Airborne Weather Radar — Page 208, Lesson 198

Let’s start with the heart of the system: the radar frequency. The whole design of an airborne weather radar is built around one idea — you want a wavelength that matches the size of the things you’re trying to see. In our case, that’s the large water droplets and wet hail that live inside severe turbulence. Those droplets are about 3 cm across. So the typical frequency adopted by most commercial systems is 9375 MHz, plus or minus 30 MHz. That frequency gives the best returns from those large droplets and wet hail in convective clouds, and it also lets us produce narrow, efficient beams. Let’s do the wavelength calculation. Wavelength, λ, equals 300 divided by 9375, in metres, which comes out to 3.2 cm. That matches the droplet size nicely. Now, why not go higher or lower? If we went higher than 9375 MHz, we’d start getting returns from smaller droplets, which would just create unnecessary clutter on the display. If we went lower, we wouldn’t get enough return signal to highlight the turbulence area. So 9375 MHz is the sweet spot. Next, let’s talk about water and ice in the radome. The radome is the protective cover over the antenna. Some of the radar’s energy gets absorbed by water and ice — just like in a microwave oven. If there’s water inside the radome or ice on the outside, that absorbed energy causes the water to evaporate and the ice to melt. The problem is, that energy is now being used up locally instead of being transmitted forward. So less energy goes out in the forward direction, you get weaker returns, and overall performance degrades. That’s a real operational consideration — a wet or icy radome quietly weakens your weather picture. Now, what is the equipment actually designed to do? It’s designed to detect clouds that are likely to produce turbulence, to highlight where the turbulence is most severe, and to indicate safe routes to avoid those areas where possible. Here’s the key relationship: the size and concentration of water droplets in a cloud is an indication of turbulence. But note — this does not apply to clear air turbulence, CAT. CAT is invisible to this radar. Also, in continuous rainfall, the shorter the distance between light and strong returns, the steeper the rainfall gradient, and the greater the likelihood of turbulence. So a tight transition from weak to strong signal is a red flag. For a given transmission power, a 3 cm wavelength gives the best returns from large water droplets. Wavelengths of 10 cm and above produce very few weather returns — that’s why we don’t use those for weather detection. Now let’s get into the colour coding, because that’s how you actually read the display. In colour weather radar systems, targets are colour-coded by rainfall intensity. Black means very light or no returns — less than 0.7 mm per hour. Green means light returns — 0.7 to 4 mm per hour. Yellow means medium returns — 4 to 12 mm per hour. Red means strong returns — greater than 12 mm per hour. And magenta indicates turbulence, due to rainfall intensity. One important note: on colour systems without magenta, the red areas may have a cyclic function. That means the red areas alternate between red and black, flashing, to draw the pilot’s attention to the severe areas. So where’s the worst turbulence? The areas of greatest potential turbulence occur where the colour zones are closest together — that’s the steepest rainfall gradient. And there are specific shapes on the display that are also associated with turbulence: U-shapes, fingers, scalloped edges, and hooks. These are all areas to avoid. Let me show you the beam coverage at varying ranges — that’s Figure 13.6. You can see the radar beam coverage at different ranges, from 0 NM out to 180 NM, with the vertical coverage in feet and the 3-degree beam angle. That gives you a sense of how the beam spreads as range increases. And Figure 13.7 shows the reflective levels of different precipitation types — that’s the basis for the colour coding we just went through. So to tie it all together: the frequency choice gives you the right wavelength to see the dangerous droplets, the radome condition affects how much energy actually gets out, and the colour display translates rainfall intensity into a picture you can read — with the steepest gradients and those specific shapes telling you where to steer clear.