[ecco-support] [EXTERNAL] Accessing 'NVEL' Field using MATLAB?

[Wang, Ou (US 329B)]

Hi Luke,

To calculate streamfunction of the AMOC accurately, one actually needs to use “UVELMASS” and “VVELMASS”, the native ocean velocity weighted by vertical grid cell area fraction. The “UVELMASS” and “VVELMASS” variables are velocities (in m/s) that
1) are on native model grid and along the native model x- and y- directions which are not necessarily eastward or northward;
2) are weighted by vertical grid cell area fraction to account for partial cells.
Both are necessary to calculate streamfunctions accurately. An example Jupyter notebook that calculates streamfunction and transport across the OSNAP lines can be found at https://github.com/ECCO-GROUP/ECCO-v4-Python-Tutorial/blob/master/Tutorials_as_Jupyter_Notebooks/ECCO_v4_Example_OSNAP.ipynb.

“UVELMASS” and “VVELMASS” can be found in the OCEAN_3D_VOLUME_FLUX datasets in podaac.jpl.nasa.gov/ECCO. Click “data” and then enter Volume in the search box on top right. Or one can click directly the following URL for monthly mean UVELMASS and VVELMASS on native model grid: https://search.earthdata.nasa.gov/search/granules?p=C1991543739-POCLOUD&pg[0][v]=f&pg[0][gsk]=-start_date&g=G1996974974-POCLOUD&q=ECCO&ac=true&tl=1643931320.62!3!!&m=-0.28125!-0.140625!1!1!0!0%2C2<https://search.earthdata.nasa.gov/search/granules?p=C1991543739-POCLOUD&pg%5b0%5d%5bv%5d=f&pg%5b0%5d%5bgsk%5d=-start_date&g=G1996974974-POCLOUD&q=ECCO&ac=true&tl=1643931320.62!3!!&m=-0.28125!-0.140625!1!1!0!0%2C2>. V4r4 also has daily mean UVELMASS and VVELMASS at https://search.earthdata.nasa.gov/search/granules?p=C1991543699-POCLOUD&pg[0][v]=f&tl=1643932834.024!3<https://search.earthdata.nasa.gov/search/granules?p=C1991543699-POCLOUD&pg%5b0%5d%5bv%5d=f&tl=1643932834.024!3>!!.

Using “NVEL” or even “NVELMASS” to calculate streamfunctions will cause large errors and therefore is not recommended. For this reason, NVEL is only provided on gridded 0.5x0.5 degree lat-lon grid, not on native model grid.

Feel free to post follow-on questions if this is not clear.

Best,
Ou

From: ecco-support <ecco-support-bounces at mit.edu> on behalf of "Dorrian, Luke J" <ldorrian3 at gatech.edu>
Reply-To: "ECCO support list, wider membership" <ecco-support at mit.edu>
Date: Wednesday, February 2, 2022 at 2:53 PM
To: "ecco-support at mit.edu" <ecco-support at mit.edu>
Cc: "Fu, Yao" <yaofu at gatech.edu>
Subject: [EXTERNAL] [ecco-support] Accessing 'NVEL' Field using MATLAB?

Hello,

My name is Luke and I am using ECCOV4 data to construct streamfunctions of the AMOC. I have been reading the data through the 'UVEL' field using ncread() on MATLAB but found that I had to readjust data to reflect true north velocities. I read in the guide that the 'NVEL' field is a diagnostic output for the model and would that to use that instead. However, when looking through the .nc file using ncdisp(), I am not able to find the 'NVEL' variable. I am using data directly from ECCO's website and was wondering how I can access this data? It doesn't seem to be listed within the file that I've downloaded. If it's not there, could you point me in the right direction to getting a file with that field? That would be very helpful as otherwise I need to adjust the data each time I incorporate different sections of data in the Southern Hemisphere.

Thanks!

Luke Dorrian
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