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In an answer to a questions about turbulence this answer mentioned that aircraft use radar to help detect the most turbulent areas of a storm so that pilots can avoid them.

How does radar do this? Does it have to do with rain inside the storm moving up and down with the wind? And if so, can radar detect wind speeds outside of a storm at all?

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  • $\begingroup$ Have you read the answers here? $\endgroup$ – fooot♦ Dec 4 '15 at 17:37
  • $\begingroup$ @fooot Have now, but neither of those answers really answer my question. They give a good general overview of weather radar, but don't really give detail on how weather radar see's turbulence. $\endgroup$ – Jay Carr Dec 4 '15 at 17:44
  • $\begingroup$ More on CAT detection by airborne UV lidar: DELICAT LIDAR system alerts pilots to air turbulence ahead. $\endgroup$ – mins Dec 5 '15 at 17:39
  • $\begingroup$ @mins You might consider spinning that into an answer and seeing how everyone votes on it. $\endgroup$ – Jay Carr Dec 6 '15 at 2:47
  • $\begingroup$ More on Low Level Wind Shear detection on airport in Japan by lidar and radar combined. Also this presentation. $\endgroup$ – mins Dec 6 '15 at 21:05
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The answer you are referring to explains how radar is used to fly through a squall line whilst avoiding the most turbulent areas.

Conventional weather radar cannot detect wind speed or turbulence directly, it only detects solid and liquid objects above a threshold size.

The most turbulent areas of thunderstorms are also those areas that contain the most water and ice. This precipitation reflects radar waves very well so the most turbulent areas have the strongest reflections and shows up as red / purple on the scope. The area downwind of the core is usually very turbulent as well.

So the weather radar is merely detecting the presence of water. It is the pilot interpreting the picture who is really detecting the turbulence.

Clear air turbulence (CAT) does not have condensated water in it and is therefore undetectable by conventional radar. There are systems such as LIDAR that can detect CAT but they generally not installed on aircraft.

As @reirab noted in a comment, a Doppler radar can be used to detect the relative speed of the particles from an aircraft. This is not used to reliably detect wind speed, but when there are large variations in wind speed (a.k.a. turbulence) the Doppler shift in the reflections from the particles causes a spectrum spread. This is an indication for turbulence. The traditional S-band airborne weather radars do not have this capability but the newer X-band radar (e.g. this Honeywell radar (PDF)) can detect wet turbulence to distance of up to 40-60 nautical miles using this technique.

LIDAR (a portmanteau of Light and RADAR) is a technology that uses light instead of radio waves. LIDAR can measure position and relative velocity of aerosols and therefore it can be used to detect turbulence. It has been used successfully to detect wind shears and map wake vortices on final approach from the ground (paperPDF).

Currently investigation (paperPDF, presentationPDF)is going on into airborne use of LIDAR to detect CAT. One of the difficulty is to detect vertical components of the turbulence. This causes very little Doppler shift but at the same time have the most effect on the aircraft (since the vertical component of turbulence causes changes in angle of attack).

As far as I know there are currently no commercial applications of LIDAR for airborne turbulence detection.

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  • $\begingroup$ Conventional radar can't detect wind directly, but it can detect the motion of particulates (like water droplets) via the Doppler effect (which is why weather radars are often called 'Doppler radars.') Since the velocity of the water droplets will usually be more or less equal to the local wind velocity, we can figure out wind speed (at least the component of it toward or away from the radar) using this effect. You're right, though, that this is harder for CAT, due to the lesser magnitude reflections from particulates. $\endgroup$ – reirab Dec 9 '15 at 21:31
  • $\begingroup$ @reirab You can't reliably make out the wind component from an airborne weather radar. When the Doppler shift causes a spread spectrum you can detect variation in wind speed when there is wet turbulence. This is used on the newer generation X-band weather radars. Many aircraft still fly around with the more conventional S-band weather radar, which doesn't have this turbulence mode. The X-band types can detect wet turbulence to up to a range of about 40-60 NM. $\endgroup$ – DeltaLima Dec 9 '15 at 22:06
  • $\begingroup$ @reirab I don't think typical weather radars do anything to measure these velocities though, or at least they are not displayed. Radar displays normally show "base reflectivity" in dBm. This has nothing to do with the velocity of the particles doing the reflecting. $\endgroup$ – Joel M. Dec 9 '15 at 22:07
  • $\begingroup$ @JoelM. I'm not sure about airborne radars, but it's done all the time on land-based weather radars. In the U.S., the National Weather Service makes graphical displays of the wind data available to the public from all of its Nexrad radars. See here, for example. This is especially useful for detecting high levels of wind shear, which can indicate rotation (and, therefore, tornado formation.) This is one of the NWS' primary tools for issuing tornado warnings. $\endgroup$ – reirab Dec 9 '15 at 22:12
  • $\begingroup$ @JoelM. Note that what is being measured here is different. For wind speeds, they're measuring Doppler shift, i.e. the frequency of the received signal (and its difference from the frequency that was originally transmitted.) It's the amplitude of the received signal that is displayed in dBm, which gives you information about precipitation intensity. Both the frequency and amplitude information are derived from the same received signal, though. $\endgroup$ – reirab Dec 9 '15 at 22:18
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The short answer is that radar cannot directly observe the wind. What radar can detect is the velocities of small particles lofted into the air. This is done by measuring the doppler shift of the energy returned to the radar. The radar can only detect the component of velocity toward or away from the radar.

The radar doesn't have to diagnose turbulence within convection, as that is assumed to be there already and is part of the nature of convection. It is much more valuable to detect clear air turbulence which does not have the visual indications that boiling cumulus growth does.

Thankfully, clear air generally isn't exactly clear. If there are aerosols or bugs that are large enough to scatter radar energy then the radar can infer their motion as the wind's motion. For small things like dust and insects this isn't a bad approximation. The return energy of these scatterers is very small and is generally below the threshold that the radar display will plot (unlike birds or bats), but the computers can interpret the data.

The detection of turbulence is algorithmic, and while I do not have knowledge of specific algorithms used, my first order guess is that they look at the variation in doppler velocity with distance from the airplane and they look at the time-series of doppler velocities at each sampled location in subsequent scans. If the variation meets some criteria this could be classed as "turbulence" and provide an alert to the pilots. The ability of such a system also depends on the physical capabilities of the radar including the wavelength the radar operates at, its sampling resolution and the sampling rate.

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    $\begingroup$ Radar actually detects clear air turbulence using changes in refractivity caused by turbulence, not the motion of particulates. No radar that I know of can see and measure the motion of objects as small as bugs, they are effectively invisible to radars of any wavelength large enough to propagate in the atmosphere. Measurement of refractivity is very difficult to do, however, and as far as I know there are very few radars that are actually designed to do it. See: journals.ametsoc.org/doi/abs/10.1175/… $\endgroup$ – Joel M. Dec 5 '15 at 0:10
  • $\begingroup$ Effectively what I'm saying is that CAT(clear air turbulence) is not typically measured with radar. It is inferred from things like convective weather and precipitation, but not measured. $\endgroup$ – Joel M. Dec 5 '15 at 0:11
  • $\begingroup$ @JoelM. I welcome an edit to my post if you want to contribute your knowledge. As far as bugs, I can't attest to airborne radar, but I think you kind find plenty of examples on S-band e.g. the WSR88D network. I've also seen similar use of lofted insects causing radar fine lines at convergence boundaries both on WSR-88D and the various incarnations of the DOW vehicles. There has even been some recent research into the dual-pol signatures of insects, e.g. journals.ametsoc.org/doi/abs/10.1175/JTECH-D-13-00247.1 $\endgroup$ – casey Dec 5 '15 at 1:35
  • $\begingroup$ it sounds like your experience in this area is greater than mine, so I can only hypothesize but I do know that radar will only be able to detect things like bugs and particulates in large, dense quantities. So while I suppose it's possible to detect turbulence in that way, it doesn't seem like a very reliable method. $\endgroup$ – Joel M. Dec 5 '15 at 14:38
  • $\begingroup$ Do you know of any radar system that is regularly used to directly detect turbulence? The only ones I've heard of using the refractivity method are ones built for other purposes, and then used to detect CAT for research. $\endgroup$ – Joel M. Dec 5 '15 at 14:40

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