D3 Weather Radars

Topic

Weather radars are active remote-sensing systems: unlike a satellite, which merely receives radiation naturally emitted or reflected by the atmosphere, a weather radar emits its own pulses of microwave radiation—with wavelengths between three and ten centimeters—and then measures the energy returned as the pulses backscatter off hydrometeors (such as raindrops, ice crystals, or hail) encountered along their path. The intensity of this backscattered energy is known as reflectivity; empirically, the higher the reflectivity measured at a given point, the higher the precipitation rate at that same location.

Weather radars do not operate in isolation but are organized into networks covering entire regions: a network of over 160 Doppler radars covers the entire United States, while a similarly extensive network of around 200 radars covers much of Europe; in both cases, measurements are updated every five to ten minutes.

Within this family of weather radars, the Doppler radar offers an additional capability: it measures the frequency shift of the backscattered wave relative to the originally emitted frequency—the Doppler effect—and uses this shift to infer the radial velocity of the hydrometeors (i.e., how fast they are moving toward or away from the radar along the beam's path). Since hydrometeors are carried by the wind, this radial velocity allows one to infer the wind speed along the direction of the beam.

However, a single Doppler radar only detects the wind component moving directly toward or away from it, rather than the full wind vector. Reconstructing the three-dimensional horizontal wind requires additional techniques: the dual-Doppler method combines measurements from two radars observing the same region from different angles, while an alternative method combines measurements from a single Doppler radar taken at different scanning angles to achieve an equivalent result. Within this same family of weather radars, dual-polarization radars address a limitation distinct from that of simple Doppler radar: instead of emitting microwave pulses in a single orientation, they transmit in both horizontal and vertical planes and compare how the hydrometeor responds to each orientation. From this comparison, they derive additional variables describing whether the hydrometeor is wider than it is tall, the extent to which it delays the signal passing through it, and the uniformity of its shape. Using these additional variables, dual-polarization radar can classify the type of hydrometeor—rain, hail, snow, or a mixture thereof—and, based on that classification, calculate a more accurate precipitation rate than a conventional radar that measures only reflectivity.

Cloud radar applies the principles of weather radar at frequencies much higher than those of conventional precipitation radar—approaching 94 gigahertz. This much higher frequency makes it sensitive to particles far smaller than raindrops or hailstones: the tiny droplets that form clouds and the smallest ice crystals, which conventional precipitation radar simply cannot detect. Thanks to this enhanced sensitivity, cloud radar reveals the internal structure of clouds that remains invisible to conventional precipitation radars.

The combined output of this entire family of weather radars—ranging from conventional reflectivity radar to Doppler, dual-polarization, and cloud radars—yields a set of three-dimensional reflectivity fields that reveal the complete structure of precipitation systems. They allow for the identification of the "bright band"—the layer where falling snow melts into rain—and generate Doppler wind vectors that describe air movement within those same precipitation systems. Thus, weather radars convert the energy they emit—and which is returned to them by hydrometeors—into a constantly updating, three-dimensional image showing where it is raining, snowing, or hailing across the territory they cover.

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