Module 2 / Weather Radar Detecting Precipitation
Project Atmosphere Canada
Project Atmosphere Canada (PAC) is a collaborative initiative of
Environment Canada and the Canadian Meteorological and
Oceanographic Society (CMOS) directed towards teachers in the
primary and secondary schools across Canada. It is designed to
promote an interest in meteorology amongst young people, and
to encourage and foster the teaching of the atmospheric
sciences and related topics in Canada in grades K-12.
Material in the Project Atmosphere Canada Teacher's Guide has
been duplicated or adapted with the permission of the American
Meteorological Society (AMS) from its Project ATMOSPHERE
teacher guides.
Acknowledgements
The Meteorological Service of Canada and the Canadian
Meteorological and Oceanographic Society gratefully
acknowledge the support and assistance of the American
Meteorological Society in the preparation of this material.
Projects like PAC don't just happen. The task of transferring the
hard copy AMS material into electronic format, editing, re-writing,
reviewing, translating, creating new graphics and finally format-
ting the final documents required days, weeks, and for some
months of dedicated effort. I would like to acknowledge the
significant contributions made by Environment Canada staff and
CMOS members across the country and those from across the
global science community who granted permission for their
material to be included in the PAC Teacher's Guide.
Eldon J. Oja
Project Leader Project Atmosphere Canada
On behalf of
Environment Canada and the Canadian Meteorological and
Oceanographic Society
All rights reserved. No part of this publication may be
reproduced, stored in a retrieval system, or transmitted, in any form or by
any means, electronic, mechanical, photocopying, recording or otherwise without
the prior written permission of the publisher.
Permission is hereby granted for the reproduction, without alteration, of materials
contained in this publication for non-commercial use in schools or in other
teacher enhancement activities on the condition their source is acknowledged.
This permission does not extend to delivery by electronic means.
Published by Environment Canada
© Her Majesty the Queen in Right of Canada, 2001
Cat. no. En56-172/2001E-IN
ISBN 0-662-31474-3
Contents
Introduction
In many ways, modern meteorology originated during World War II. The extensive
expansion of aviation at that time led to the formation of networks of balloon-borne
instruments to take upper air measurements, the training of vast numbers of
meteorologists, the expansion of weather observing locations around the world,
the utilization of electronic computers, and the early development of rockets
for launching satellites. It also was the impetus for developing radar, an acronym
for Radio Detection and Ranging.
Radar began as a tool to detect aircraft. Radio waves in the microwave band
beamed outward at the speed of light are reflected back from objects which they
strike. One-half of the total time needed to travel to and return from the target
multiplied by the speed of light determines the target's distance. Watching
the target's movement for a few minutes then gives the target speed and direction
in comparison to the radar station. This use of radar is basic to modern aircraft
safety.
Early radar observations revealed echoes from "shower-clouds”. The
first significant use of radar to track weather occurred in 1942 in England
when a thunderstorm with hail was followed for eleven kilometres. Today this
is one of the major uses for modern radars, with sophisticated analysis of the
return signal being performed to study and predict the development of hail and
other forms of severe weather.
Precipitation is not the only target that reflects the radar beam. Almost all
radars detect strong echoes that are created by stray signal reflections off
trees, hills, buildings and even lakes in the vicinity of the radar site. These
are known as "ground clutter" or anomalous propagation and can fool
the meteorologist who is watching the radar if he or she is not careful. Ground
clutter is usually caused by reflection from close-by targets, but is occasionally
found at much longer distances if atmospheric conditions are right.
The post-war period saw the development of techniques to estimate precipitation
rates from the strength of the echoes detected by the radar. The relationship
between the types of echoes and the associated weather also became better known.
The Meteorological Service of Canada developed a national network of radars
to provide warning capability for severe thunderstorms, tornadoes, and hurricanes.
More recent development produced Doppler radar. Doppler radar is a system that
sends out a series of rapid microwave pulses and measures the movement of precipitation
droplets in the interval between the pulses. This enables the computer to reconstruct
the internal air motion within and around the area of precipitation, and often
provides vital clues about the nature and strength of the weather system. Meteorologists
use Doppler radar to detect circulations inside thunderstorms that indicate
the development of tornadoes, wind flow in a large winter storm, or damaging
winds from a decaying thunderstorm. For hazardous weather situations such as
hurricanes, tornadoes and thunderstorms, recognition of the developing wind
pattern may allow warnings that can save lives.
Radar is one of science's major tools for observing weather and precipitation
on a scale and time frame that makes it possible for the Meteorological Service
of Canada to provide detailed information to the public when it is most needed.
Basic Understandings
- Radar, short for Radio Detection and Ranging, transmits microwaves as a
focussed signal designed to detect precipitation particles in the atmosphere
(rain, snow, and hail).
- Radar energy travels through the atmosphere at the speed of light in a narrow
beam. The radar's antenna directs the beam around the horizon and up and down
at various angles until most of the atmosphere within a given distance around
the radar has been scanned.
- After a radar sends out a signal, it "listens" for returning signals.
A returning signal, called an echo, occurs when the transmitted signal strikes
and reflects off objects (raindrops, ice, snow, trees, buildings, mountains,
birds, or even insects) within its path.
- Part of the reflected signal is received back at the radar. The display of
the strength of the signals returned from echoes is called reflectivity. Reflectivity
can be correlated to the intensity of the echo and in turn, the amount and type
of precipitation that is falling.
- The time from transmission of a signal to the receipt of an echo determines
the distance to a target. The direction the antenna is aimed determines the
direction to the target.
- Modern weather radars can also evaluate the returned signal to detect target
motion toward or away from the radar.
- A computer attached to the radar stores the values of reflectivity from
each distance and direction as the radar beam spirals around the horizon and
up through several elevation angles until a volume of the sky is covered for
about 350 kilometres around the site.
- Selections of the stored reflectivity values can be displayed on a monitor
to show a horizontal view of the atmosphere at any level or a vertical slice
up thorough the atmosphere in a particular direction.
- The reflectivity data generally indicate only those cloud particles large
enough to fall as precipitation. Not all of this precipitation will reach the
ground, as it is common for falling rain to evaporate after it leaves the cloud.
- Each horizontal image can show reflectivity for i) any elevation angle,
ii) at a constant height, or iii) the greatest value at that location from any
elevation. Each vertical image can show the height of echoes along any direction.
- Since the radar beam is normally bent downward by the atmosphere as it travels,
radar views generally extend far beyond the visual horizon.
- The range of horizontal coverage depends on atmospheric effects, the curvature
of the earth, and the radar beam characteristics.
- Radar reflectivity values are displayed on screen by assigning colours to
indicate precipitation intensity ranges.
- These intensities can be related to precipitation amounts over a period
of time by collecting and adding individual radar images.
- The computer connected to the new weather radars can alert operators for
patterns that may indicate hail, flash flooding and tornadoes, or the operators
may detect these patterns themselves.
- Successive radar images can be animated to illustrate storm development,
structure, and movement.
- Precipitation echoes generally occur in cells, lines, or areas. Regions
of most intense precipitation are usually in the centre of the echoes.
- Snow returns a weak echo, rain a stronger echo and wet hail a very strong
signal. Water droplets comprising clouds are usually too small to be detected
by normal radar operations.
- Now radars are so sensitive that even dust, birds, insects and sudden changes
in atmospheric temperature and humidity can be seen.
- As noted above, not all echoes are caused by meteorological phenomena. Buildings,
hills and trees near the radar transmitter may return a signal. As a result,
a reflectivity pattern of strong, non-moving echoes is often displayed
near the radar site. These are called ground clutter or anomalous propagation.
- The shape, size, and strength of a radar echo can lead to the detection
of hazardous weather situations, especially those involving thunderstorms.
- Individual thunderstorm cells may exist along cold fronts or squall lines.
The cells may join to form clusters of severe thunderstorms. Patterns such as
these tend to show strong reflectivities indicating possible heavy rains or
hail.
- Tornadoes may show hook-shaped echoes or pendants in reflectivity displays.
- Vertical slices of the atmosphere constructed from the radar scans are very
useful because they show the pattern of reflectivity through the depth of the
atmosphere. One of the elements revealed by such a display is the vertical extent
of thunderstorms; the highest (or biggest) thunderstorms are most likely to
have severe weather such as hail or tornadoes associated with them.
- The spiralling bands of heavy thunderstorms within hurricanes show up clearly
in reflectivity patterns because of the huge amount of precipitation they contain.
- Occasionally a band of very high reflectivities will appear on the radar.
This is called "bright banding" and is related to an area in the clouds
where snow is melting into rain. The melting/wet snow has a much higher reflectivity
than snow and a higher reflectivity than rain. The "bright band" occurs
at the altitude where the temperature is around 0 degrees Celsius, i.e. temperature
in the upper reaches of the cloud is below freezing and temperature of the cloud
closer to ground is above freezing. The weather forecaster must be aware of
this process so as not to confuse the bright band with an intense area of precipitation.
- Radar displays require careful interpretation. For example, the radar beam,
usually less curved than the Earth's surface, may pass over the tops of more
distant targets. Low level precipitation (especially snow) is often unseen beyond
a certain distance (100 km or so) because it is below the radar beam.
- Heavy precipitation between the radar site and distant targets can weaken
the radar signal so echoes of rain areas further away can be distorted or undetected.
This is caused by the absorption and scattering of the radar beam by the intervening
precipitation.
- Unusual temperature and humidity patterns may distort echoes and give false
impressions of their size and shape.
- The radar beam spreads out as distance from the radar increases. This can
cause distortion in the shapes and sizes of echoes, making distant objects appear
weaker and larger than in reality.
- As the distance between the radar site and the precipitation increases,
the ability of the radar to accurately detect the occurring precipitation decreases.
Radar scans this entire volume by raising and lowering the beam as the antenna
rotates.
Activity
Activity - Weather Radar Investigation
Introduction
Radar is an important weather observing tool used to locate areas of precipitation
and to maintain a watch on the severity of storms. Meteorologists interpret
bright images, called echoes, appearing on weather radar screens in order to
acquire information about rain and snow areas.
Approach
- Construct a Radar Screen slide chart by following the directions
(Radar Screen Construction) given in the
accompanying material.
- Complete the radar activity entitled "Interpreting a Radar Precipitation
Display".
Additional Activities
- Go to the Environment Canada web site
http://weatheroffice.ec.gc.ca
Navigate to the RADAR
page and examine a variety of weather radar images. Next select a recent
weather radar image and compare the main features on the radar image with
those features found on the weather chart and satellite image for the same
time. Weather charts and satellite images can be found on the same web page
by navigating to the appropriate menu selections
- Set up simulated radar in a darkened classroom. Use a
flashlight to represent the radar and hang mobiles of smooth and crumpled aluminium-foil
pieces to depict areas of rain or snow. Swing the flashlight beam around or
up and down to search for the precipitation areas. Set up a co-ordinate system
to describe directions and distances to the "echo" sources.
Radar Screen Construction
Materials: Manila file folders, tape, scissors,
transparent material, ruler, coloured overhead marking pen or grease pencil,
stapler, pencil
Procedure:
Cut the RADAR DISPLAY VIEWS on the accompanying sheet along the dotted lines
and attach by tape to a piece of manila folder cut to the same size. Mark and
cut a 9 cm square viewing window in the lower front corner of a file folder.
(see diagram above) Secure the sleeve by stapling across
the folder 13 cm from the bottom fold. Tape transparent material across the
viewing window. Cut out the DISTANCE SCALE on the attached sheet and tape it
to the left-hand side of the file folder. Cut out the pullers and tape at each
end of the Radar-view strip. Insert the radar-view strip in the sleeve. In the
lower right hand corner, find the black dot representing the radar site. Make
a permanent reference mark on the transparent material representing this radar
site. The Radar Screen is now ready for use.
A version of the Radar Screen can be made for overhead projection. Cut the viewing
window completely through a Radar Screen sleeve. Reproduce the radar displays
on transparent material and cut the transparent sleeve insert to size.
Environment Canada's King City weather radar located 40 km north of Toronto
showing heavy precipitation echoes over southern Ontario on June 2, 1998
Activity - Interpreting a Radar Precipitation Display
Upon completing this activity, you should be able to:
- Locate areas of precipitation by interpreting a radar
display.
- Track and determine changes over time as precipitation
echoes move through a weather radar's field of view.
- Relate the intensity of radar echoes to the areas with
the greatest amount of rainfall.
Introduction
Day or night, clear or cloudy, meteorologists need to observe weather at great
distances. Radar especially designed for weather observation makes it possible
to locate areas of precipitation and to maintain watches on the severity of
storms as weather happens.
Weather radars detect water and ice particles in and below clouds that are large
enough to fall as rain, snow, or hail. Their fields of view stretch far beyond
the visible horizon, sometimes showing the tops of thunderstorms 350 kilometres
away. Their returned signals can be interpreted to determine how intense the
precipitation is, the size and shape of the precipitation area, its development,
and how fast and in what direction it is moving. In addition, a trained meteorologist
can infer from the radar data the conditions that forewarn of the existence
of hazardous weather such as tornadoes, heavy downpours, and hurricanes.
From the relationship between the intensity of radar echoes and the rate of
rainfall, the total amount of rain at a location can be estimated by computer
addition of the rainfall over a period of time. The determination of rainfall
totals over an hour, several hours, or even the duration of the storm is important
in judging the possibility of flash flooding in a river or stream valley. Localized
heavy rainfall events, such as the Saguenay flood event, can cause significant
erosion and represent a serious threat to life and property in many parts of
Canada.
Method
The Radar Screen slide chart presents precipitation "Reflectivity"
views at two successive times during a day when there was precipitation occurring.
The irregular shapes appearing in the image represent precipitation areas. They
are contoured and shaded to denote levels of intensity, which in turn are related
to precipitation rates. The darkest hatched areas surrounded by the most contours
indicate the most intense rainfall. The "4 p.m. Precipitation Total"
is also shown for the storms up until the time of the 4 p.m. reflectivity image.
Adjust the slide insert until the 3 p.m. view is centred in the Radar Screen
window.
Questions
1. Look at the 3 p.m. Reflectivity view. The location of the radar is
depicted as a dot (•) in the echo shown in the lower right-hand corner.
Distance can be measured in the horizontal view by the markings appearing along
the boundaries of the view at 10-kilometer intervals. Find the strongest echo
beyond the one immediately surrounding the radar's location. How far away and
in what direction from the radar site is it? Also, how many levels of intensity
does this echo contain?
_______________________________________________
_______________________________________________
2. The centres of the more intense radar echoes are tracked by computer. The
individual storm cells are given a storm identification, in this case A, B,
and C. The past positions of the storm cells are denoted by dots connected with
lines to the present echo location. Computer projections of future individual
cell positions are given by crosses connected with lines for 15-minute time
increments. In what general directions have these intense cells been moving
in the previous 30 minutes? Are they projected to continue in the same direction
or turn to a new direction? If so, what direction?
3. Tape a piece of tracing paper or transparency material over the Radar Screen
window. Mark the radar's location and trace the outlines of each echo in the
3 p.m. Reflectivity view. Next, pull the slide insert until the 4
p.m. Reflectivity view is centred. Trace the 4 p.m. echoes. Have any of
the echoes changed location or shape? If so, which ones? Is there any echo that
did not change? If so, where it is located?
_______________________________________________
_______________________________________________
4. Near radar sites, it is possible to get intense non-moving echoes because
the radar signal is reflected from nearby stationary objects. Do any of your
echoes fit this pattern, (yes or no)? If so, what is this echo called?
________________________________________________
5. Precipitation echoes will continue to move across the radar field of view.
In the 4 p.m. view, how many levels of intensity are now contained in the most
intense storm? __________. Generally, the more intense the rain or snow, the
brighter the radar echo. Based upon this intensity, has the precipitation area
experienced an (increase, decrease, or no change) in rainfall rate?
6. Note the curved feature at the southwest end of the most intense cell (located
approximately in the centre of the view). The hook-like protrusion is called
a "hook echo" and occurs when rain is being wrapped around a quickly
rotating column of air. Name this severe weather feature. ____________________________.
If you spotted this feature as a radar operator, what action would you consider
taking?
7. From the projected track of the storm cells indicated in the 3 p.m. view
and the current position at 4 p.m., how well do you think the computer forecast
did? Explain your answer.
_______________________________________________
_______________________________________________
_______________________________________________
8. Compare the movement of individual storm cells as compared to the entire
line. Do individual cells move in the same direction as the line itself? __________.
Which direction is the line moving? __________. In which general direction are
the cells moving? ______________________________.
9. Finally, pull out the slide until the 4 P.m. Precipitation Total view is
centred in the window. The levels shown in this image depict the total amount
of precipitation (usually in millimetres) that has fallen at any location during
the time echoes were detected by the radar and compiled by the computer. How
do the greatest rainfall amounts compare to the most intense reflectivity echoes
from the 3 and 4 p.m. views?
_______________________________________________
_______________________________________________
_______________________________________________
10. If a hydrologic forecaster has prior knowledge of streams, local typography,
and locations of homes and business areas, how would the forecaster use the
rainfall total information to alert people to possible flood danger? What factors
do you think are important for rainfall runoff?
_______________________________________________
_______________________________________________
_______________________________________________
Created :
2002-06-06
Modified :
2003-07-09
Reviewed :
2003-07-09
Url of this page : http://www.msc.ec.gc.ca
/education/teachers_guides/module2_weather_radar_detecting_precipitation_e.html
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Environment Canada's World Wide Web Site.
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