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I'm playing around with NOAA's GFS forecast data and I see that it contains both a "0 °C isotherm level" and a "Highest tropospheric freezing level".

Could someone tell me the difference between these two levels? I'd assume they are the same except in the case of an inversion.

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  • $\begingroup$ One of the effects is the change of freezing point with pressure: physics.stackexchange.com/questions/60170/… $\endgroup$
    – arkaia
    Commented Apr 6, 2016 at 15:53
  • $\begingroup$ Unfortunately that's not relevant in this situation, the temperature difference in freezing point is very small on atmospheric scales. $\endgroup$
    – os1
    Commented Apr 7, 2016 at 7:29
  • $\begingroup$ If you read the physics.stackexchange, that is exactly what they say. $\endgroup$
    – arkaia
    Commented Apr 7, 2016 at 12:05
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    $\begingroup$ From browsing a few OPeNDAP servers that provide both the "highest tropospheric freezing level" and the "0C isotherm level", that's exactly what I see (that the difference between those levels is rather small). $\endgroup$
    – David Hammen
    Commented Apr 7, 2016 at 12:55
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    $\begingroup$ I understand "0 °C isotherm level" as "Lowest tropospheric freezing level", so it is lowest versus highest. If temperature in the troposphere profile crosses 0 °C just once, their altitude is the same. But due various inversion layers, temperature can cross 0 °C multiple times. $\endgroup$
    – Poutnik
    Commented Feb 3, 2022 at 13:42

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These are both derived fields from the Unified PostProcessor (UPP). Honestly the code there is a mess and I can't tell exactly where those levels are being defined, but just from the meta descriptions it seems like the "0 °C isotherm level" is the lowest height of 0˚C in the atmosphere (above ground level) for a given grid point, and the "Highest tropospheric freezing level" is the highest such level below the derived tropopause height. So that backs up what Poutnik's answer suggested.

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Various processes affect the freezing point. Both CCN (cloud condensation nuclei) characteristics and drop size affect the freezing point.

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