Shapes worth knowing
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.
A note on scope
Modules 2 and 4 taught you what a A thunderstorm with a single rotating updraft that can persist for hours. The storm type that produces most strong tornadoes, very large hail and the most damaging winds. is and where its rotation lives. This lesson does not re-teach that. It teaches what those structures look like on a How much of the radar pulse bounced back from whatever the beam was passing through on that tilt. It is the familiar green-to-red precipitation picture, and it describes what is up in the beam, not what is reaching the ground. display, which is a different skill, and it is careful throughout about the gap between seeing a shape and knowing what is under it.
A hook-shaped appendage on the reflectivity image, wrapped around the back of a supercell. One of the oldest radar clues that a storm is rotating. and A concave bite out of the reflectivity on the inflow side of a storm, where air being drawn in is eroding the precipitation. A radar clue that the storm has a strong, organised updraft.
The famous one. A hook-shaped appendage on the rear right flank of a supercell, formed as precipitation wraps around the The rotating updraft inside a supercell, typically a few miles across. Radar can see it aloft; whether anything is rotating at the ground is a separate question.. The inflow notch is the related feature: a concave bite out of the reflectivity where inflow is eroding the echo.
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.
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.
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 A current of air flowing into the back of a line of storms and descending toward the leading edge. Where it reaches the ground it pushes the gust front forward, which is what makes a line bow. 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.
A small intense roughly circular reflectivity maximum at the tip of a hook echo, from lofted debris. On reflectivity alone it is indistinguishable from a small precipitation core, so it needs the dual-pol fields to confirm.
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.
You see a narrow spike of weak reflectivity extending away from the radar behind a 66 dBZ core. What does it tell you?
The spike always points the same way relative to one thing in particular.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.