<p>Just leaving this for future readers. I finally found in some ECMWF <a href="https://apps.ecmwf.int/codes/grib/param-db?id=182" rel="noreferrer">documentation</a> (referring to a different variable) that: "<em>The ECMWF Integrated Forecasting System convention is that downward fluxes are positive. Therefore, negative values indicate evaporation and positive values indicate condensation</em>". This applies also to other variables.
Thanks to Daniel and Spencer for their help.</p>

CC BY-SA 4.0
2019-08-28T12:00:35.583Z
2019-08-28T12:00:35.583Z
Nemesi
<p>Please, let me know if this question is too technical/specific for Earth Science. In case, I will remove it and eventually post it elsewhere.</p>

<p>I am processing some data extracted from ERA5 to set up a large scale hydrological model. We are calculating the evapotranspiration using the typical <a href="http://www.fao.org/3/X0490E/x0490e08.htm#calculation%20procedure" rel="nofollow noreferrer">P-M formula</a>, using as input the usual variables: wind speed, solar radiation, temperature, pressure, relative humidity and so on. In order to check the consistency of our calculations with the  variables provided by the ECMWF, I downloaded also some evaporation related variables. I'm struggling to understand the data related to the <code>mean evaporation rate</code> variable data. ERA5 are available at hourly time steps at ~0.28 degrees spatial resolution. For the <code>mean evaporation rate</code>, Minimum, Mean, and Maximum values are provided expressed in <code>kg x sqmt x sec-1</code>. Data look like this:</p>

<p><a href="https://i.sstatic.net/bF8vW.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/bF8vW.jpg" alt="enter image description here"></a></p>

<p>The <code>ncdump</code> is the following:</p>

<p><a href="https://i.sstatic.net/VG0uZ.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/VG0uZ.jpg" alt="enter image description here"></a></p>

<p>and plotting the mean values for a hour during the day, would look like this:</p>

<p><a href="https://i.sstatic.net/OSAsB.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/OSAsB.jpg" alt="enter image description here"></a></p>

<p>What looks not clear to me is why the mean evaporation rate is negative: I would expect a positive value in the wet regions and zero over the deserts. I tried to look at the <a href="https://confluence.ecmwf.int/display/CKB/ERA5+data+documentation" rel="nofollow noreferrer">ERA5 data documentation</a>, but the issue is not addressed there. Am I misinterpreting something? </p>

CC BY-SA 4.0
2019-08-26T12:06:33.777Z
Could you provide the `standard_name` or `long_name` of the variable? If it is something like "flux from the surface", it should be negative because the water content of the surface is reduced by it. If it is "flux into the atmosphere", it should be positive because the water content of the atmosphere increases.
daniel.heydebreck
CC BY-SA 4.0
2019-08-26T12:13:34.487Z
hello Daniel, the long name is just `Mean Evaporation Rate`, I edited the question adding the header of the `ncdump`
Nemesi
CC BY-SA 4.0
2019-08-26T16:00:55.760Z
@daniel.neumann: The fact that the Maximum values are positive make me doubt about the logic you suggested ("If it is something like "flux from the surface", it should be negative because the water content of the surface is reduced by it"). In general your suggestion would make sense, but, in this case, the max values should be lower than the other ones (at least during the sunny hours).
Nemesi
CC BY-SA 4.0
2019-08-26T22:29:18.563Z
I'll bet this is actually a _net_ evaporation rate.
Spencer
CC BY-SA 4.0
2019-08-27T07:26:24.663Z
hi @Spencer: still, how comes mean values are negative over a tropical area during the sunny hours? Then, take into account that the value refers to hourly time steps and they provide Min, Mean, and Max...
Nemesi
CC BY-SA 4.0
2019-08-27T15:08:17.183Z
@Nemesis: If we look at the values over land and invert the sign they look like expected, don't they?
daniel.heydebreck
CC BY-SA 4.0
2019-08-27T15:09:10.663Z
Which version of cdo are you using? Maybe it is a buggy version that cannot handle the offset and scale values?
daniel.heydebreck
CC BY-SA 4.0
2019-08-27T15:37:14.600Z
@daniel.neumann: After testing the values and looking also at other variables, I now think that you are right. CDO version is the latest `1.9.7.1`. I was mislead by the `-infov` output. I thought it was giving three values per cell, while it gives the Min, Mean, and Max values for the whole grid. So the negative values are water loss from the ground, positive values are condensation of vapor to the ground. I wish ECMWF made the metadata better. Thanks a lot for your inputs
Nemesi
CC BY-SA 4.0
2019-08-27T16:06:40.500Z
Found it! "The ECMWF Integrated Forecasting System convention is that downward fluxes are positive. Therefore, negative values indicate evaporation and positive values indicate condensation" (https://apps.ecmwf.int/codes/grib/param-db?id=182). My bad!
Nemesi
CC BY-SA 4.0
2019-08-28T11:33:37.890Z
Maybe you can write an answer on your own and accept it?
daniel.heydebreck
<evaporation><era>
ERA5 mean evaporation rates data - Why is that negative?
Nemesi
# ERA5 mean evaporation rates data - Why is that negative?

(Asked by: username_0 on 2019-08-26)

Please, let me know if this question is too technical/specific for Earth Science. In case, I will remove it and eventually post it elsewhere.

I am processing some data extracted from ERA5 to set up a large scale hydrological model. We are calculating the evapotranspiration using the typical [P-M formula](http://www.fao.org/3/X0490E/x0490e08.htm#calculation%20procedure), using as input the usual variables: wind speed, solar radiation, temperature, pressure, relative humidity and so on. In order to check the consistency of our calculations with the variables provided by the ECMWF, I downloaded also some evaporation related variables. I'm struggling to understand the data related to the `mean evaporation rate` variable data. ERA5 are available at hourly time steps at ~0.28 degrees spatial resolution. For the `mean evaporation rate`, Minimum, Mean, and Maximum values are provided expressed in `kg x sqmt x sec-1`. Data look like this:

[![enter image description here](https://i.sstatic.net/bF8vW.jpg)](https://i.sstatic.net/bF8vW.jpg)

The `ncdump` is the following:

[![enter image description here](https://i.sstatic.net/VG0uZ.jpg)](https://i.sstatic.net/VG0uZ.jpg)

and plotting the mean values for a hour during the day, would look like this:

[![enter image description here](https://i.sstatic.net/OSAsB.jpg)](https://i.sstatic.net/OSAsB.jpg)

What looks not clear to me is why the mean evaporation rate is negative: I would expect a positive value in the wet regions and zero over the deserts. I tried to look at the [ERA5 data documentation](https://confluence.ecmwf.int/display/CKB/ERA5+data+documentation), but the issue is not addressed there. Am I misinterpreting something?

**Tags:** evaporation, era

> **Comments:**
> **username_1**: Could you provide the `standard_name` or `long_name` of the variable? If it is something like "flux from the surface", it should be negative because the water content of the surface is reduced by it. If it is "flux into the atmosphere", it should be positive because the water content of the atmosphere increases.  
> **username_0**: hello Daniel, the long name is just `Mean Evaporation Rate`, I edited the question adding the header of the `ncdump`  
> **username_0**: @daniel.neumann: The fact that the Maximum values are positive make me doubt about the logic you suggested ("If it is something like "flux from the surface", it should be negative because the water content of the surface is reduced by it"). In general your suggestion would make sense, but, in this case, the max values should be lower than the other ones (at least during the sunny hours).  
> **username_2**: I'll bet this is actually a _net_ evaporation rate.  
> **username_0**: hi @username_2: still, how comes mean values are negative over a tropical area during the sunny hours? Then, take into account that the value refers to hourly time steps and they provide Min, Mean, and Max...  
> **username_1**: @Nemesis: If we look at the values over land and invert the sign they look like expected, don't they?  
> **username_1**: Which version of cdo are you using? Maybe it is a buggy version that cannot handle the offset and scale values?  
> **username_0**: @daniel.neumann: After testing the values and looking also at other variables, I now think that you are right. CDO version is the latest `1.9.7.1`. I was mislead by the `-infov` output. I thought it was giving three values per cell, while it gives the Min, Mean, and Max values for the whole grid. So the negative values are water loss from the ground, positive values are condensation of vapor to the ground. I wish ECMWF made the metadata better. Thanks a lot for your inputs  
> **username_0**: Found it! "The ECMWF Integrated Forecasting System convention is that downward fluxes are positive. Therefore, negative values indicate evaporation and positive values indicate condensation" (https://apps.ecmwf.int/codes/grib/param-db?id=182). My bad!  
> **username_1**: Maybe you can write an answer on your own and accept it?  

---

## 1 answers

### Answer by username_0 on 2019-08-28. Score: 7 | Accepted answer

Just leaving this for future readers. I finally found in some ECMWF [documentation](https://apps.ecmwf.int/codes/grib/param-db?id=182) (referring to a different variable) that: "*The ECMWF Integrated Forecasting System convention is that downward fluxes are positive. Therefore, negative values indicate evaporation and positive values indicate condensation*". This applies also to other variables.
Thanks to Daniel and username_2 for their help.


---

