CFSv2 air pressure

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Tromp, Reinier

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Aug 31, 2022, 10:31:01 AMAug 31
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Dear Sir/Madam,

 

I am trying to figure out what exactly the units of the air pressure data are for the CFSv2 model.

 

In my netcdf it mentions Pa, but in the OpenDAP it mentions hPa. When I look at the actual data though, it seems to hover between -50 and 50 (unsure about the unit), which does not make a lot of sense.

 

I was wondering if you would be able to help me out. Is there a document available that describes the units? I also noticed the scale factor and offset, but am in the dark how these come into play.

 

Thank you!

 

Cheers,

 

 

 

Logo WSP

 

 

Reinier Tromp

Project Manager

P.Eng, M.Sc., PMP, ir.

(He/Him)

 

 

 

 

 

T+ 1 (289) 982 4733

 

 

 

WSP Canada

100 Commerce Valley Dr W

Thornhill, Ontario

L3T 0A1 Canada

 

 

 

 

 

wsp.com

 

 





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Michael McDonald

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Sep 1, 2022, 11:13:10 AMSep 1
to Tromp, Reinier, fo...@hycom.org
This should all be in the THREDDS metadata listing of every NetCDF object, e.g.,



* airprs: Grid
coordinates: Date
time_average_bounds: 0.0, 0.0
long_name:  surface pressure
standard_name: surface_air_pressure
units: Pa
scale_factor: 0.00390625
add_offset: -33.41318

Tromp, Reinier

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Sep 1, 2022, 12:55:51 PMSep 1
to Michael McDonald, fo...@hycom.org

Hi Michael,

 

I did see that, I just can’t make sense of it when I actually look at the data. The surface air pressure is hovering between -50 and +50 Pa (or hPa if you look at the unit notation in the netcdf file). In real life, the surface air pressure should be in the order of  101,325 Pa or 1,013.25 hPa, which I can’t correlate with the data from CFSv2.

 

Therefore I was wondering, how do the “scale_factor” and “add_offset” come into play here? What do those parameters mean exactly?

 

Thank you,

 

 

 

Logo WSP

 

 

Reinier Tromp

Project Manager

P.Eng, M.Sc., PMP, ir.

(He/Him)

 

 

 

 

 

T+ 1 (289) 982 4733

 

 

 

WSP Canada

100 Commerce Valley Dr W

Thornhill, Ontario

L3T 0A1 Canada

 

 

 

 

 

wsp.com

Erick Olvera

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Sep 1, 2022, 1:39:30 PMSep 1
to Tromp, Reinier, Michael McDonald, HYCOM.org Forum
Hello,

We experienced the same issue previously, this entry in the forum may be helpful.


Best,
Erick

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Michael McDonald

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Sep 1, 2022, 1:49:24 PMSep 1
to Tromp, Reinier, fo...@hycom.org
Reinier,
You didn't specify "how" you downloaded/acquired this CFSv2 data, as this can impact the format/state of the data. 

For example, if you use NCSS to subset and export to CSV, then the java netcdf does not perform any data conversions and the raw Netcdf value (sans any “scale_factor” and “add_offset”) is provided in the CSV. A user must then convert this data (locally/offline) with the “scale_factor” and “add_offset”. 

If you download the entire NetCDF file and use any NetCDF-supported tools that automatically unpack this data (performing the “scale_factor” and “add_offset” in the background for you) then the final output value that you see in your plot/application should be the correct value. 

e.g.,

and here in the forum,






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Michael McDonald
HYCOM.org Administrator

Tromp, Reinier

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Sep 2, 2022, 11:03:07 AMSep 2
to Erick Olvera, Michael McDonald, HYCOM.org Forum

Thank you! This was already what I had done because it felt like it made sense, but I am happy to hear it is the way it is supposed to be done. I am confused though why this is not exported or mentioned anywhere in the datasets themselves.

 

Thanks again!

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