<p>Since it discusses the pressure values setup first, then says:</p>
<blockquote>
<p>Also <em><strong>provided</strong></em> are the Geopotential and Geometric heights, the
temperature and density of the level <strong>based on</strong> the 1976 version of the
International Civil Aviation Organization (ICAO) Standard Atmosphere...</p>
</blockquote>
<p>I would take that to indicate that the vertical coordinates are defined by pressure, which is how I've always believed it to be.  And then the other values are just estimates from the standard atmosphere approximation.</p>
<p>In addition <a href="https://rda.ucar.edu/datasets/ds115.4/docs/levels.hybrid.html" rel="nofollow noreferrer">https://rda.ucar.edu/datasets/ds115.4/docs/levels.hybrid.html</a> looks to give more details on the vertical setup...</p>
<blockquote>
<p>The vertical coordinate parameters <strong>define</strong> the edges of the atmospheric
layers <strong>in terms of the surface pressure</strong> (in Pascals). The parameters
can be considered as being true at the mid-point (in pressure) of a
layer for each point. The formula to use to determine edges of the
layers is <span class="math-container">$P_{k+1/2} = A_{k+1/2} + B_{k+1/2}P_s$</span>, where <span class="math-container">$P_s$</span> is the surface pressure and <span class="math-container">$A$</span> and <span class="math-container">$B$</span> are defined in the table.</p>
</blockquote>
<p>So others who work on modeling may be able to offer further input on the setup, but from my casual understanding, it looks to definitely be based on pressure :-)</p>

CC BY-SA 4.0
2021-03-27T20:47:30.517Z
2021-03-28T05:14:22.663Z
2021-03-28T05:14:22.663Z
JeopardyTempest
<p>I am trying to understand the actual meaning of the <a href="https://www.ecmwf.int/en/forecasts/documentation-and-support/137-model-levels" rel="nofollow noreferrer">ECMWF L137 model levels</a>. In the link I provided, both pressures (in hPa) and heights (in m)  are given for every level. But which actually <em>defines</em> the model level, and which is merely included as an indicative value?</p>

<atmosphere-modelling>
Meaning of ECMWF model levels
Drubbels
# Meaning of ECMWF model levels

(Asked by: username_0 on 2021-03-27)

I am trying to understand the actual meaning of the [ECMWF L137 model levels](https://www.ecmwf.int/en/forecasts/documentation-and-support/137-model-levels). In the link I provided, both pressures (in hPa) and heights (in m) are given for every level. But which actually *defines* the model level, and which is merely included as an indicative value?

**Tags:** atmosphere-modelling


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## 1 answers

### Answer by username_1 on 2021-03-27. Score: 2

Since it discusses the pressure values setup first, then says:

> Also ***provided*** are the Geopotential and Geometric heights, the
> temperature and density of the level **based on** the 1976 version of the
> International Civil Aviation Organization (ICAO) Standard Atmosphere...

I would take that to indicate that the vertical coordinates are defined by pressure, which is how I've always believed it to be. And then the other values are just estimates from the standard atmosphere approximation.

In addition <https://rda.ucar.edu/datasets/ds115.4/docs/levels.hybrid.html> looks to give more details on the vertical setup...

> The vertical coordinate parameters **define** the edges of the atmospheric
> layers **in terms of the surface pressure** (in Pascals). The parameters
> can be considered as being true at the mid-point (in pressure) of a
> layer for each point. The formula to use to determine edges of the
> layers is $P\_{k+1/2} = A\_{k+1/2} + B\_{k+1/2}P\_s$, where $P\_s$ is the surface pressure and $A$ and $B$ are defined in the table.

So others who work on modeling may be able to offer further input on the setup, but from my casual understanding, it looks to definitely be based on pressure :-)


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