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ymcmrs / ICAMS

Python based toolbox for InSAR troposphere Correction using global Atmospheric Models that considers the spatial Stochastic properties of the troposphere (ICAMS).
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Geopotential height and geometrical height #2

Closed kanglcn closed 1 year ago

kanglcn commented 1 year ago

Hi Yunmeng,

Thanks for making this great package free!

I have a quick question about the ERA5 data product usage. The height data in ERA5 is the geopotential height, but this package treats it as normal geometrical height with respect to geoid. Do you think their difference is neglectable?

I am not very familiar with this package so please don't hesitate to correct me if I am wrong.

Thanks.

ymcmrs commented 1 year ago

Hi Kang,

The geopotential height has been transformed to geometrical height directly when read the parameters (see _utils.py  Line 300).

To make the input DEM has a same reference as that of the weather model, we also make a geoid correction (assume a constant geoid height), e.g., see tropo_icams_date.py Line 400.

Hope this helps!

Cheers

Yunmeng

From: Kang Liang @.> Reply-To: ymcmrs/ICAMS @.> Date: Monday, September 19, 2022 at 1:57 PM To: ymcmrs/ICAMS @.> Cc: Subscribed @.> Subject: [ymcmrs/ICAMS] Geopotential height and geometrical height (Issue #2)

Hi Yunmeng,

Thanks for making this great package free!

I have a quick question about the ERA5 data product usage. The height data in ERA5 is the geopotential height, but this package treats it as normal geometrical height with respect to geoid. Do you think their difference is neglectable?

I am not very familiar with this package so please don't hesitate to correct me if I am wrong.

Thanks.

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kanglcn commented 1 year ago

Hi Yunmeng,

In my understanding, what ERA5 product directly provides is geopotential, please look at https://en.wikipedia.org/wiki/Geopotential_height. The geometric height cannot be directly obtained by simply dividing geopotential by a constant gravitational acceleration since the gravitational acceleration is a function of height.

Thanks.

ymcmrs commented 1 year ago

Hi Kang,

Glad to know you are thinking of this.

Yes, gravitational acceleration is a function of height. But note, g is correlated with R=r+h (r means earth radius, and h means height), and also water vapor, which mainly cause the InSAR-delay, is mostly distributed below 5km, so we can assume the changes in R is very limited.

Here is all about making complex but less important problem simple.

Yunmeng

From: Kang Liang @.> Reply-To: ymcmrs/ICAMS @.> Date: Tuesday, September 20, 2022 at 3:50 AM To: ymcmrs/ICAMS @.> Cc: Yunmeng Cao @.>, Comment @.***> Subject: Re: [ymcmrs/ICAMS] Geopotential height and geometrical height (Issue #2)

Hi Yunmeng,

In my understanding, what ERA5 product directly provides is geopotential, please look at https://en.wikipedia.org/wiki/Geopotential_height. The geometric height cannot be directly obtained by simply dividing geopotential by a constant gravitational acceleration since the gravitational acceleration is a function of height.

Thanks.

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kanglcn commented 1 year ago

Got it. Thanks for your explanation! Kang