How to use geopotential to mask out atmospheric variables below model terrain?

Hi all,

I am working with ERA5 reanalysis data on pressure levels and try to mask out atmospheric variables that are below the model terrain by using the invariant geopotential from the ERA5 reanalysis data on single levels  (see user forum post by Michela Giusti ).

I have noted that the geopotential in ERA5 reanalysis data on pressure levels varies over time.

That means I observe different values for the geopotential at different hours (and dates) for the same location (coordinates).

For instance, I observe the following values in my data: 

Shouldn't the geopotential be invariant within the same pressure level and coordinates (as it is the case for the geopotential in the ERA5 reanalysis data on single levels ) ?

In other words, why does the geopotential vary over time in the ERA5 reanalysis data on pressure levels but is time-invariant in the ERA5 reanalysis data on single levels ?

Furthermore, how can I mask out atmospheric variables below the model terrain based on changing geopotentials in the ERA5 reanalysis on pressure levels data?