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