Shapes worth knowing
- View
Hook echo, inflow notch, weak echo region, three-body scatter spike, bow echo and debris ball. Six shapes, and six limits on what they mean.
In this lesson you will
- Identify the hook echo and inflow notch and state what they do and do not indicate.
- Describe the weak echo region and bounded weak echo region.
- Identify the three-body scatter spike and state its criteria and meaning.
- Identify bow echo structure and the debris ball.
A note on scope
Modules 2 and 4 taught you what a supercell is and where its rotation lives. This lesson does not re-teach that. It teaches what those structures look like on a reflectivity display, which is a different skill, and it is careful throughout about the gap between seeing a shape and knowing what is under it.
Hook echo and inflow notch
The famous one. A hook-shaped appendage on the rear right flank of a supercell, formed as precipitation wraps around the mesocyclone. The inflow notch is the related feature: a concave bite out of the reflectivity where inflow is eroding the echo.
That precipitation is being wrapped around a rotating updraft. That is genuinely significant and it is all it means. A hook echo is not a tornado, plenty of hooks never produce one, and the hook you are looking at is the precipitation field some thousands of feet up. Module 8 lesson 4 had the operational version of this: the notch is also the worst place on the storm to be.
Weak echo regions
- Weak echo region
- An area of low reflectivity beneath high reflectivity aloft, on the inflow flank. The updraft is strong enough that precipitation has not yet formed or has been carried up and over rather than falling out.
- Bounded weak echo region
- The same thing enclosed on all sides, a vault of low reflectivity surrounded by high. It is seen on a vertical cross-section rather than a plan view, and it is one of the clearer radar indications of a very strong, persistent updraft.
For a spotter these matter mostly as vocabulary: they are terms a forecaster may use on a net, and they indicate updraft strength rather than any surface hazard. Module 5 is the lesson on what a strong updraft means for hail size.
The three-body scatter spike
The most useful signature in this lesson and the least known. A spike of weak reflectivity extending radially away from the radar, directly behind a very intense core. It is an artifact: energy bounces off large hail down to the ground, back up to the hail and then to the radar, and the extra travel time makes the radar place it further away than it is.
| Property | Value |
|---|---|
| Core reflectivity required | above about 63 dBZ |
| Spike length | roughly 10 to 30 km beyond the core |
| Direction | always radially away from the radar, never any other direction |
| Typical lead on the largest surface hail | about 10 to 30 minutes |
Lemon 1998 describes it as a sufficient but not necessary condition for large hail: seeing one is strong evidence, and not seeing one proves nothing.
It is not hail falling where the spike is. There is nothing there at all; the spike is a timing artifact and the hail is back in the core. And it is not a feature of the storm, so it points differently depending on which radar you are looking at, which is a neat way to confirm you have identified one correctly.
Lines: bow echoes and what they carry
Module 2 lesson 8 covers the physics. On reflectivity what you are looking for is a segment of a line that has bowed forward, often with a notch of weaker reflectivity behind the apex where the rear-inflow jet is punching in. A line of them makes the line-echo wave pattern.
The operational meaning is straightforward and the limit is the usual one: the bow apex is where the strongest straight-line winds are expected, and circulations can spin up along the leading edge, especially near the ends of the bow. What the reflectivity picture cannot tell you is whether any of them is on the ground.
Debris ball
A small, intense, roughly circular reflectivity maximum at the tip of a hook. It is lofted debris returning energy, and it is genuinely significant: debris implies something at the surface picked it up.
On reflectivity alone a debris ball is indistinguishable from a small intense precipitation core, and a spotter who calls one from a single reflectivity image is guessing. Lesson 7 gives you the field that separates debris from rain unambiguously, and it is the reason the modern answer to this question is a four-panel and not a picture.
Check yourself
You see a narrow spike of weak reflectivity extending away from the radar behind a 66 dBZ core. What does it tell you?
- Large hail is very likely in the core. The spike itself is an artifact and nothing is falling there
- Hail is falling along the length of the spike
- A tornado is likely, since the signature indicates lofted debris
- The radar has a hardware fault producing a false radial
Hint
The spike always points the same way relative to one thing in particular.
Answer
Large hail is very likely in the core. The spike itself is an artifact and nothing is falling there
A three-body scatter spike, and it needs a core above about 63 dBZ. The energy took a longer path via the ground and back, so the radar places it too far away: there is nothing physically there. It is strong evidence of large hail in the core, and says nothing about rotation.
Sources
- Lemon 1998, Wea. Forecasting 13(2), 327–340: the radar three-body scatter spike, an operational large-hail signature
- NOAA JetStream, the online school for weather: radar
- NWS Warning Decision Training Division, radar and warning guidance
- NWS Weather Spotter's Field Guide