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TOPIC: Why Reflectivity?

Why Reflectivity? 1 month 5 days ago #564

  • ON3CHD
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Hi David.
Via qtvlm I can now download the GFS grb that does include the reflectivity.
It looks like they use the clear air mode which does create also negative values of dbz.
Can you telll me if I am right in considering for my thunderstorms forecasting that these negative value are equivalent to a 0 dbz as they might be in the wrf file?
Thanks in advance fy views on the subject.

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Why Reflectivity? 1 month 5 days ago #565

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Hello,

Equation used to calculate base or composite radar reflectivity allow negative values. But if you look at the NOAA charts (eg. radar.weather.gov/Conus/centgrtlakes_lite_loop.php ) you will notice that negative and 0 value dbZ have the same white color.

To indicate possible thunderstorms value above 40-50 dbZ are needed ( openskiron.org/en/understanding-thunderstorm-indicators )

David will confirm that in wrf negative values are most probably converted to be equivalent to 0
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Why Reflectivity? 1 month 5 days ago #566

Hi Claude,
The new GFS is now a non-hydrostatic model and its physics suite, like WRF, determines the amount of water there is suspended in the atmosphere in various forms. The models differ in how they determine this and may not agree on the amounts of water, but the representation as simulated radar reflectivity will most likely be using the same physical coefficients for the reflection of radar energy by the type and density of water in the atmosphere.
My guess is that WRF filters out anything below 0 dBZ as not relevant while the new GFS appears to cut off at -20 dBZ. I think that it is safe to assume that this is arbitrary and in any case, as Dominique mentioned, values of 40 dBZ and above are the ones that need to raise concern.
David
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Why Reflectivity? 1 month 5 days ago #567

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www.google.com/url?sa=t&source=web&rct=j...&cshid=1568581900689

Thanks David, thanks Dominique.Maybe this link might help us to better understand the differences betwenn the model used.
Unless mistaking I have never seen a WRF giving negative dbz.BR

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Last edit: by ON3CHD.

Why Reflectivity? 1 month 5 days ago #568

The models unlike the real radar do not suffer from power loss over increased range and provide a lossless image of reflectivity over the entire forecast area. There is no attempt to imitate the loss features of the radar. Therefore I find difficulty in grasping why GFS might try and imitate clear air mode and work in two reflectivity modes - clean air and regular mode. It does not make sense as this would cause having two values of reflectivity for the same amount of hydrometeors. It is more reasonable that they have just provide a more sensitive threshold (and it does appear as a threshold as it cuts off at -20 dBZ).
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Why Reflectivity? 1 month 5 days ago #569

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Thank you for your explanation and time devoted.

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