ADVANCED RADAR APPLICATIONS IN
METEOROLGY
Nurgül Ertem
110000531
Radar in Meteorology
Radar stands for Radio Detection and Ranging. It refers to the use of radio waves to
detect objects and determine the distance (range) to the object
.
Radars, however, will be our focus here. They have particularly become important in
meteorology because of the following reasons.
• (i) They can see through fog, cloud, rain and other types of atmospheric conditions
which light cannot pass through.
• (ii) They can observe many places in the sky almost simultaneously.
• (iii) They can run continuously, often without operators being present (computer
controlled).
• (iv) They can operate during both day and night.
• (v) The data from them can be easily stored to computer and then subjected to many
types of sophisticated analysis.
• (vi) Modern solid-state radars often need little maintenance, or replacement of
parts, so the largest expense is often the setup costs.
• (vii) Radars are not restricted to ground-level studies, and can often observe several
kilometres upward into the atmosphere
Signal Processing and Product
Generation
•
The processing of radar data generally involve two distinct steps. The first step,
called signal processing, is the extraction of raw radar parameters like echo strength
(reflectivity) or Doppler velocity from the radar signals coming out of the receiver.
The second step, called data processing or product generation, is the further
processing of raw radar parameters in order to obtain information that is useful for
meteorological or hydrological purposes. In general, these two steps are done by
different computers, signal processing being done at the radar site, while product
generation can be done everywhere the data are sent to.
Simple Radar System
antenna
transmitter
106 W
display
T/R switch
receiver
10-14 W
Radar System
Transmitter--produces high power pulses at desired frequency.
Pulses may be 1 microsecond in duration.
Receiver--detects, amplifies and converts (digitizes) received
voltages from each
pulse as a function of range
Antenna--radiates transmitted power in narrow beam for maximum
“gain”
--receives backscattered signal from targets
T/R switch--switches antenna between transmitter and receiver at
high rate, typically once every millisecond
Receivers are designed to detect very weak signals, on the order of
10-13 to 10-14 watts. Transmitted power is typically 106 watts, peak
power.
Radar Systems
More advanced radars provide either frequency agility or
polarization agility.
Frequency agility--transmit and receive multiple frequencies (near
in frequency to each other) to increase sampling rate
Polarization agility--alternatively transmit either horizontal or
vertical polarization to provide wealth of information on particle
phase, shape and orientation
Sending and Receiving Signals
detecting a target
• The radar creates an
electromagnetic energy pulse
which is focused by an antenna
and transmitted through the
atmosphere. Objects in the path of
this electromagnetic pulse, called
targets, scatter the
electromagnetic energy. Some of
that energy is scattered back
toward the radar.
• The receiving antenna (which is
normally also the transmitting
antenna) gathers this backscattered radiation and feeds it to
a device called a receiver.
RADAR
•
Radar weather information are used to
determine current atmospheric
conditions, but meteorologists must
integrate data from many sources to
get a complete and accurate picture of
atmospheric conditions.
Meteorologists feed current data into
computer models to help them predict
weather conditions (forecast) and
make critical decisions
•
Radar could be used to answer the
following questions about the line of
precipitation. What is the intensity of
the precipitation (in this case snow)?
What direction and at what speed are
the snow bands moving? Will the
snow bands become stronger or
weaker?
COLORED RADAR
•
•
The location of the colored radar
echoes indicate where precipitation is
falling and the various colors indicate
the intensity of the precipitation
through the color code in the lower
left corner of the image.
The example radar image above
shows several strong thunderstorms
moving through Illinois and Indiana
on April 20, 1996. Regions of light
and dark blue indicate regions of
lighter precipitation while areas of
red and pink indicate strong, to
occasionally severe thunderstorms.
Normally, it is difficult to distinguish
precipitation type on the basis of the
radar reflectivity alone. Snow and
light drizzle both produce radar
reflectivity with about the same
value. Melting snow and moderate
rain also have similar values. Very
high reflectivities (the grays on the
scale on the image above) are always
associated with hail.
Radar in meteorology
Weather radars play a vital role in short term weather forecasting
and for meteorological research. They are being used routinely in
meteorology to monitor storms and follow their evolution, as well
as observe winds and detect regions where severe weather might
develop. Specialists in radar meteorology, with backgrounds in
meteorology, engineering, and computer science, work to improve
the use of radar as a meteorological instrument.
For example, our large S-band Doppler radar is used for weather
surveillance around the Montreal area. Part of the Canadian radar
network, it is used by the local weather office to monitor weather
in real-time. Its data are used in a variety of applications, from
severe weather detection to sewer flow forecasting.
How Does Radar Work?
single antenna
Return
back
target
send
short pulses of energy
The antenna rotates about a vertical axis, scanning the horizon in all directions
Radar - How Radar Works
•
Radar works by transmitting a pulse
of electromagnetic energy.
•
Objects (raindrops, ice, snow, birds,
insects, terrain, and buildings) reflect
that energy. Part of the reflected
energy is received back at the radar.
Once the radar receives the reflected
signal, computer programs and
meteorologists interpret the signal to
determine where it is precipitating.
Radar Observatory
•
•
Marshall Radar Observatory itself where the main radar facility is located. It
consists of a dual-wavelength, dual-polarization, Doppler scanning radar system.
With its 9 m antenna sitting on top of a tower, it is one of the most sophisticated
weather radar in the world. The first element of the system is a dual-polarization,
Doppler, long wavelength (S-band) radar which measures the intensity, the velocity,
and the shape of weather targets (rain, snow, hail, etc.) up to a range of 250 km. It
is complemented by a shorter wavelength (X-band) radar and two additional
receivers located elsewhere which help us obtain a more complete picture of the
weather. This radar is used 24 hours a day to monitor the storms around Montreal.
After the radar signals have been processed and interpreted, the data are sent to the
weather office and the downtown campus where they are made available on the
web in real-time.
Two other radars are located in Ste-Anne de Bellevue. One is a small dualpolarization radar called X-Polito, while the other is a vertically pointing radar
(VPR). X-Polito is a prototype low-cost radar developed to measure rainfall over
short distances. The VPR is a research radar used primarily to study the formation
of precipitation.
Interpreting Doppler Radar
Velocities
To understand Doppler radial velocity patterns, one first has to
consider the geometry of a radar scan. Normally the radar beam is
pointed at an elevation angle greater than zero so that the beam, as
it moves away from the radar, moves higher and higher above the
surface of the earth. Because of this geometry, radar returns
originating from targets near the radar represent the low-level wind
field, while returns from distant targets represent the wind field at
higher levels.
Interpreting Doppler Radar Velocities
speed shear wind patterns
• On a radar PPI display, the
distance away from the radar at
the center of the display
represents both a change in
horizontal distance and a change
in vertical distance. To determine
the wind field at a particular
elevation above the radar, one
must examine the radial velocities
on a ring at a fixed distance from
the radar. The exact elevation
represented by a particular ring
depends upon the elevation angle
of the radar beam.
DOPPLER
•
Doppler velocity patterns (right)
correspond to vertical wind profiles
(left), where the wind barbs indicate
wind speed and direction from the
ground up to 24,000 feet. Negative
Doppler velocities (blue-green) are
toward the radar and positive (yellowred) are away. The radar location is at
the center of the display
•
For this first example, wind direction
is constant with height, but wind
speed increases from 20 knots at the
ground to 40 knots at 24,000 feet.
Note on the radial velocity field that
the maximum inbound velocity is to
the west and maximum outbound to
the east while to the north and south
the radar measures zero radial
velocity. This is because the winds are
perpendicular to the radar beam when
viewed to the north or south.
dual-polarization
•
In general, weather radars send and
receive microwaves at one
polarization, usually horizontal. By
transmitting and/or receiving radar
waves at more than one polarization,
additional information can be
obtained on the nature of the targets
•
The most common dual-polarization
scheme is the transmission and
reception of horizontally and
vertically polarized waves.
WSR-88D Radar Imagery
detecting precipitation
• The word radar is an acronym from "Radio Detection and Ranging". Radar
images are useful for locating precipitation. As a Magnetic Resonance
Imaging (MRI) scan examines the inside of a human body, a radar
examines the inside of a cloud. A radar sends a pulse of energy into the
atmosphere and if any precipitation is intercepted by the energy, part of the
energy is scattered back to the radar. These returned signals, called "radar
echoes", are assembled to produce radar images.
Radar Instrumentation
•
King City C-band doppler scanning
•
Portable C-Band Doppler Scanning
Radar
•
Portable 915 MHz Wind Profiler
radar
•
Portable X-Band Doppler Scanning
Radar
Radar Instrumentation
•
Portable/airborne Ka-Band 35 GHz
Cloud Radar
•
Joss-Waldvogel Disdrometer
RADAR IMAGES
REFLECTIVITY /
REFLECTIVITE
The radar only makes measurements if
sufficient scattering targets (eg:
rain, snow, etc) are present,
although the tops of mountains are
also frequently picked up (they
can often be recognized by their
zero velocity on the radial velocity
image)
Reflectivity CAPPI
İN CONVECTİON
Interpretation of the radar reflectivity scale
Type and intensity
Reflectivity
Drizzle or clear air targets (bugs, etc.)
0 dBZ
Very light rain or snow
A few raindrops or snowflakes
10 dBZ
Light rain or snow
Typical of spring/fall: 1-2 mm/hr
25 dBZ
Moderate precipitation
Strong for spring/fall: 5 mm/hr
35 dBZ
Heavy rain
Summer showers: 20 mm/hr
45 dBZ
Very heavy rain or hail
Peak of thunderstorms: 100 mm/hr
55 dBZ
Reflectivity CAPPI
•
The reflectivity CAPPI (Constant Altitude
Plan Position Indicator - radar lingo for a
horizontal section at constant altitude - ) is
the most often used product for displaying
precipitation intensity around the McGill
radar. It shows the intensity of all the
echoes received by the radar, those we
want (like precipitation) as well those we
don't (like ground targets)
•
align="justify"In this case, a large
area of precipitation is approaching
from the North West. It is made of
light stratiform precipitation (in
green) with several embedded
showers and thundershowers (in
warmer colors). Some of these
thundershowers actually spawned
tornadoes
Surface refractivity
•
In this example, the refractivity field
measured by radar (bottom) is
contrasted with simultaneous weather
observations over a range of 45 km.
Two air masses can be identified, a
drier one to the north (10°C dew point
temperature) and a wetter one to the
south (14°C dew point temperature).
The refractivity computed using
surface observations (shown in
brackets in the upper window) match
well the refractivity measured by
radar. While the presence of a
gradient in moisture could have been
inferred from surface observations
alone, the radar measured refractivity
allow the precise determination of the
position of the boundary between the
two air masses (shown with heavy
dashes).
McGill S-band radar
• This radar is used for
weather surveillance,
providing data in real-time
to various users including
the local weather office,
as well as for
meteorological research
and the development of
automated algorithms of
weather detection and
identification.
National Weather Service Doppler Radars
Radar Images
•
PPI, antenin belirli bir yükseklik
açısında (vertical elevation) sabit
tutulmasıyla elde edilen bir üründür.
Yatayda (azimut) 0-360° tarama
yaparak elektromanyetik dalga
gönderilir. Bu görüntüde, radarın
tespit ettiği ekoların reflektivite
değerlerine göre radarın kaplama
alanı içerisinde yer alan hedeflerin
gerçek koordinatları ve varsa yağışlı
bölgeler belirlenir. Görüntünün
sağında bulunan renk skalası dBZ
cinsinden reflektivite değerlerini
gösterir
Yağışın Cinsi
dBZ
YağışMiktarı mm/saat
Dolu ile birlikte Yoğun
ve Şiddetli Gürültülü
Sağanak Yağış
55>
Şiddetli Gök Gürültülü
Sağanak Yağış
50-54
51 ile 100
Mutedil veya Şiddetli
Yağmur veya Karla
Karışık Yağmur
45-49
26 ile 50
Mutedil Yağmur
veya Karla Karışık
Yağmur
40-44
13 ile 25
Hafif Yağmur , Mutedil
veya Kuvvetli Kar
30-39
3 ile 12
Çok Hafif Yağmur veya
Hafif Kar
15-29
0.1 ile 2.9
Çisenti veya açık hava
hedefleri (böcek,toz
vb.)
<15
>100
0 ile İz
• KIZILÖTESİ
GÖRÜNÜR
Renklendirilmiş
CONCLUSION
Some natural disasters (meteorologically) such as avalanche,
flood, tornado have occured in many regions of the world and
they have caused to the death of huming beings and bringing
greate ecenomic losses.
Therefore, radars have a great importance in determining of the
natural disasters and its early warning systems.
THANKS