What a dBZ is
A logarithmic scale, a conversion that is an assumption rather than a measurement, and the difference between the two reflectivity products your app offers.
The Z, and the d and the B
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. 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 The unit radar reflectivity is measured in. Roughly, 20 is drizzle, 40 is a proper thunderstorm core and 60 or more usually means hail. is, and it has one consequence that matters constantly.
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 The assumed conversion from radar reflectivity to rainfall rate. Several exist because the right one depends on the drop size distribution, which the radar cannot independently determine..
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.
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
Approximate and overlapping, and deliberately so: the boundaries are not sharp and anybody quoting them to the decibel has misunderstood the scale.
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.
Your app shows a 62 dBZ core on composite reflectivity over your town. What can you legitimately conclude?
Two words in the question are doing work: "composite", and "column".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.