Velocity, and what it cannot see
- View
Radar measures motion along the beam and nothing else. Everything confusing about the velocity display follows from that one sentence.
In this lesson you will
- Explain the radial-velocity-only restriction and the color convention.
- Distinguish base from storm-relative velocity.
- Identify a velocity couplet and explain what rotational velocity is.
- Identify aliasing, range folding and the zero isodop.
One component only
This is the whole lesson and everything else is consequence. The radar can only measure the part of a target's motion that is along the beam, toward it or away from it. Motion across the beam is completely invisible to it.
Green is toward the radar. Red is away from it. Nothing else. Red does not mean strong, dangerous, or outbound-tornado. A 70 knot wind blowing directly across the beam displays as zero, in whatever color zero is, while a gentle breeze straight down the beam displays as a real number. The colors are about geometry, not severity.
Base against storm-relative
- Base velocity
- What the radar measured: the ground-relative motion along the beam. Includes the storm bodily moving along, which on a fast-moving storm can dominate everything else.
- Storm-relative velocity
- The same field with the storm motion subtracted out. What is left is the circulation within the storm, which is what you actually want to look at. A rotation that is hidden in base velocity because the whole storm is racing along can be obvious in storm-relative.
It is only as good as the storm motion that was subtracted. If the assumed motion is wrong, the storm-relative field is wrong, and it can invent an apparent circulation or hide a real one. Forecasters know this and adjust it. An app that picks the motion for you may not have picked well.
Couplets
A rotating column, seen from the side by an instrument that can only measure along-beam motion, shows as strong inbound velocity immediately beside strong outbound velocity. That adjacency is the signature. Wide separation between the extremes is a broad circulation or none at all; tight adjacency between two adjacent radar samples is the thing forecasters call gate-to-gate shear.
Rotational velocity is the standard way to put a number on it: the average of the inbound and outbound peaks. If the strongest inbound is 50 knots and the strongest outbound is 40, the rotational velocity is 45 knots.
| What is known | What follows |
|---|---|
| Probability of tornado damage rises as low-level rotational velocity rises | Well established in the literature, and it is a probability rather than a threshold |
| About 45 knots at the lowest tilt is used as an impact-based warning tag threshold | The one specific number worth knowing, and it is a warning-issuance figure rather than a spotter figure |
| Values inflate or wash out with range | Beam broadening averages a tight circulation away. The same tornado gives a much smaller number at 100 km than at 30 |
This course deliberately publishes no table converting rotational velocity to a damage rating. Those tables exist, they bin differently between studies, and nothing module 7 asks of a spotter requires estimating a damage rating from a radar app.
A couplet is rotation aloft. It is not a tornado on the ground. Lesson 2 told you how high the beam is at range; the couplet you are looking at may be a mile up. Many couplets never produce a tornado. Some tornadoes occur under unimpressive couplets. You cannot see the ground on this product, and lesson 9 is about what that means for what you say.
Three artifacts that fool people
- Velocity aliasing
- The radar can only measure speeds unambiguously up to a limit. Above it, the value wraps around and displays as a strong velocity of the opposite sign. The giveaway is an abrupt transition from extreme inbound to extreme outbound with no sensible values between, often in a patch rather than a tight couplet. Software usually corrects it and sometimes fails.
- Range folding
- Returns from beyond the unambiguous range assigned to the wrong distance, displayed as blocks of missing data, purple on many color scales. Purple is absence of data, not severity. It is one of the more consequential misreadings in amateur use.
- The zero isodop
- The line through the display where the wind is blowing across the beam and the measured velocity is therefore zero. Its shape encodes how the wind turns with height, which is a forecaster skill. For a spotter it is worth recognizing so it is not mistaken for a boundary or a feature in the storm.
Check yourself
A velocity display shows a tight pair of strong green and strong red pixels immediately adjacent, 90 km from the radar. What can you report?
- Nothing from this alone. It is rotation aloft; go and look, and report what you can see
- A tornado, since gate-to-gate shear is the definitive tornado signature
- A tornado moving away from the radar, since the red side indicates outbound motion
- Strong straight-line winds, since the colors show air moving in opposite directions
Hint
Work out roughly how high the beam is at 90 km, using module 1 lesson 1.
Answer
Nothing from this alone. It is rotation aloft; go and look, and report what you can see
At 90 km the lowest beam is around 4,000 feet up, so a couplet there is a circulation most of a mile above the ground. It is a good reason to pay attention and it is not an observation of anything at the surface. The third option misreads the color convention, which is about direction relative to the radar and nothing else.
Sources
- Thompson et al. 2017, Wea. Forecasting 32(4), 1509–1528: tornado damage rating probabilities derived from WSR-88D data
- NOAA JetStream, the online school for weather: radar
- NWS Warning Decision Training Division, radar and warning guidance
- Smith et al. 2015, Wea. Forecasting: WSR-88D tornado intensity estimates and conditional probability of damage rating