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

**URL:** <https://forum.ecmwf.int/t/how-to-use-geopotential-to-mask-out-atmospheric-variables-below-model-terrain/1826>\
**Category:** C3S - Datasets and Usage\
**Tags:** c3s, era5\
**Created:** [17 October 2022 11:00 UTC](https://forum.ecmwf.int/t/how-to-use-geopotential-to-mask-out-atmospheric-variables-below-model-terrain/1826 "2022-10-17T11:00:04Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Felix\_Bracht](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/f/ec9cab/32.png) [@Felix\_Bracht](https://forum.ecmwf.int/u/Felix_Bracht)\
**Post date:** [17 October 2022 11:00 UTC](https://forum.ecmwf.int/t/how-to-use-geopotential-to-mask-out-atmospheric-variables-below-model-terrain/1826/1 "2022-10-17T11:00:04Z")

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Hi all,

I am working with_&nbsp;_ **_ERA5 reanalysis data on pressure levels_&nbsp;** and try to mask out&nbsp;atmospheric variables that are below the model terrain by using the invariant geopotential&nbsp;from the **_ERA5 reanalysis data on single levels_&nbsp;** &nbsp;(see [user forum post](https://confluence.ecmwf.int/x/mq4kCQ) by [Michela Giusti](https://confluence.ecmwf.int/display/~usmg) ).

I have noted that the geopotential in&nbsp; **_ERA5 reanalysis data on pressure levels&nbsp;_** 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:&nbsp;

| level | time | geopotential | longitude | latitude |
| 1 | 01.01.2020 10:00 | 458472 | -100 | 40 |
| 1 | 01.01.2020 22:00 | 460877 | -100 | 40 |
| ... | ... | ... | ... | ... |

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_&nbsp;) ?

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 **&nbsp;** ERA5 reanalysis on pressure levels data?