For weather radars, the target of choice is precipitation (rain, snow, hail, etc.). Insects and "clear air echoes" can also provide useful meteorological information. 3-D fields of precipitation can be obtained in the vicinity of a weather radar by performing a complete rotation of the antenna at successively higher elevation angles. Typically, the range of meteorological radars is of the order of 150-300 km. Beyond this range, the radar looks too high in the atmosphere because of the curvature of the earth.
This short article explains the most popular methods of visualizing data from scanning radars.
When making measurements, the radar antenna is set at a given elevation and then rotated over a full 360°. At McGill, this process is repeated at 24 different elevations providing 3D information about precipitation. Grouped together, the 24 conical sectors form a 'volume scan'. A volume scan can be completed in approximately 5 minutes.
The following figure displays the altitude of the radar beam depending on the horizontal distance. Each of the thin blue lines represent one of the 24 angles of the operational scanning strategy utilized on the McGill S-band radar. For simplicity the curvature of the earth and diffraction by the atmosphere have been neglected.
In the early days of radars, it was customary to look at radar measurements from a single rotation of the antenna at a given elevation. This type of display is called a Plan Position Indicator (PPI). It is equivalent to displaying data along one of the blue lines of the above graph. What is seen is a conical sector projected on a 2D map. Because the altitude of measurements increases with range, the interpretation of PPIs may sometimes be difficult. For example, on the same image, one will often observe rain at close range and low levels, followed by snow beyond at higher levels.
This is illustrated in the figure below displaying a 4.9º PPI of a stratiform weather system. The quantity displayed is called Reflectivity. To a first approximation, reflectivity can be thought of as a measure of the intensity of precipitation with warmer colors indicating greater intensity.
The radar is at the center of the picture. The ring of stronger reflectivity ≈40 km away from the radar is a characteristic feature of many PPI displays. This enhancement of reflectivity is observed when snowflakes that form in cold air start melting and turn into rain. This melting layer is thus an indication of the altitude at which the atmosphere reach 0°C and it is often called the 'bright band'. At horizontal ranges closer than 40 km measurements are made in the rain. Further away, it is snow that is being measured.
PPIs at different elevations can be studied side by side to get a picture of the 3D structure of precipitation. This method of displaying radar data is not the most practical as it requires a little bit of imagination and a lot of training.
The caveats of PPis were addressed by the introduction of Constant Altitude Plan Position Indicator (CAPPI). This type of display combines information from multiple PPIs to produce a 2D map of radar measurements at a given altitude above ground. As illustrated in the graph at the top of this page, a CAPPI can be produced by using parts of the PPIs (thin blue line) that intersect with the thick orange line.
CAPPIs from the McGill radar are displayed in the real-time imagery section. An example is also given below.
In this figure, a banded system passes over the McGill radar. All measurements are made in the rain at approximately 1.5 km above the ground. Such maps are very straightforward to interpret. In addition this type of display makes it possible to merge measurements from many different radars to form mosaics. Most of the time successive CAPPIs are animated to highlight the movement of precipitation. atmospheric features. This is by far the most common product of weather radars.
One way to study winds is to look at CAPPIs of Doppler velocity. Doppler velocity is measured using the Doppler effect of moving targets on the radar echo frequency. Only the radial (along the beam) part of velocity is measured by a weather radar. In other words, a radar only sees velocity towards or away from it.
If we take the example of a constant wind from the north, strong approaching velocities will be observed when the radar looks north, strong receding velocities when the radar looks south, and no velocity when the radar looks east or west. This information can then be displayed (see figure on the right), using colder colors for increasingly strong approaching velocities and warmer colors for increasingly strong receding velocities.
Radial velocity images are usually more complicated than in this example because the wind is rarely uniform and because only in regions with targets (like rain, bugs, etc.) will you obtain information.
These complications can be fully appreciated by looking at a true CAPPI of Doppler velocity measurements shown below. In this figure, the wind is blowing away from the radar in the North-East quadrant while it is blowing towards it from the south.
Despite these limitations, Doppler information is valuable to weather forecasters especially in severe weather where rotation signatures (indicative of risk of tornado) and divergence signatures (indicative of strong downdrafts) can be identified. These two features can be detected by automatic algorithms.
Alone, Doppler velocity measurements do not enable one to infer the complete wind field around a radar. However, this information can be used in conjunction with an atmospheric model to do so.
Many other products of scanning radars also exist. For example one can add the reflectivities of many volume scans to produce maps of precipitation accumulation for a given period. These maps, such as the one below, can be useful for flood water management, hydrology studies, climatology studies...
Radar visualization software also allow the production of cross-sections of radar volume scans. This allows the direct observation of the vertical structure of weather patterns. In the cross-section depicted below we can clearly observe the melting layer signature below 5km. A direct comparison between this figure and the one at the very top of this page can be made.