The term "radar", which has been in use since the 1940s, is an acronym formed from the English term "RAdio Detection And Ranging". It defines a system of spatial location which uses energy from the HF, VHF, SHF and microwave portion of the electromagnetic spectrum.
Weather radar systems generally use a paraboloidal antenna and, much like a searchlight, reveal objects caught in their beam. Radar detects and locates objects equally well by day or night, under clear or overcast sky conditions.
By selecting an appropriate frequency for weather radar operation, it is possible to penetrate through light precipitation and see beyond showers and small-scale meteorological events. Meteorologists use radar primarily to detect and locate precipitation, both suspended in clouds and falling, and to measure its intensity.
Microwave pulses from the transmitter, focused into a narrow beam by the antenna, sweep the sky as the antenna slowly rotates while pointing at different elevation angles. When the energy emitted by the radar antenna strikes particles of precipitation, such as drops of water, snowflakes, sleet or hail, it is reflected with intensity proportional to the number and size of the particles.
When operating in conventional mode, Environment Canada's radars transmit bursts of 2 microsecond duration at a repetition rate of 1200 Hz. Two Doppler modes are used, where the pulse duration is 0.8 micro-second and the repetition rate is either 890 or 1100 Hz. Using a combination of both repetition rates enables the use of Doppler data at longer ranges than would otherwise be possible.
The intensity of the returned signal is digitized along with the antenna position information for each transmitted pulse. It is subsequently processed and portrayed to the meteorologist on a 16-colour geographically-referenced CAPPI display showing precipitation intensity levels in dBz and as rain or snow rates (mm or cm per hour). A CAPPI (Constant Altitude Plan Position Indicator) is a horizontal radar projection showing reflectivity data at a specified altitude.
The returned-echo intensity signal is digitized along with the antenna position information for each pulse. It is subsequently processed and portrayed to the meteorologist on a 16-colour geographically-referenced CAPPI display showing precipitation intensity levels in dBz and as rain or snow rates (mm or cm per hour). A CAPPI (Constant Altitude Plan Position Indicator) is a horizontal radar projection showing reflectivity data at a specified altitude.
1 km CAPPI, Exeter radar, Nov 17, 2000
This 1 km CAPPI from the Exeter radar Nov 17 2000 shows bands of snow squalls coming in off Lake Huron, typical of when a cold northwesterly flow crosses the relatively warm waters of the Great Lakes, particularly in late November and December. The squalls nearly always form in lines in the direction of the low level winds.
We can see the cells of stronger radar returns embedded in the lines. The snowfall accumulation rate estimated from radar is up to 2 cm per hour in the squall 80 km (2 rings) north of the radar. Radar typically underestimates this snowfall. Accumulations of 5-10 cm per hour are not uncommon in these squalls.
Some radars use software which can target a portion of the sky and compile a vertical profile of precipitation. This is true of all of the radars which form the National Radar Program network operated by Environment Canada. Using the cross-section tool, it is possible to analyse precipitation inside a thunderstorm, among other things, this gives the vertical precipitation rate, on the basis of which violent storms can be identified quickly.
Arbitrary Cross-Section, Exeter Radar, 000Z 13 Aug, 2003
VAD, Exeter radar, 1350Z 12 Jan 2005.
The frame on the left is height versus direction, the middle is height versus wind speed in m/s, and the right graph is reflectivity. The values are averages around the radar circle at the various heights. These are calculated by 3 separate radar scans as indicated by the 3 colours.
Doppler radars not only detect the intensity and location of precipitation at several levels, but also measure its speed and direction of movement within the area swept, whether it is moving towards the site or away from it.
Doppler radars accordingly make it possible to detect the rotation of precipitation, called mesocyclonic movement, which is one of the signs associated with the development of tornadoes.
Although the resolution of MSC's radar is 1km, tornadoes are generally too small and too ephemeral to be detected.
The images below, however, show characteristic reflectivity and Doppler versions of a tornadic event.
Tornadoes which occur in Canada average approximately 10 to 100 m in diameter. They normally occur close to the ground and last for no more than a few tens of seconds to a few minutes. Since they emerge from the base of the clouds and because of the curvature of the Earth, they are almost always below the radar beam.
Doppler radar data are useful in assisting research into the initial conditions conducive to the formation of tornadoes. Doppler data can be used to systematically predict their arrival and assist in the issuance of warnings defining their trajectory.
The image above and below are from the Carvel radar, and were captured on Aug 12 2003.
At the time a small tornado is reported to have touched down with this cell. This is quite unique because the storm was only about 20 km away from the radar. These radar pictures are zoomed in on the storm. The upper image shows reflectivity. We see the dramatic hook shape associated with this cell, typical of a tornadic storm.
In the Dopplerized image shown below, we see a small area of bright green indicating radial velocities towards the radar up to 36 m/s. The velocities just to the north are near zero. This "velocity couplet" indicates the cyclonic (counter-clockwise) circulation located on the southeastern edge of our reflectivity hook. This is a classic example of what a tornado looks like on Doppler radar.
Doppler radar is highly effective in the case of major thunderstorms and mesocyclones several kilometres in diameter, since it measures rotation inside convection cells, determines the violence of the phenomena and issues increasingly accurate warning messages.
Data generated by Doppler radar about precipitation can also identify wind swings, gust fronts and sometimes even low level wind shear (LLWS), when this does not occur too close to the ground.
Below is a Doppler image captured by the Halifax radar during hurricane Juan.
The beige area to the southeast of the radar indicates winds at the height of the radar beam well in excess of 50 m/s (180 km/hr) that were crossing the Halifax area at this time.
In addition to the capability of directly measuring the intensity of precipitation, radar data can be used to detect other phenomena, such as the structure of violent thunderstorms and associated gusts, or to compile short-term forecasts on the movement of precipitation.
The use of mathematical algorithms on Doppler data makes it possible to determine the speed of thunderstorm gusts; this helps meteorologists to rapidly issue warning messages.
Environment Canada 's radars are installed close to the most populated areas and those most affected by violent weather . Their primary purpose is the early detection of developing precipitation and thunderstorms. They have a range of 256 km radius around the site in conventional mode and a Doppler range of 128 km radius from the site.
By comparing a series of radar images of the same type at precise intervals, forecasters can observe the development of a disturbance, follow its trajectory, determine the intensity of precipitation, issue alerts or warnings, and fine-tune their forecasts.
Radar is also used in research to study the development process of precipitation in clouds, specifically heavy rain, hail and violent thunderstorms.
|Foxwarren||Eastern Saskatchawan/Western Manitoba||MB||XFW||C||98E|
|Holyrood||Eastern Newfoundland and Labrador||NF&L;
|Jimmy Lake||NW Saskatchawan/NE Alberta||SN||WHN||C||98E|
|King City||Southern Ontario||ON||WKR||C||98A|
|Marble Mountain||Western Newfoundland and Labrador||NF&L;||XME||C||98A|
|Marion Bridge||Cape Breton||NS||XMB||C||98E|
|Montreal River Harbour||The Soo||ON||WGJ||C||98E|
|Mt. Silver Star||Eastern B.C.||BC||XSS||C||98A|
|Prince George||Northern B.C.||BC||XPG||C||98R|
|Spirit River||Grand Prarie||AB||WWW||C||98E|
|Val d'irène||Lower St. Lawrence||QC||XAM||C||98A|
- Explain briefly how a weather radar works.
- What can be observed using radar?
- What is the range of Environment Canada's radar?
- Explain why Environment Canada's radars cannot directly detect a tornado.
- What is a 1.5 km CAPPI?
- What can you measure with Doppler radar?
- What conditions will enable you to detect low-level wind shear (LLWS) using Doppler radar?
- Indicate (yes or no) if the following objects and phenomena can be detected with conventional or Doppler radar:
- hail in the clouds
- wind shear
- aircraft in flight
- aurora borealis
- snow showers
- blowing snow
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Environment Canada's World Wide Web Site.