Echo tops vs. Top?

[Jason Harris]

I'm wondering why the echo tops tend to be about 10,000 feet higher than the "top" measurement. For example, I'm looking at cell F0 in Florida on GR3 right now near Volusia county that has a "Top" of 28, 400 on the "Storm Attributes" table but when one clicks on the "tops" up on the menu above the radar viewing panel for echo tops the graphic clearly displays for that same cell the green colors of 35-40 and 40-45 . So what exactly are the differences in the meaning and method of these measurements?

*And I did do a search but didn't see a thread for this exact topic.

 

[Greg Stumpf]

I'm wondering why the echo tops tend to be about 10,000 feet higher than the "top" measurement. For example, I'm looking at cell F0 in Florida on GR3 right now near Volusia county that has a "Top" of 28, 400 on the "Storm Attributes" table but when one clicks on the "tops" up on the menu above the radar viewing panel for echo tops the graphic clearly displays for that same cell the green colors of 35-40 and 40-45 . So what exactly are the differences in the meaning and method of these measurements?

The "Top" attribute in the Storms Cell ID and Tracking (SCIT) algorithm (in the Storm Attributes Table) is based on the height of the highest two-dimensional feature with at least 10 km^2 of reflectivity > 30 dBZ.

The Echo Tops product displays the height of the last 18 dBZ level in the vertical profile of dBZ within that grid cell. The traditional ET algorithm used a 4 km^2 grid cell. The Enhanced Echo Tops (EET) algorithm uses a polar grid of 1 degree x 1 km.

And then, an operator defined echo top is based on your base data analysis using whatever parameters you decide.

 

[Peter Wharton]

That explains the technical data behind various definitions of "top" but doesn't resolve what information is most useful - what would be a "standard" definition of the top of a storm for comparative purposes?

 

[Greg Stumpf]

That explains the technical data behind various definitions of "top" but doesn't resolve what information is most useful - what would be a "standard" definition of the top of a storm for comparative purposes?

AFAIK, there is no standard definition. The "top" of a thunderstorm is quite "fuzzy". So, you choose what works best for your application of the problem.

Top of the precipitation? Top of the updraft? Top of the cloud material? Top of the atmospheric perturbation? Top of the electric field?

 

[Jeff Duda]

I think you can probably get a good idea for how high the top of a storm actually is just by looking at the higher elevation scans on a storm and seeing where certain dBZ levels cut off (such as the 50 dBZ or 30 dBZ contour). Does it really matter which top you use? Any given thunderstorm that has a top measured by the height of the 18 dBZ echo, top of any positive dBZ echo, the 50 dBZ echo, or that calculated by some algorithm is going to be the same size no matter which calculation you use to put a number on it. As long as you use the same value for all storms so that you're comparing apples to apples it doesn't matter.

 

[Brad Nelson]

Like Jeff mentioned, it really depends on what threshold you use for the dbz cut-off but just using a typical 30/18 dbz relationship, here's what you'd get:

Storm Tops: The height of the top of the 30 dBz reflectivity.

Echo Tops: The height of the top of the 18 dBz reflectivity.

Comparing storm to storm this is certainly apples to apples but some programs may have different dbz thresholds and, therefore, could have different storm and echo tops.

 

[Greg Stumpf]

Storm Tops: The height of the top of the 30 dBz reflectivity.

Echo Tops: The height of the top of the 18 dBz reflectivity.

No, not exactly. See post #2.

 

[Jason Harris]

Greg wrote:

Top of the precipitation? Top of the updraft? Top of the cloud material? Top of the atmospheric perturbation? Top of the electric field?

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What measurement will get you the best idea of the top of the updraft? Can one detect the overshooting top on radar?

 

[Darren Addy]

Has the technology for measuring the 18.5 dBZ echo top improved much since Tim Vasquez wrote the first edition of The Weather Map Book? I don't have his new edition yet, but the first edition makes it sound like multiple technological limitations make the echo top data extremely unreliable.

 

[Jeff Duda]

I think measuring the 18.5 dBZ storm top is pretty straightforward. You just find the highest level (elevation scan) at which 18.5 dBZ or greater of reflectivity can be found, then do some sort of interpolation (probably linear) to determine where between that elevation angle and the next one the actual 18.5 dBZ isosurface would be.

 

[Darren Addy]

This NEXRAD report uses "echo top" to mean "cloud top" or "assumed cloud top" and discusses inconsistencies between pilot-reported heights and those calculated by radar.

The Vasquez book I referred to earlier talks about this same problem and has a good graphic showing a storm that has an actual height of 45,000 feet being read differently, depending upon it's distance from the radar. In the graphic, the 45,000 ft. storm is read as: 30,000, 20,000, 40,000, 30,000, and 30,000 as it draws closer. (Only the middle distance was even close to accurate). The caption says this problem is worst in VCP 21.

Volume Coverage Patterns 11 and 21: These VCPs are used in the Precipitation Mode to better sample the vertical structure of convective weather echoes and to provide better temporal resolution. VCP 11 provides better vertical sampling of weather echoes than VCP 21 and is usually preferred in situations where convective precipitation is within 60 nmi of the antenna. The VCP 11 has 14 elevation angles and completes 16 azimuthal scans in 5 minutes while VCP 21 has 9 elevation angles and completes 11 azimuthal scans in 6 minutes

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