RadarScope displays data from 30 radars operated by Environment Canada. These radars are based on different technology than the NEXRAD radars in the United States. The product suite is more limited, and the ground clutter is often more of a problem due to both technological differences and surrounding terrain.
Doppler radars work by bouncing radio waves off particles in the air. Those particles could be raindrops, hail, snow, or even dust and insects. The amount of energy that bounces off of those particles and returns to the radar is called “reflectivity” and is represented by the variable “Z”. Reflectivity covers a wide range of signal strength, from very weak to very strong, so it is measured on a decibel (logarithmic) scale in units of dBZ, or decibels of Z. The higher the dBZ value, the larger the number and/or size of the particles the radar beam is seeing.
The dBZ values increase as the strength of the signal returned to the radar increases. The scale of dBZ values is related to the intensity of rainfall. It is important to remember, however, that the radar shows only areas of returned energy and not necessarily precipitation. So the presence of a return, especially a very weak return below 20 dBZ, doesn’t always mean that it’s raining.
The colors along the bottom of the map correspond to precipitation types and intensities. When you move your cursor across the squares, RadarScope will display a value for each color. NEXRAD radars can’t distinguish between different types of precipitation with absolute certainty. However, reflectivity values can be somewhat roughly associated with different precipitation types:
The Canadian base reflectivity product has a resolution of 0.5 kilometers and a range of 112 kilometers.
The Canadian long-range reflectivity product in RadarScope is synthesized from two products we receive from Environment Canada: a short-range reflectivity that filters out most ground clutter, and a long-range product with a 1-kilometer resolution that extends to 230 kilometers.
Precipitation Depiction is a proprietary DTN product combining super-res reflectivity data and surface observations to show a depiction of the current hydrometeor type. Rain is displayed using a somewhat traditional color palette for reflectivity, snow is presented in shades of blue, and wintry mix is shown in shades of red. The determination of rain, mix, or snow is an approximation based on multiple data sources and subject to error.
Doppler velocity products indicate storm motion toward or away from the radar. The velocity products in RadarScope use the Doppler effect to determine how fast the particles in the air are moving relative to the radar itself. Negative values (green in RadarScope) indicate motion toward the radar, while positive values (red in RadarScope) indicate motion away from the radar. They can be difficult to interpret without training and experience, but Doppler velocity products can be used to detect the overall movement of a storm as well as relative motion within the storm itself, such as rotation. Note that the radar can only detect the component of the velocity vector along the radar beam, so this isn't a full picture of the wind field. But it gives you a fairly good idea which way a storm is heading. The Canadian base velocity product has a resolution of 0.5 kilometers and a range of 112 kilometers.
Environment Canada is in the midst of a multi-year project to replace all of the radars across Canada. These radars are based on newer technology and provide an improved and expanded product suite. RadarScope will support these new radars as they come online and the data are made available to us. RadarScope currently displays data from five of these radars.
Doppler radars work by bouncing radio waves off particles in the air. Those particles could be raindrops, hail, snow, or even dust and insects. The amount of energy that bounces off of those particles and returns to the radar is called “reflectivity” and is represented by the variable “Z”. Reflectivity covers a wide range of signal strength, from very weak to very strong, so it is measured on a decibel (logarithmic) scale in units of dBZ, or decibels of Z. The higher the dBZ value, the larger the number and/or size of the particles the radar beam is seeing.
The dBZ values increase as the strength of the signal returned to the radar increases. The scale of dBZ values is related to the intensity of rainfall. It is important to remember, however, that the radar shows only areas of returned energy and not necessarily precipitation. So the presence of a return, especially a very weak return below 20 dBZ, doesn’t always mean that it’s raining.
The colors along the bottom of the map correspond to precipitation types and intensities. When you move your cursor across the squares, RadarScope will display a value for each color. NEXRAD radars can’t distinguish between different types of precipitation with absolute certainty. However, reflectivity values can be somewhat roughly associated with different precipitation types:
The Canadian super-res reflectivity product has a resolution of 0.5 kilometers by 0.5 degrees, it provides the highest-resolution reflectivity available from Canadian radars to a distance of 230 kilometers from the radar. Super-res reflectivity is available at four different tilts or beam angles, with tilt 1 being the lowest to ground level.
Doppler velocity products indicate storm motion toward or away from the radar. The velocity products in RadarScope use the Doppler effect to determine how fast the particles in the air are moving relative to the radar itself. Negative values (green in RadarScope) indicate motion toward the radar, while positive values (red in RadarScope) indicate motion away from the radar. They can be difficult to interpret without training and experience, but Doppler velocity products can be used to detect the overall movement of a storm as well as relative motion within the storm itself, such as rotation. Note that the radar can only detect the component of the velocity vector along the radar beam, so this isn’t a full picture of the wind field. But it gives you a fairly good idea which way a storm is heading. The Canadian super-res velocity product has a resolution of 0.5 kilometers and a range of 230 kilometers.
Within any volume that is sampled by a radar, there can be a wide range of motions being observed. Spectrum width is a measure of that variation. Higher values of spectrum width correlate to a wider range of velocities being observed (turbulent flow; e.g. mesovortices); lower values indicate a narrower range (smooth flow; e.g. straight-line winds). With proper interpretation, spectrum width can provide an indication of turbulence, which can be helpful in identifying conditions associated with severe thunderstorm activity.
The Differential Reflectivity (ZDR) product shows the difference in returned energy between the horizontal and vertical pulses of the radar. Differential Reflectivity is defined as the difference between the horizontal and vertical reflectivity factors in dBZ units. Its values can range from -7.9 to +7.9 in units of decibels (dB).
Positive values indicate that the targets are larger horizontally than they are vertically, while negative values indicate that the targets are larger vertically than they are horizontally. Values near zero suggest that the target is spherical, with the horizontal and vertical size being nearly the same. Canadian Differential Reflectivity is available in at 0.5-kilometer resolution.
Differential Reflectivity values are biased toward larger particles. Stated differently, the larger the particle, the more it contributes to the resulting reflectivity factor. Hence while raindrops are normally wider than they are tall which would tend to yield a positive ZDR value, a scattering of large hailstones in the same volume of air being observed will yield a ZDR value closer to 0, because the spherical shape of the larger objects contributes more to the final reflectivity value. If the base reflectivity product is indicating high dBZ values whereas differential reflectivity is returning values near zero, then the volume in question is likely filled with a mixture of hail and rain.
The Correlation Coefficient (CC) product is defined as the measure of how similarly the horizontally and vertically polarized pulses are behaving within a pulse volume. Its values range from 0 to 1 and are unitless, with higher values indicating similar behavior and lower values conveying dissimilar behavior. The CC will be high as long as the magnitude or angle of the radar’s horizontal and vertical pulses undergo similar change from pulse to pulse, otherwise it will be low. It is available in 0.5-kilometer resolution.
Correlation Coefficient serves well at discerning echoes of meteorological significance. Non-meteorological echoes (such as birds, insects, and ground clutter) produce a complex scattering pattern which causes the horizontal and vertical pulses of the radar to vary widely from pulse to pulse, yielding CC values typically below 0.8. Hail and melting snow are non-uniform in shape and thus cause a scattering effect as well, but these meteorological echoes have more moderate CC values ranging from 0.8 to 0.97. Uniform meteorological echoes such as found in rain and hail yield well-behaved scatter patterns, and their CC from pulse to pulse generally exceeds 0.97.
The accuracy of the Correlation Coefficient product degrades with distance from the radar. The CC will also decrease when multiple types of hydrometeors are present within a pulse volume, thus a volume with rain and hail will yield a lower CC than the same volume with solely rain.
Differential phase shift in general (technically classified as propagation differential phase shift) is the difference between the horizontal and vertical pulses of the radar as they propagate through a medium such as rain or hail and are subsequently attenuated (slow down). Due to differing shapes and concentration, most targets do not cause equal phase shifting in the horizontal and vertical pulses. When the horizontal phase shift is greater than the vertical the differential phase shift is positive, otherwise it is negative. Stated differently, horizontally oriented targets will produce a positive differential phase shift, whereas vertically-oriented targets product a negative differential phase shift.
While this correspondence between positive values (horizontal) and negative values (vertical) is analogous to Differential Reflectivity (ZDR), there is a key distinction: differential phase is dependent on particle concentration. That is, the more horizontally oriented targets are present within a pulse volume, the greater the positive differential phase shift. Thus a high concentration of small raindrops could yield a higher differential phase value than a smaller concentration of larger raindrops. Differential phase shifting is largely unaffected by the presence of hail, and shifts in snow and ice crystals are typically near zero degrees. Non-meteorological echoes (birds, insects, and so forth) produce highly variable differential phase shifts.
Specific Differential Phase (KDP) is defined as the range derivative of the differential phase shift along a radial. Its possible values range from -2 to 7 in units of degrees per kilometer. It is available in 0.5 km resolution. It is best used to detect heavy rain. Areas of heavy rain will typically have high KDP due to the size or concentration of the drops. Hail and snow/ice crystals have no preferential orientation and will yield KDP values near zero degrees. Non-meteorological echoes will result in noisy KDP values. KDP is not calculated for areas in which the Correlation Coefficient (CC) is less than 0.9, which will result in gaps in the rendered data.
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