
as	O
drops	O
and	O
flakes	O
is	O
subject	O
to	O
the	O
air	O
currents	O
,	O
and	O
scanning	O
radar	B-Application
can	O
pick	O
up	O
the	O
horizontal	O
component	O
of	O
this	O
motion	O
,	O
thus	O
giving	O
the	O
possibility	O
to	O
estimate	O
the	O
wind	O
speed	O
and	O
direction	O
where	O
precipitation	O
is	O
present	O
.	O
</s>
<s>
A	O
target	O
's	O
motion	O
relative	O
to	O
the	O
radar	B-Application
station	I-Application
causes	O
a	O
change	O
in	O
the	O
reflected	O
frequency	O
of	O
the	O
radar	B-Application
pulse	O
,	O
due	O
to	O
the	O
Doppler	O
effect	O
.	O
</s>
<s>
With	O
velocities	O
of	O
less	O
than	O
70-metre/second	O
for	O
weather	O
echos	O
and	O
radar	B-Application
wavelength	O
of	O
10cm	O
,	O
this	O
amounts	O
to	O
a	O
change	O
only	O
0.1	O
ppm	O
.	O
</s>
<s>
Doppler	B-Application
weather	I-Application
radars	I-Application
use	O
this	O
phase	O
difference	O
(	O
pulse	O
pair	O
difference	O
)	O
to	O
calculate	O
the	O
precipitation	O
's	O
motion	O
.	O
</s>
<s>
This	O
speed	O
is	O
called	O
the	O
radial	B-Protocol
Doppler	O
velocity	O
because	O
it	O
gives	O
only	O
the	O
radial	B-Protocol
variation	O
of	O
distance	O
versus	O
time	O
between	O
the	O
radar	B-Application
and	O
the	O
target	O
.	O
</s>
<s>
For	O
a	O
10cm	O
radar	B-Application
such	O
as	O
the	O
NEXRAD	B-Algorithm
,	O
the	O
unambiguous	O
velocity	O
range	O
would	O
be	O
doubled	O
.	O
</s>
<s>
In	O
a	O
uniform	O
rainstorm	O
moving	O
eastward	O
,	O
a	O
radar	B-Application
beam	O
pointing	O
west	O
will	O
"	O
see	O
"	O
the	O
raindrops	O
moving	O
toward	O
itself	O
,	O
while	O
a	O
beam	O
pointing	O
east	O
will	O
"	O
see	O
"	O
the	O
drops	O
moving	O
away	O
.	O
</s>
<s>
In	O
the	O
synoptic	O
scale	O
interpretation	O
,	O
the	O
user	O
can	O
extract	O
the	O
wind	O
at	O
different	O
levels	O
over	O
the	O
radar	B-Application
coverage	O
region	O
.	O
</s>
<s>
As	O
the	O
beam	O
is	O
scanning	O
360	O
degrees	O
around	O
the	O
radar	B-Application
,	O
data	O
will	O
come	O
from	O
all	O
those	O
angles	O
and	O
be	O
the	O
radial	B-Protocol
projection	O
of	O
the	O
actual	O
wind	O
on	O
the	O
individual	O
angle	O
.	O
</s>
<s>
One	O
can	O
then	O
calculate	O
the	O
direction	O
and	O
the	O
strength	O
of	O
the	O
motion	O
of	O
particles	O
as	O
long	O
as	O
there	O
is	O
enough	O
coverage	O
on	O
the	O
radar	B-Application
screen	O
.	O
</s>
<s>
As	O
the	O
radar	B-Application
only	O
sees	O
the	O
radial	B-Protocol
component	O
and	O
has	O
a	O
certain	O
elevation	O
from	O
ground	O
,	O
the	O
radial	B-Protocol
velocities	O
are	O
contaminated	O
by	O
some	O
fraction	O
of	O
the	O
falling	O
speed	O
.	O
</s>
<s>
In	O
the	O
velocity	O
data	O
,	O
there	O
could	O
be	O
smaller	O
zones	O
in	O
the	O
radar	B-Application
coverage	O
where	O
the	O
wind	O
varies	O
from	O
the	O
one	O
mentioned	O
above	O
.	O
</s>
<s>
These	O
may	O
only	O
cover	O
few	O
square	O
kilometers	O
but	O
are	O
visible	O
by	O
variations	O
in	O
the	O
radial	B-Protocol
speed	O
.	O
</s>
<s>
This	O
causes	O
the	O
water	O
molecule	O
dipole	O
to	O
be	O
oriented	O
in	O
that	O
direction	O
;	O
so	O
,	O
radar	B-Application
beams	O
are	O
,	O
generally	O
,	O
polarized	O
horizontally	O
in	O
order	O
to	O
receive	O
the	O
maximal	O
signal	O
reflection	O
.	O
</s>
<s>
High	O
values	O
,	O
near	O
one	O
,	O
indicate	O
homogeneous	O
precipitation	O
types	O
,	O
while	O
lower	O
values	O
indicate	O
regions	O
of	O
mixed	O
precipitation	O
types	O
,	O
such	O
as	O
rain	O
and	O
snow	O
,	O
or	O
hail	O
,	O
or	O
in	O
extreme	O
cases	O
debris	O
aloft	O
,	O
usually	O
coinciding	O
with	O
a	O
tornado	B-Language
debris	O
signature	O
and	O
a	O
tornado	B-Algorithm
vortex	I-Algorithm
signature	I-Algorithm
.	O
</s>
<s>
With	O
more	O
information	O
about	O
particle	O
shape	O
,	O
dual-polarization	O
radars	B-Application
can	O
more	O
easily	O
distinguish	O
airborne	O
debris	O
from	O
precipitation	O
,	O
making	O
it	O
easier	O
to	O
locate	O
tornados	O
.	O
</s>
<s>
With	O
this	O
new	O
knowledge	O
added	O
to	O
the	O
reflectivity	O
,	O
velocity	O
,	O
and	O
spectrum	O
width	O
produced	O
by	O
Doppler	B-Application
weather	I-Application
radars	I-Application
,	O
researchers	O
have	O
been	O
working	O
on	O
developing	O
algorithms	O
to	O
differentiate	O
precipitation	O
types	O
,	O
non-meteorological	O
targets	O
,	O
and	O
to	O
produce	O
better	O
rainfall	O
accumulation	O
estimates	O
.	O
</s>
<s>
NOAA	O
established	O
a	O
test	O
deployment	O
for	O
dual-polametric	O
radar	B-Application
at	O
NSSL	O
and	O
equipped	O
all	O
its	O
10cm	O
NEXRAD	B-Algorithm
radars	B-Application
with	O
dual-polarization	O
,	O
which	O
was	O
completed	O
in	O
April	O
2013	O
.	O
</s>
<s>
In	O
2004	O
,	O
ARMOR	B-Algorithm
Doppler	I-Algorithm
Weather	I-Algorithm
Radar	I-Algorithm
in	O
Huntsville	O
,	O
Alabama	O
was	O
equipped	O
with	O
a	O
SIGMET	O
Antenna	O
Mounted	O
Receiver	O
,	O
giving	O
Dual-Polarmetric	O
capabilities	O
to	O
the	O
operator	O
.	O
</s>
<s>
McGill	O
University	O
J	O
.	O
S	O
.	O
Marshall	O
Radar	B-Application
Observatory	O
in	O
Montreal	O
,	O
Canada	O
has	O
converted	O
its	O
instrument	O
(	O
1999	O
)	O
and	O
the	O
data	O
were	O
used	O
operationally	O
by	O
Environment	O
Canada	O
in	O
Montreal	O
until	O
its	O
closure	O
in	O
2018	O
.	O
</s>
<s>
Another	O
Environment	O
Canada	O
radar	B-Application
,	O
in	O
King	O
City	O
(	O
North	O
of	O
Toronto	O
)	O
,	O
was	O
dual-polarized	O
in	O
2005	O
;	O
it	O
uses	O
a	O
5cm	O
wavelength	O
,	O
which	O
experiences	O
greater	O
attenuation	O
.	O
</s>
<s>
Environment	O
Canada	O
is	O
converting	O
graually	O
all	O
of	O
its	O
radars	B-Application
to	O
dual-polarization	O
.	O
</s>
<s>
Météo-France	O
is	O
planning	O
on	O
incorporating	O
dual-polarizing	O
Doppler	O
radar	B-Application
in	O
its	O
network	O
coverage	O
.	O
</s>
<s>
Various	O
methods	O
of	O
displaying	O
data	O
from	O
radar	B-Application
scans	O
have	O
been	O
developed	O
over	O
time	O
to	O
address	O
the	O
needs	O
of	O
its	O
users	O
.	O
</s>
<s>
Since	O
data	O
are	O
obtained	O
one	O
angle	O
at	O
a	O
time	O
,	O
the	O
first	O
way	O
of	O
displaying	O
them	O
has	O
been	O
the	O
Plan	O
Position	O
Indicator	O
(	O
PPI	O
)	O
which	O
is	O
only	O
the	O
layout	O
of	O
radar	B-Application
return	O
on	O
a	O
two	O
dimensional	O
image	O
.	O
</s>
<s>
Importantly	O
,	O
the	O
data	O
coming	O
from	O
different	O
distances	O
to	O
the	O
radar	B-Application
are	O
at	O
different	O
heights	O
above	O
ground	O
.	O
</s>
<s>
This	O
is	O
very	O
important	O
as	O
a	O
high	O
rain	O
rate	O
seen	O
near	O
the	O
radar	B-Application
is	O
relatively	O
close	O
to	O
what	O
reaches	O
the	O
ground	O
but	O
what	O
is	O
seen	O
from	O
160km	O
away	O
is	O
about	O
1.5km	O
above	O
ground	O
and	O
could	O
be	O
far	O
different	O
from	O
the	O
amount	O
reaching	O
the	O
surface	O
.	O
</s>
<s>
It	O
is	O
thus	O
difficult	O
to	O
compare	O
weather	O
echoes	O
at	O
different	O
distances	O
from	O
the	O
radar	B-Application
.	O
</s>
<s>
PPIs	O
are	O
affected	O
by	O
ground	O
echoes	O
near	O
the	O
radar	B-Application
.	O
</s>
<s>
When	O
looking	O
at	O
the	O
general	O
motion	O
of	O
the	O
rain	O
to	O
extract	O
wind	O
at	O
different	O
altitudes	O
,	O
it	O
is	O
better	O
to	O
use	O
data	O
relative	O
to	O
the	O
radar	B-Application
.	O
</s>
<s>
To	O
avoid	O
some	O
of	O
the	O
PPI	O
problems	O
,	O
the	O
constant-altitude	O
plan	O
position	O
indicator	O
(	O
CAPPI	B-Algorithm
)	O
has	O
been	O
developed	O
by	O
Canadian	O
researchers	O
.	O
</s>
<s>
It	O
is	O
a	O
horizontal	O
cross-section	O
through	O
radar	B-Application
data	O
.	O
</s>
<s>
This	O
way	O
,	O
one	O
can	O
compare	O
precipitation	O
on	O
an	O
equal	O
footing	O
at	O
difference	O
distance	O
from	O
the	O
radar	B-Application
and	O
avoid	O
ground	O
echoes	O
.	O
</s>
<s>
Although	O
data	O
are	O
taken	O
at	O
a	O
certain	O
height	O
above	O
ground	O
,	O
a	O
relation	O
can	O
be	O
inferred	O
between	O
ground	O
stations	O
 '	O
reports	O
and	O
the	O
radar	B-Application
data	O
.	O
</s>
<s>
CAPPIs	B-Algorithm
call	O
for	O
a	O
large	O
number	O
of	O
angles	O
from	O
near	O
the	O
horizontal	O
to	O
near	O
the	O
vertical	O
of	O
the	O
radar	B-Application
to	O
have	O
a	O
cut	O
that	O
is	O
as	O
close	O
as	O
possible	O
at	O
all	O
distance	O
to	O
the	O
height	O
needed	O
.	O
</s>
<s>
Even	O
then	O
,	O
after	O
a	O
certain	O
distance	O
,	O
there	O
is	O
n't	O
any	O
angle	O
available	O
and	O
the	O
CAPPI	B-Algorithm
becomes	O
the	O
PPI	O
of	O
the	O
lowest	O
angle	O
.	O
</s>
<s>
The	O
zigzag	O
line	O
on	O
the	O
angles	O
diagram	O
above	O
shows	O
the	O
data	O
used	O
to	O
produce	O
1.5km	O
and	O
4km	O
height	O
CAPPIs	B-Algorithm
.	O
</s>
<s>
Since	O
the	O
CAPPI	B-Algorithm
uses	O
the	O
closest	O
angle	O
to	O
the	O
desired	O
height	O
at	O
each	O
point	O
from	O
the	O
radar	B-Application
,	O
the	O
data	O
can	O
originate	O
from	O
slightly	O
different	O
altitudes	O
,	O
as	O
seen	O
on	O
the	O
image	O
,	O
in	O
different	O
points	O
of	O
the	O
radar	B-Application
coverage	O
.	O
</s>
<s>
Reflectivity	O
data	O
being	O
relatively	O
smooth	O
with	O
height	O
,	O
CAPPIs	B-Algorithm
are	O
mostly	O
used	O
for	O
displaying	O
them	O
.	O
</s>
<s>
Velocity	O
data	O
,	O
on	O
the	O
other	O
hand	O
,	O
can	O
change	O
rapidly	O
in	O
direction	O
with	O
height	O
and	O
CAPPIs	B-Algorithm
of	O
them	O
are	O
not	O
common	O
.	O
</s>
<s>
It	O
seems	O
that	O
only	O
McGill	O
University	O
is	O
producing	O
regularly	O
Doppler	O
CAPPIs	B-Algorithm
with	O
the	O
24	O
angles	O
available	O
on	O
their	O
radar	B-Application
.	O
</s>
<s>
However	O
,	O
some	O
researchers	O
have	O
published	O
papers	O
using	O
velocity	O
CAPPIs	B-Algorithm
to	O
study	O
tropical	O
cyclones	O
and	O
development	O
of	O
NEXRAD	B-Algorithm
products	O
.	O
</s>
<s>
There	O
does	O
n't	O
seem	O
to	O
have	O
regular	O
use	O
of	O
CAPPI	B-Algorithm
for	O
them	O
although	O
the	O
SIGMET	O
company	O
offer	O
a	O
software	O
capable	O
to	O
produce	O
those	O
types	O
of	O
images	O
.	O
</s>
<s>
The	O
American	O
National	O
Weather	O
Service	O
is	O
using	O
such	O
Composite	O
as	O
their	O
scanning	O
scheme	O
can	O
vary	O
from	O
4	O
to	O
14	O
angles	O
,	O
according	O
to	O
their	O
need	O
,	O
which	O
would	O
make	O
very	O
coarse	O
CAPPIs	B-Algorithm
.	O
</s>
<s>
Another	O
important	O
use	O
of	O
radar	B-Application
data	O
is	O
the	O
ability	O
to	O
assess	O
the	O
amount	O
of	O
precipitation	O
that	O
has	O
fallen	O
over	O
large	O
basins	O
,	O
to	O
be	O
used	O
in	O
hydrological	O
calculations	O
;	O
such	O
data	O
is	O
useful	O
in	O
flood	O
control	O
,	O
sewer	O
management	O
and	O
dam	O
construction	O
.	O
</s>
<s>
The	O
computed	O
data	O
from	O
radar	B-Application
weather	O
may	O
be	O
used	O
in	O
conjunction	O
with	O
data	O
from	O
ground	O
stations	O
.	O
</s>
<s>
To	O
produce	O
radar	B-Application
accumulations	O
,	O
we	O
have	O
to	O
estimate	O
the	O
rain	O
rate	O
over	O
a	O
point	O
by	O
the	O
average	O
value	O
over	O
that	O
point	O
between	O
one	O
PPI	O
,	O
or	O
CAPPI	B-Algorithm
,	O
and	O
the	O
next	O
;	O
then	O
multiply	O
by	O
the	O
time	O
between	O
those	O
images	O
.	O
</s>
<s>
Aviation	O
is	O
a	O
heavy	O
user	O
of	O
radar	B-Application
data	O
.	O
</s>
<s>
Most	O
country	O
weather	B-Application
radars	I-Application
are	O
scanning	O
enough	O
angles	O
to	O
have	O
a	O
3D	O
set	O
of	O
data	O
over	O
the	O
area	O
of	O
coverage	O
.	O
</s>
<s>
To	O
know	O
the	O
vertical	O
structure	O
of	O
clouds	O
,	O
in	O
particular	O
thunderstorms	O
or	O
the	O
level	O
of	O
the	O
melting	O
layer	O
,	O
a	O
vertical	O
cross-section	O
product	O
of	O
the	O
radar	B-Application
data	O
is	O
available	O
to	O
meteorologist	O
.	O
</s>
<s>
When	O
a	O
weather	B-Application
radar	I-Application
is	O
scanning	O
in	O
only	O
one	O
direction	O
vertically	O
,	O
it	O
obtains	O
high	O
resolution	O
data	O
along	O
a	O
vertical	O
cut	O
of	O
the	O
atmosphere	O
.	O
</s>
<s>
This	O
is	O
different	O
from	O
the	O
vertical	O
cross	O
section	O
mentioned	O
above	O
by	O
the	O
fact	O
that	O
the	O
radar	B-Application
is	O
making	O
a	O
vertical	O
cut	O
along	O
specific	O
directions	O
and	O
does	O
not	O
scan	O
over	O
the	O
entire	O
360	O
degrees	O
around	O
the	O
site	O
.	O
</s>
<s>
This	O
kind	O
of	O
sounding	O
and	O
product	O
is	O
typically	O
only	O
available	O
on	O
research	O
radars	B-Application
.	O
</s>
<s>
Over	O
the	O
past	O
few	O
decades	O
,	O
radar	B-Application
networks	O
have	O
been	O
extended	O
to	O
allow	O
the	O
production	O
of	O
composite	O
views	O
covering	O
large	O
areas	O
.	O
</s>
<s>
For	O
instance	O
,	O
all	O
major	O
countries	O
such	O
as	O
the	O
United	O
States	O
,	O
Canada	O
,	O
much	O
of	O
Europe	O
and	O
Japan	O
,	O
produce	O
images	O
that	O
include	O
all	O
of	O
their	O
radars	B-Application
.	O
</s>
<s>
In	O
fact	O
,	O
such	O
a	O
network	O
can	O
consist	O
of	O
different	O
types	O
of	O
radar	B-Application
with	O
different	O
characteristics	O
like	O
beam	O
width	O
,	O
wavelength	O
and	O
calibration	O
.	O
</s>
<s>
These	O
differences	O
have	O
to	O
be	O
taken	O
into	O
account	O
when	O
matching	O
data	O
across	O
the	O
network	O
,	O
particularly	O
to	O
decide	O
what	O
data	O
to	O
use	O
when	O
two	O
radars	B-Application
cover	O
the	O
same	O
point	O
.	O
</s>
<s>
If	O
one	O
uses	O
the	O
stronger	O
echo	O
but	O
it	O
comes	O
from	O
the	O
most	O
distant	O
radar	B-Application
,	O
one	O
uses	O
returns	O
that	O
are	O
from	O
higher	O
altitude	O
coming	O
from	O
rain	O
or	O
snow	O
that	O
might	O
evaporate	O
before	O
reaching	O
the	O
ground	O
(	O
virga	O
)	O
.	O
</s>
<s>
If	O
one	O
uses	O
data	O
from	O
the	O
closest	O
radar	B-Application
,	O
it	O
might	O
be	O
attenuated	O
passing	O
through	O
a	O
thunderstorm	O
.	O
</s>
<s>
Composite	O
images	O
of	O
precipitations	O
using	O
a	O
network	O
of	O
radars	B-Application
are	O
made	O
with	O
all	O
those	O
limitations	O
in	O
mind	O
.	O
</s>
<s>
To	O
help	O
meteorologists	O
spot	O
dangerous	O
weather	O
,	O
mathematical	O
algorithms	O
have	O
been	O
introduced	O
in	O
the	O
weather	B-Application
radar	I-Application
treatment	O
programs	O
.	O
</s>
<s>
Vertically	B-Algorithm
Integrated	I-Algorithm
Liquid	I-Algorithm
(	O
VIL	O
)	O
is	O
an	O
estimate	O
of	O
the	O
total	O
mass	O
of	O
precipitation	O
in	O
the	O
clouds	O
.	O
</s>
<s>
Potential	O
wind	O
gust	O
,	O
which	O
can	O
estimate	O
the	O
winds	O
under	O
a	O
cloud	O
(	O
a	O
downdraft	O
)	O
using	O
the	O
VIL	O
and	O
the	O
height	O
of	O
the	O
echotops	O
(	O
radar	B-Application
estimated	O
top	O
of	O
the	O
cloud	O
)	O
for	O
a	O
given	O
storm	O
cell	O
.	O
</s>
<s>
The	O
algorithm	O
is	O
searching	O
for	O
a	O
"	O
doublet	O
"	O
of	O
inbound/outbound	O
velocities	O
with	O
the	O
zero	O
line	O
of	O
velocities	O
,	O
between	O
the	O
two	O
,	O
along	O
a	O
radial	B-Protocol
line	O
from	O
the	O
radar	B-Application
.	O
</s>
<s>
TVS	O
or	O
Tornado	B-Algorithm
Vortex	I-Algorithm
Signature	I-Algorithm
algorithm	O
is	O
essentially	O
a	O
mesocyclone	O
with	O
a	O
large	O
velocity	O
threshold	O
found	O
through	O
many	O
scanning	O
angles	O
.	O
</s>
<s>
This	O
algorithm	O
is	O
used	O
in	O
NEXRAD	B-Algorithm
to	O
indicate	O
the	O
possibility	O
of	O
a	O
tornado	B-Language
formation	O
.	O
</s>
<s>
This	O
algorithm	O
detects	O
variation	O
of	O
wind	O
velocities	O
from	O
point	O
to	O
point	O
in	O
the	O
data	O
and	O
looking	O
for	O
a	O
doublet	O
of	O
inbound/outbound	O
velocities	O
with	O
the	O
zero	O
line	O
perpendicular	O
to	O
the	O
radar	B-Application
beam	O
.	O
</s>
<s>
The	O
animation	O
of	O
radar	B-Application
products	O
can	O
show	O
the	O
evolution	O
of	O
reflectivity	O
and	O
velocity	O
patterns	O
.	O
</s>
<s>
A	O
new	O
popular	O
presentation	O
of	O
weather	B-Application
radar	I-Application
data	O
in	O
United	O
States	O
is	O
via	O
Radar	B-Application
Integrated	O
Display	O
with	O
Geospatial	O
Elements	O
(	O
RIDGE	O
)	O
in	O
which	O
the	O
radar	B-Application
data	O
is	O
projected	O
on	O
a	O
map	O
with	O
geospatial	O
elements	O
such	O
as	O
topography	O
maps	O
,	O
highways	O
,	O
state/county	O
boundaries	O
and	O
weather	O
warnings	O
.	O
</s>
<s>
It	O
is	O
frequently	O
used	O
in	O
conjunction	O
with	O
animations	O
of	O
radar	B-Application
data	O
over	O
a	O
time	O
period	O
.	O
</s>
<s>
Radar	B-Application
data	O
interpretation	O
depends	O
on	O
many	O
hypotheses	O
about	O
the	O
atmosphere	O
and	O
the	O
weather	O
targets	O
,	O
including	O
:	O
</s>
<s>
The	O
first	O
assumption	O
is	O
that	O
the	O
radar	B-Application
beam	O
is	O
moving	O
through	O
air	O
that	O
cools	O
down	O
at	O
a	O
certain	O
rate	O
with	O
height	O
.	O
</s>
<s>
As	O
the	O
index	O
of	O
refraction	O
of	O
air	O
decreases	O
faster	O
than	O
normal	O
the	O
radar	B-Application
beam	O
bends	O
toward	O
the	O
ground	O
instead	O
of	O
continuing	O
upward	O
.	O
</s>
<s>
Eventually	O
,	O
it	O
will	O
hit	O
the	O
ground	O
and	O
be	O
reflected	O
back	O
toward	O
the	O
radar	B-Application
.	O
</s>
<s>
The	O
extreme	O
of	O
this	O
problem	O
is	O
when	O
the	O
inversion	O
is	O
very	O
strong	O
and	O
shallow	O
,	O
the	O
radar	B-Application
beam	O
reflects	O
many	O
times	O
toward	O
the	O
ground	O
as	O
it	O
has	O
to	O
follow	O
a	O
waveguide	O
path	O
.	O
</s>
<s>
This	O
will	O
create	O
multiple	O
bands	O
of	O
strong	O
echoes	O
on	O
the	O
radar	B-Application
images	O
.	O
</s>
<s>
If	O
we	O
want	O
to	O
reliably	O
estimate	O
the	O
precipitation	O
rate	O
,	O
the	O
targets	O
have	O
to	O
be	O
10	O
times	O
smaller	O
than	O
the	O
radar	B-Application
wave	O
according	O
to	O
Rayleigh	O
scattering	O
.	O
</s>
<s>
This	O
is	O
because	O
the	O
water	O
molecule	O
has	O
to	O
be	O
excited	O
by	O
the	O
radar	B-Application
wave	O
to	O
give	O
a	O
return	O
.	O
</s>
<s>
This	O
is	O
relatively	O
true	O
for	O
rain	O
or	O
snow	O
as	O
5	O
or	O
10cm	O
wavelength	O
radars	B-Application
are	O
usually	O
employed	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
very	O
small	O
targets	O
such	O
as	O
cloud	O
droplets	O
are	O
too	O
small	O
to	O
be	O
excited	O
and	O
do	O
not	O
give	O
a	O
recordable	O
return	O
on	O
common	O
weather	B-Application
radars	I-Application
.	O
</s>
<s>
As	O
demonstrated	O
at	O
the	O
start	O
of	O
the	O
article	O
,	O
radar	B-Application
beams	O
have	O
a	O
physical	O
dimension	O
and	O
data	O
are	O
sampled	O
at	O
discrete	O
angles	O
,	O
not	O
continuously	O
,	O
along	O
each	O
angle	O
of	O
elevation	O
.	O
</s>
<s>
In	O
the	O
figure	O
to	O
the	O
left	O
,	O
at	O
the	O
top	O
is	O
a	O
view	O
of	O
a	O
thunderstorm	O
taken	O
by	O
a	O
wind	B-Algorithm
profiler	I-Algorithm
as	O
it	O
was	O
passing	O
overhead	O
.	O
</s>
<s>
Compare	O
this	O
with	O
a	O
simulated	O
view	O
of	O
what	O
a	O
regular	O
weather	B-Application
radar	I-Application
would	O
see	O
at	O
60km	O
,	O
in	O
the	O
bottom	O
of	O
the	O
figure	O
.	O
</s>
<s>
Not	O
only	O
the	O
coarser	O
resolution	O
of	O
the	O
radar	B-Application
blur	O
the	O
image	O
but	O
the	O
sounding	O
incorporates	O
area	O
that	O
are	O
echo	O
free	O
,	O
thus	O
extending	O
the	O
thunderstorm	O
beyond	O
its	O
real	O
boundaries	O
.	O
</s>
<s>
This	O
shows	O
how	O
the	O
output	O
of	O
weather	B-Application
radar	I-Application
is	O
only	O
an	O
approximation	O
of	O
reality	O
.	O
</s>
<s>
The	O
image	O
to	O
the	O
right	O
compares	O
real	O
data	O
from	O
two	O
radars	B-Application
almost	O
colocated	O
.	O
</s>
<s>
The	O
TDWR	B-Algorithm
has	O
about	O
half	O
the	O
beamwidth	O
of	O
the	O
other	O
and	O
one	O
can	O
see	O
twice	O
more	O
details	O
than	O
with	O
the	O
NEXRAD	B-Algorithm
.	O
</s>
<s>
The	O
radar	B-Application
beam	O
has	O
a	O
distribution	O
of	O
energy	O
similar	O
to	O
the	O
diffraction	O
pattern	O
of	O
a	O
light	O
passing	O
through	O
a	O
slit	O
.	O
</s>
<s>
When	O
a	O
secondary	O
lobe	O
hits	O
a	O
reflective	O
target	O
such	O
as	O
a	O
mountain	O
or	O
a	O
strong	O
thunderstorm	O
,	O
some	O
of	O
the	O
energy	O
is	O
reflected	O
to	O
the	O
radar	B-Application
.	O
</s>
<s>
thumb|250px|Idealized	O
energy	O
distribution	O
of	O
a	O
radar	B-Application
beam	O
(	O
Central	O
lobe	O
at	O
0	O
and	O
secondary	O
lobes	O
on	O
each	O
side	O
)	O
thumb|250px|Diffraction	O
by	O
a	O
circular	O
slit	O
simulating	O
the	O
energy	O
viewed	O
by	O
weather	O
targets	O
thumb|300px|The	O
strong	O
echoes	O
are	O
returns	O
of	O
the	O
central	O
peak	O
of	O
the	O
radar	B-Application
from	O
a	O
series	O
of	O
small	O
hills	O
(	O
yellow	O
and	O
reds	O
pixels	B-Algorithm
)	O
.	O
</s>
<s>
Other	O
objects	O
can	O
be	O
misinterpreted	O
as	O
rain	O
or	O
snow	O
by	O
weather	B-Application
radars	I-Application
.	O
</s>
<s>
As	O
such	O
,	O
fine	O
line	O
patterns	O
within	O
weather	B-Application
radar	I-Application
imagery	O
,	O
associated	O
with	O
converging	O
winds	O
,	O
are	O
dominated	O
by	O
insect	O
returns	O
.	O
</s>
<s>
Bird	O
migration	O
,	O
which	O
tends	O
to	O
occur	O
overnight	O
within	O
the	O
lowest	O
2000	O
metres	O
of	O
the	O
Earth	O
's	O
atmosphere	O
,	O
contaminates	O
wind	O
profiles	O
gathered	O
by	O
weather	B-Application
radar	I-Application
,	O
particularly	O
the	O
WSR-88D	B-Algorithm
,	O
by	O
increasing	O
the	O
environmental	O
wind	O
returns	O
by	O
30	O
–	O
60km/hr	O
.	O
</s>
<s>
Other	O
objects	O
within	O
radar	B-Application
imagery	O
include	O
:	O
</s>
<s>
The	O
rotating	O
blades	O
of	O
windmills	B-Application
on	O
modern	O
wind	O
farms	O
can	O
return	O
the	O
radar	B-Application
beam	O
to	O
the	O
radar	B-Application
if	O
they	O
are	O
in	O
its	O
path	O
.	O
</s>
<s>
In	O
some	O
conditions	O
,	O
the	O
radar	B-Application
can	O
even	O
see	O
toward	O
and	O
away	O
velocities	O
that	O
generate	O
false	O
positives	O
for	O
the	O
tornado	B-Algorithm
vortex	I-Algorithm
signature	I-Algorithm
algorithm	O
on	O
weather	B-Application
radar	I-Application
;	O
such	O
an	O
event	O
occurred	O
in	O
2009	O
in	O
Dodge	O
City	O
,	O
Kansas	O
.	O
</s>
<s>
As	O
with	O
other	O
structures	O
that	O
stand	O
in	O
the	O
beam	O
,	O
attenuation	O
of	O
radar	B-Application
returns	O
from	O
beyond	O
windmills	B-Application
may	O
also	O
lead	O
to	O
underestimation	O
.	O
</s>
<s>
Microwaves	O
used	O
in	O
weather	B-Application
radars	I-Application
can	O
be	O
absorbed	O
by	O
rain	O
,	O
depending	O
on	O
the	O
wavelength	O
used	O
.	O
</s>
<s>
For	O
10cm	O
radars	B-Application
,	O
this	O
attenuation	O
is	O
negligible	O
.	O
</s>
<s>
That	O
is	O
the	O
reason	O
why	O
countries	O
with	O
high	O
water	O
content	O
storms	O
are	O
using	O
10cm	O
wavelength	O
,	O
for	O
example	O
the	O
US	O
NEXRAD	B-Algorithm
.	O
</s>
<s>
For	O
a	O
5cm	O
radar	B-Application
,	O
absorption	O
becomes	O
important	O
in	O
heavy	O
rain	O
and	O
this	O
attenuation	O
leads	O
to	O
underestimation	O
of	O
echoes	O
in	O
and	O
beyond	O
a	O
strong	O
thunderstorm	O
.	O
</s>
<s>
Canada	O
and	O
other	O
northern	O
countries	O
use	O
this	O
less	O
costly	O
kind	O
of	O
radar	B-Application
as	O
the	O
precipitation	O
in	O
such	O
areas	O
is	O
usually	O
less	O
intense	O
.	O
</s>
<s>
The	O
images	O
above	O
show	O
how	O
a	O
strong	O
line	O
of	O
echoes	O
seems	O
to	O
vanish	O
as	O
it	O
moves	O
over	O
the	O
radar	B-Application
.	O
</s>
<s>
To	O
compensate	O
for	O
this	O
behaviour	O
,	O
radar	B-Application
sites	O
are	O
often	O
chosen	O
to	O
somewhat	O
overlap	O
in	O
coverage	O
to	O
give	O
different	O
points	O
of	O
view	O
of	O
the	O
same	O
storms	O
.	O
</s>
<s>
Shorter	O
wavelengths	O
are	O
even	O
more	O
attenuated	O
and	O
are	O
only	O
useful	O
on	O
short	O
range	O
radar	B-Application
.	O
</s>
<s>
Many	O
television	O
stations	O
in	O
the	O
United	O
States	O
have	O
5cm	O
radars	B-Application
to	O
cover	O
their	O
audience	O
area	O
.	O
</s>
<s>
Knowing	O
their	O
limitations	O
and	O
using	O
them	O
with	O
the	O
local	O
NEXRAD	B-Algorithm
can	O
supplement	O
the	O
data	O
available	O
to	O
a	O
meteorologist	O
.	O
</s>
<s>
Due	O
to	O
the	O
spread	O
of	O
dual-polarization	O
radar	B-Application
systems	I-Application
,	O
robust	O
and	O
efficient	O
approaches	O
for	O
the	O
compensation	O
of	O
rain	O
attenuation	O
are	O
currently	O
implemented	O
by	O
operational	O
weather	O
services	O
.	O
</s>
<s>
Attenuation	O
correction	O
in	O
weather	B-Application
radars	I-Application
for	O
snow	O
particles	O
is	O
an	O
active	O
research	O
topic	O
.	O
</s>
<s>
A	O
radar	B-Application
beam	O
's	O
reflectivity	O
depends	O
on	O
the	O
diameter	O
of	O
the	O
target	O
and	O
its	O
capacity	O
to	O
reflect	O
.	O
</s>
<s>
However	O
,	O
during	O
the	O
melting	O
process	O
,	O
the	O
radar	B-Application
wave	O
"	O
sees	O
"	O
something	O
akin	O
to	O
very	O
large	O
droplets	O
as	O
snow	O
flakes	O
become	O
coated	O
with	O
water	O
.	O
</s>
<s>
On	O
a	O
PPI	O
,	O
this	O
will	O
show	O
up	O
as	O
an	O
intense	O
ring	O
of	O
precipitation	O
at	O
the	O
altitude	O
where	O
the	O
beam	O
crosses	O
the	O
melting	O
level	O
while	O
on	O
a	O
series	O
of	O
CAPPIs	B-Algorithm
,	O
only	O
the	O
ones	O
near	O
that	O
level	O
will	O
have	O
stronger	O
echoes	O
.	O
</s>
<s>
An	O
opposite	O
problem	O
is	O
that	O
drizzle	O
(	O
precipitation	O
with	O
small	O
water	O
droplet	O
diameter	O
)	O
tends	O
not	O
to	O
show	O
up	O
on	O
radar	B-Application
because	O
radar	B-Application
returns	O
are	O
proportional	O
to	O
the	O
sixth	O
power	O
of	O
droplet	O
diameter	O
.	O
</s>
<s>
It	O
is	O
assumed	O
that	O
the	O
beam	O
hits	O
the	O
weather	O
targets	O
and	O
returns	O
directly	O
to	O
the	O
radar	B-Application
.	O
</s>
<s>
Most	O
of	O
it	O
is	O
weak	O
,	O
and	O
multiple	O
reflections	O
diminish	O
it	O
even	O
further	O
so	O
what	O
can	O
eventually	O
return	O
to	O
the	O
radar	B-Application
from	O
such	O
an	O
event	O
is	O
negligible	O
.	O
</s>
<s>
However	O
,	O
some	O
situations	O
allow	O
a	O
multiple-reflected	O
radar	B-Application
beam	O
to	O
be	O
received	O
by	O
the	O
radar	B-Application
antenna	O
.	O
</s>
<s>
For	O
instance	O
,	O
when	O
the	O
beam	O
hits	O
hail	O
,	O
the	O
energy	O
spread	O
toward	O
the	O
wet	O
ground	O
will	O
be	O
reflected	O
back	O
to	O
the	O
hail	O
and	O
then	O
to	O
the	O
radar	B-Application
.	O
</s>
<s>
These	O
two	O
images	O
show	O
what	O
can	O
be	O
achieved	O
to	O
clean	O
up	O
radar	B-Application
data	O
.	O
</s>
<s>
As	O
mentioned	O
,	O
radar	B-Application
data	O
are	O
an	O
average	O
of	O
the	O
scanned	O
volume	O
by	O
the	O
beam	O
.	O
</s>
<s>
A	O
program	O
by	O
the	O
Center	O
for	O
Collaborative	O
Adaptive	O
Sensing	O
of	O
the	O
Atmosphere	O
(	O
CASA	O
)	O
aims	O
to	O
supplement	O
the	O
regular	O
NEXRAD	B-Algorithm
(	O
a	O
network	O
in	O
the	O
United	O
States	O
)	O
using	O
many	O
low	O
cost	O
X-band	B-Algorithm
(	O
3cm	O
)	O
weather	B-Application
radar	I-Application
mounted	O
on	O
cellular	O
telephone	O
towers	O
.	O
</s>
<s>
These	O
radars	B-Application
will	O
subdivide	O
the	O
large	O
area	O
of	O
the	O
NEXRAD	B-Algorithm
into	O
smaller	O
domains	O
to	O
look	O
at	O
altitudes	O
below	O
its	O
lowest	O
angle	O
.	O
</s>
<s>
Using	O
3cm	O
radars	B-Application
,	O
the	O
antenna	O
of	O
each	O
radar	B-Application
is	O
small	O
(	O
about	O
1	O
meter	O
diameter	O
)	O
but	O
the	O
resolution	O
is	O
similar	O
at	O
short	O
distance	O
to	O
that	O
of	O
NEXRAD	B-Algorithm
.	O
</s>
<s>
The	O
attenuation	O
is	O
significant	O
due	O
to	O
the	O
wavelength	O
used	O
but	O
each	O
point	O
in	O
the	O
coverage	O
area	O
is	O
seen	O
by	O
many	O
radars	B-Application
,	O
each	O
viewing	O
from	O
a	O
different	O
direction	O
and	O
compensating	O
for	O
data	O
lost	O
from	O
others	O
.	O
</s>
<s>
To	O
mitigate	O
the	O
different	O
demands	O
,	O
scanning	O
strategies	O
have	O
been	O
developed	O
according	O
to	O
the	O
type	O
of	O
radar	B-Application
,	O
the	O
wavelength	O
used	O
and	O
the	O
most	O
common	O
weather	O
situations	O
in	O
the	O
area	O
considered	O
.	O
</s>
<s>
One	O
example	O
of	O
scanning	O
strategies	O
is	O
offered	O
by	O
the	O
US	O
NEXRAD	B-Algorithm
radar	B-Application
network	O
which	O
has	O
evolved	O
over	O
time	O
.	O
</s>
<s>
In	O
2008	O
,	O
it	O
added	O
extra	O
resolution	O
of	O
data	O
,	O
and	O
in	O
2014	O
,	O
additional	O
intra-cycle	O
scanning	O
of	O
the	O
lowest	O
level	O
elevation	O
(	O
MESO-SAILS	B-Algorithm
)	O
.	O
</s>
<s>
With	O
5	O
to	O
10	O
minutes	O
between	O
complete	O
scans	O
of	O
weather	B-Application
radar	I-Application
,	O
much	O
data	O
is	O
lost	O
as	O
a	O
thunderstorm	O
develops	O
.	O
</s>
<s>
A	O
Phased-array	O
radar	B-Application
is	O
being	O
tested	O
at	O
the	O
National	O
Severe	O
Storms	O
Lab	O
in	O
Norman	O
,	O
Oklahoma	O
,	O
to	O
speed	O
the	O
data	O
gathering	O
.	O
</s>
<s>
A	O
team	O
in	O
Japan	O
has	O
also	O
deployed	O
a	O
phased-array	O
radar	B-Application
for	O
3D	O
NowCasting	O
at	O
the	O
RIKEN	O
Advanced	O
Institute	O
for	O
Computational	O
Science	O
(	O
AICS	O
)	O
.	O
</s>
<s>
Aircraft	O
application	O
of	O
radar	B-Application
systems	I-Application
include	O
weather	B-Application
radar	I-Application
,	O
collision	O
avoidance	O
,	O
target	O
tracking	O
,	O
ground	O
proximity	O
,	O
and	O
other	O
systems	O
.	O
</s>
<s>
For	O
commercial	O
weather	B-Application
radar	I-Application
,	O
ARINC	O
708	O
is	O
the	O
primary	O
specification	O
for	O
weather	B-Application
radar	I-Application
systems	O
using	O
an	O
airborne	O
pulse-Doppler	B-Algorithm
radar	I-Algorithm
.	O
</s>
<s>
Unlike	O
ground	O
weather	B-Application
radar	I-Application
,	O
which	O
is	O
set	O
at	O
a	O
fixed	O
angle	O
,	O
airborne	O
weather	B-Application
radar	I-Application
is	O
being	O
utilized	O
from	O
the	O
nose	O
or	O
wing	O
of	O
an	O
aircraft	O
.	O
</s>
<s>
To	O
compensate	O
for	O
this	O
,	O
the	O
antenna	O
is	O
linked	O
and	O
calibrated	O
to	O
the	O
vertical	O
gyroscope	B-Application
located	O
on	O
the	O
aircraft	O
.	O
</s>
<s>
In	O
doing	O
this	O
the	O
pilot	O
is	O
able	O
to	O
adjust	O
the	O
radar	B-Application
so	O
that	O
it	O
will	O
point	O
towards	O
the	O
weather	O
system	O
of	O
interest	O
.	O
</s>
<s>
If	O
the	O
airplane	O
is	O
at	O
a	O
low	O
altitude	O
,	O
the	O
pilot	O
would	O
want	O
to	O
set	O
the	O
radar	B-Application
above	O
the	O
horizon	O
line	O
so	O
that	O
ground	O
clutter	O
is	O
minimized	O
on	O
the	O
display	O
.	O
</s>
<s>
If	O
the	O
airplane	O
is	O
at	O
a	O
very	O
high	O
altitude	O
,	O
the	O
pilot	O
will	O
set	O
the	O
radar	B-Application
at	O
a	O
low	O
or	O
negative	O
angle	O
,	O
to	O
point	O
the	O
radar	B-Application
towards	O
the	O
clouds	O
wherever	O
they	O
may	O
be	O
relative	O
to	O
the	O
aircraft	O
.	O
</s>
<s>
If	O
the	O
airplane	O
changes	O
attitude	O
,	O
the	O
stabilizer	O
will	O
adjust	O
itself	O
accordingly	O
so	O
that	O
the	O
pilot	O
does	O
n't	O
have	O
to	O
fly	O
with	O
one	O
hand	O
and	O
adjust	O
the	O
radar	B-Application
with	O
the	O
other	O
.	O
</s>
<s>
There	O
are	O
two	O
major	O
systems	O
when	O
talking	O
about	O
the	O
receiver/transmitter	O
:	O
the	O
first	O
is	O
high-powered	O
systems	O
,	O
and	O
the	O
second	O
is	O
low-powered	O
systems	O
;	O
both	O
of	O
which	O
operate	O
in	O
the	O
X-band	B-Algorithm
frequency	O
range	O
(	O
8,000	O
–	O
12,500	O
MHz	O
)	O
.	O
</s>
<s>
These	O
systems	O
consist	O
of	O
magnetrons	B-Architecture
that	O
are	O
fairly	O
expensive	O
(	O
approximately	O
$	O
1,700	O
)	O
and	O
allow	O
for	O
considerable	O
noise	O
due	O
to	O
irregularities	O
with	O
the	O
system	O
.	O
</s>
<s>
Also	O
,	O
since	O
there	O
are	O
fewer	O
irregularities	O
throughout	O
the	O
system	O
,	O
the	O
low-powered	O
radars	B-Application
can	O
be	O
used	O
to	O
detect	O
turbulence	O
via	O
the	O
Doppler	O
Effect	O
.	O
</s>
<s>
Digital	O
radar	B-Application
systems	I-Application
have	O
capabilities	O
far	O
beyond	O
their	O
predecessors	O
.	O
</s>
<s>
Thunderstorms	O
are	O
identified	O
by	O
matching	O
raw	O
precipitation	O
data	O
received	O
from	O
the	O
radar	B-Application
pulse	O
,	O
to	O
a	O
preprogrammed	O
template	O
.	O
</s>
<s>
Usually	O
,	O
it	O
must	O
show	O
signs	O
of	O
horizontal	O
organization	O
and	O
vertical	O
continuity	O
:	O
and	O
have	O
a	O
core	O
or	O
a	O
more	O
intense	O
center	O
identified	O
and	O
tracked	O
by	O
digital	O
radar	B-Application
trackers	O
.	O
</s>
<s>
Using	O
Doppler	B-Application
weather	I-Application
radar	I-Application
is	O
not	O
limited	O
to	O
determining	O
the	O
location	O
and	O
velocity	O
of	O
precipitation	O
.	O
</s>
<s>
The	O
radio	O
waves	O
from	O
the	O
radars	B-Application
bounce	O
off	O
rain	O
and	O
birds	O
alike	O
(	O
or	O
even	O
insects	O
like	O
butterflies	B-Application
)	O
.	O
</s>
<s>
The	O
US	O
National	O
Weather	O
Service	O
,	O
for	O
instance	O
,	O
has	O
reported	O
having	O
flights	O
of	O
birds	O
appear	O
on	O
their	O
radars	B-Application
as	O
clouds	O
and	O
then	O
fade	O
away	O
when	O
the	O
birds	O
land	O
.	O
</s>
<s>
The	O
U.S.	O
National	O
Weather	O
Service	O
St.	O
Louis	O
has	O
even	O
reported	O
monarch	O
butterflies	B-Application
appearing	O
on	O
its	O
radars	B-Application
.	O
</s>
<s>
Different	O
programs	O
in	O
North	O
America	O
use	O
regular	O
weather	B-Application
radars	I-Application
and	O
specialized	O
radar	B-Application
data	O
to	O
determine	O
the	O
paths	O
,	O
height	O
of	O
flight	O
,	O
and	O
timing	O
of	O
migrations	O
.	O
</s>
<s>
This	O
is	O
useful	O
information	O
in	O
planning	O
windmill	B-Application
farm	O
placement	O
and	O
operation	O
,	O
to	O
reduce	O
bird	O
fatalities	O
,	O
improve	O
aviation	O
safety	O
and	O
other	O
wildlife	O
management	O
.	O
</s>
<s>
In	O
Europe	O
,	O
there	O
have	O
been	O
similar	O
developments	O
and	O
even	O
a	O
comprehensive	O
forecast	O
program	O
for	O
aviation	O
safety	O
,	O
based	O
on	O
radar	B-Application
detection	O
.	O
</s>
<s>
The	O
red-green	O
feature	O
at	O
the	O
upper	O
left	O
is	O
the	O
motion	O
of	O
clouds	O
near	O
the	O
radar	B-Application
itself	O
,	O
and	O
a	O
signature	O
of	O
falling	O
meteorites	O
is	O
inside	O
the	O
yellow	O
ellipse	O
at	O
image	O
center	O
.	O
</s>
<s>
The	O
intermixed	O
red	O
and	O
green	O
pixels	B-Algorithm
indicate	O
turbulence	O
,	O
in	O
this	O
case	O
arising	O
from	O
the	O
wakes	O
of	O
falling	O
,	O
high-velocity	O
meteorites	O
.	O
</s>
<s>
It	O
is	O
in	O
dark	O
flight	O
that	O
falling	O
meteorites	O
typically	O
fall	O
through	O
the	O
interaction	O
volume	O
of	O
most	O
types	O
of	O
radars	B-Application
.	O
</s>
<s>
It	O
has	O
been	O
demonstrated	O
that	O
it	O
is	O
possible	O
to	O
identify	O
falling	O
meteorites	O
in	O
weather	B-Application
radar	I-Application
imagery	O
.	O
</s>
<s>
This	O
is	O
especially	O
useful	O
for	O
meteorite	O
recovery	O
,	O
as	O
weather	B-Application
radars	I-Application
are	O
part	O
of	O
widespread	O
networks	O
and	O
scan	O
the	O
atmosphere	O
continuously	O
.	O
</s>
<s>
Furthermore	O
,	O
the	O
meteorites	O
cause	O
local	O
wind	O
turbulence	O
,	O
which	O
is	O
noticeable	O
on	O
Doppler	O
outputs	O
,	O
and	O
fall	O
nearly	O
vertically	O
so	O
their	O
resting	O
place	O
on	O
the	O
ground	O
is	O
close	O
to	O
their	O
radar	B-Application
signature	O
.	O
</s>
