How old is that picture
- View
What a volume scan is, what the low-level supplementary cuts buy you, and the arithmetic that converts data age into distance on the ground.
In this lesson you will
- Explain what a volume coverage pattern is and roughly how long a scan takes.
- Explain what supplementary low-level scanning adds.
- Distinguish Level II from Level III data.
- Convert data latency into a distance, and state the safety implication.
A volume scan is a stack of cones
The radar does not take a picture. It sweeps a full circle at one elevation, tilts up, sweeps again, and repeats through a set of elevations. That set is a volume coverage pattern, and one complete pass through it is a volume scan. What your app shows as a single image is assembled from a sweep that took time to make.
| Pattern type | Roughly | When it is used |
|---|---|---|
| Fast severe-weather patterns | about 4.5 to 5 minutes | Convection. The priority is revisiting low levels often |
| Deeper severe patterns | about 7 minutes | Convection where sampling the full depth matters more |
| Shallow and clear-air patterns | about 7 to 10 minutes | Winter, light stratiform, and quiet days. Slower and more sensitive |
Deliberately given as ranges. The operational set of patterns and their exact timings are revised by the agency from time to time, and a spotter who has memorised a list from a three-year-old handout is worse off than one who knows the principle. The principle is: severe weather patterns take four to five minutes, quiet-weather patterns take longer.
More elevation angles means slower updates, not better ones. Every extra tilt is time the radar is not spending back at the bottom of the storm. The whole history of scanning strategy is a negotiation between sampling the depth of the storm and revisiting the part of it nearest the ground.
Going back to the bottom, more often
Which is what the supplementary low-level scanning schemes exist to fix. Rather than waiting a full volume to look at the lowest tilt again, the radar inserts extra low-level sweeps inside the volume.
- One extra low-level cut roughly halves the time between looks at the bottom of the storm.
- Up to three extra cuts can bring the lowest-tilt update down to somewhere around 75 to 90 seconds.
- Terminating the high tilts early when there is nothing up there shortens the volume without losing anything, and buys the same effect a different way.
It does not shorten the volume scan; it inserts extra low-level looks inside it. Module 1 lesson 1 makes the same correction because it is the commonest misreading of this feature. You get the bottom of the storm more often. You do not get the whole storm more often.
The reason any of this exists is in the record. The service assessment for the 2011 Joplin tornado found that "limited scans at lowest elevation slices during this time impacted the WFO's ability to ascertain the magnitude of the tornado." Lesson 10 returns to that event.
Level II and Level III
- Level II
- The full-resolution base data: reflectivity, velocity, spectrum width and the dual-polarisation fields, at every elevation, as the radar recorded them. Large, complete, and what research and the better desktop applications use.
- Level III
- Derived and reduced products generated from Level II: individual tilts, algorithm output, precipitation totals. Smaller, quicker to move, and most public-facing displays are built from it. It arrives after Level II because it is made from it.
The arithmetic that matters
Here is the calculation nobody does and everybody should. Your app shows an image. That image is old by the sum of several delays: the sweep itself took time, the volume had to finish and be assembled, products had to be generated, and the data had to reach your phone.
| Storm speed | It has moved this far in 5 minutes | And in 10 |
|---|---|---|
| 20 mph | 1.7 miles | 3.3 miles |
| 35 mph | 2.9 miles | 5.8 miles |
| 50 mph | 4.2 miles | 8.3 miles |
| 60 mph | 5.0 miles | 10.0 miles |
Simple arithmetic rather than a published figure, and that is the point: you can do it in your head. Five minutes is optimistic for the total age of a consumer radar image; ten is not unusual.
The storm is not where the screen says it is. At 50 mph it is somewhere up to four miles further along than the picture shows, and you cannot tell in which of those miles it actually is. Module 8 said a spotting position needs an escape route and a margin; this is the number that sets the margin. Anybody navigating relative to a storm using an app alone is navigating relative to where it used to be.
Two habits follow. Read the timestamp on every image, every time, and treat an app that does not show you one as unusable for this purpose. And watch the trend rather than the frame: three frames tell you where a storm is going, which partly compensates for the fact that no frame tells you where it is now.
Check yourself
A squall line is moving at 55 mph. Your radar app image is timestamped 8 minutes ago. How far from the displayed position should you assume the line actually is?
- About 7 miles further along its track, with real uncertainty about exactly where
- Within about half a mile, since radar updates roughly every 5 minutes
- Exactly where it is displayed. The timestamp is the time of display
- It cannot be estimated without knowing the volume coverage pattern
Hint
55 mph is a little under a mile a minute.
Answer
About 7 miles further along its track, with real uncertainty about exactly where
Just over 7 miles. The arithmetic is deliberately simple because you need it in your head rather than on paper. The fourth option is the trap: the scan pattern affects how often you get a new image, not how far the storm has traveled since the last one.
Sources
- Boettcher et al. 2022, Wea. Forecasting 37(2), 153–171: the influence of WSR-88D intra-volume scanning strategies on thunderstorm observations and warnings
- NWS Radar Operations Center
- NWS Warning Decision Training Division, radar and warning guidance
- NWS Central Region, service assessment: Joplin, Missouri tornado, 22 May 2011