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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, 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:

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The ncdump is the following:

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

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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, but the issue is not addressed there. Am I misinterpreting something?

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  • $\begingroup$ 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. $\endgroup$
    – daniel.heydebreck
    Aug 26, 2019 at 12:06
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    $\begingroup$ hello Daniel, the long name is just Mean Evaporation Rate, I edited the question adding the header of the ncdump $\endgroup$
    – Nemesi
    Aug 26, 2019 at 12:13
  • $\begingroup$ @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). $\endgroup$
    – Nemesi
    Aug 26, 2019 at 16:00
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    $\begingroup$ @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 $\endgroup$
    – Nemesi
    Aug 27, 2019 at 15:37
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    $\begingroup$ Found it! "The ECMWF Integrated Forecasting System convention is that downward fluxes are positive. Therefore, negative values indicate evaporation and positive values indicate condensation" (apps.ecmwf.int/codes/grib/param-db?id=182). My bad! $\endgroup$
    – Nemesi
    Aug 27, 2019 at 16:06

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Just leaving this for future readers. I finally found in some ECMWF documentation (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 Spencer for their help.

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