What a dBZ is
- View
A logarithmic scale, a conversion that is an assumption rather than a measurement, and the difference between the two reflectivity products your app offers.
In this lesson you will
- Explain what dBZ is and why the scale is logarithmic.
- Explain what a Z-R relationship does and why more than one exists.
- Associate dBZ ranges with precipitation intensity and type.
- Distinguish base from composite reflectivity and state the pitfall.
The Z, and the d and the B
Reflectivity spans an enormous range: the returned power from drizzle and from a hail core differ by a factor of millions. So it is expressed on a logarithmic scale, in decibels relative to a reference. That is what dBZ is, and it has one consequence that matters constantly.
A step of 10 dBZ is a factor of ten in returned power. So 55 dBZ is not "a bit more than" 45 dBZ, it is ten times the return. Somebody comparing two storms as though the numbers were on a linear scale will badly underrate the difference between them.
From reflectivity to rainfall, which is an assumption
The radar measures returned power. It does not measure rainfall. Converting one to the other requires an assumption about the size distribution of the drops, and that assumption is called a Z-R relationship.
| Relationship | Used for |
|---|---|
| Z = 300 R^1.4 | The NWS default for convective rainfall |
| Z = 250 R^1.2 | Tropical systems, where the drop distribution is different |
| Z = 200 R^1.6 | Marshall-Palmer, the classic stratiform relationship |
Three different answers from the same returned power. Which is correct depends on what kind of rain it actually is, which the radar cannot independently determine, which is why forecast offices switch relationships by regime.
What this does to flood estimation is module 6 lesson 6, which covers quantitative precipitation estimation, the warm-rain low bias, the bright band and beam overshoot in full. This lesson only needs you to know that a rainfall number derived from radar is a conversion carrying an assumption, and that the assumption is sometimes wrong.
One detail worth having, because it explains something you will see: rainfall algorithms limit the reflectivity they will accept, somewhere around the low 50s dBZ. Above that the return is probably hail rather than rain, and feeding it into a rainfall relationship would invent a downpour that is not happening.
Reading the scale
| Roughly | Usually means |
|---|---|
| Below 20 dBZ | Drizzle, light snow, cloud, or non-weather targets |
| 20 to 40 dBZ | Stratiform rain. Steady, widespread, unremarkable |
| 40 to 55 dBZ | Convective rain. Heavy, showery, thunderstorm cores |
| Above about 60 dBZ | Hail is likely somewhere in that column |
Approximate and overlapping, and deliberately so: the boundaries are not sharp and anybody quoting them to the decibel has misunderstood the scale.
High dBZ does not mean tornado. It means a lot of returned power, which means big or numerous targets, which means heavy rain or hail. The strongest returns in a supercell are usually in the hail core, which is not where the tornado is. Red on the screen is not danger of a particular kind; it is a number.
Base against composite
- Base reflectivity
- What the radar saw on one elevation tilt, usually the lowest. A real slice through the atmosphere at a known height, which climbs with range exactly as lesson 2 described.
- Composite reflectivity
- The maximum value found anywhere in the vertical column, collapsed onto a map. Useful for finding the strongest part of a storm quickly, and it tells you nothing about what height that value came from.
A composite image can show a fierce-looking 65 dBZ core that is entirely aloft, with far less reaching the ground. Composite reflectivity is not what is happening at the surface, and a spotter comparing it against what they can see out of the window will conclude the radar is wrong when the radar is answering a different question. If your app offers both, know which one you are looking at.
Check yourself
Your app shows a 62 dBZ core on composite reflectivity over your town. What can you legitimately conclude?
- Something is producing a very strong return somewhere in that column, likely hail, at an unknown height
- Hail is falling at the surface over your town right now
- A tornado is likely, since this is among the highest reflectivity values seen
- About 62 mm of rain per hour is falling, from the standard conversion
Hint
Two words in the question are doing work: "composite", and "column".
Answer
Something is producing a very strong return somewhere in that column, likely hail, at an unknown height
Composite reflectivity is the column maximum with the height discarded, so a high value tells you a strong return exists somewhere in that column and not that it is reaching the ground. High reflectivity indicates hail or heavy rain rather than rotation, and the last option applies a rainfall conversion to a return that the algorithms themselves would cap as probable hail.