Transports for a section

[HandmannP]

Dear everybody,

I am working with the output of a high resolution model and am currently computing transports along a non-zonal/ non-meridional section.

There are the two possibilities to extract the sections either with cdfsections or cdf_xtrac_brokenline.

cdf sections already turned the velocities perpendicular and parallel to the section - here a great number of points on each part of the section is necessary. Here the grid is then
derived via interpolation of the original e3u and e3v onto the section points.

On the other hand cdf_xtrac_brokenline extracts the closest u or v point closest to the point along the chosen section - it also extracts the grids in horizontal and vertical direction.

I did both methods and get very unexpected peaks due to overestimation of the velocities at breaks of the subsections./ overestimation of transports due to too great grid in the vertical - How do you usually tackle this issue?

Could it be that there is some kind of critical angle between two following sections wich lead to this effect?

How do you usually derive transports along a section in a model?
For Show sections wich only consist of one line/ very low deviations from a line there is no problem-
but scaling up and going to more sophisticated structures this effect has a big influence.
Because I do model - observations comparison I need to stick to the given array-section.

I would be very happy about any input concerning this issue since I have already spend a lot of time on this.

Thank you already!!

[molines]

Hello, In cdftools, only cdf_xtrac_brokenline is supported. cdf_sections was there for a while but it was never at the CDFTOOLS standards. The main point in cdf_xtrac_brokenline, is that no velocity interpolation is done. The brokenline is buildt in such a way that it joins F points ( on the C-grid), hence having either U or V segments. We end up with a pseudo zonal section [unfold] ( in fact a succession of U and V if the section is oblique) where vomecrty is the velocity accross the section. Then we can use cdftransport (using -self option, so that the metrics is read from the broken line file. ). I should say that this is the most accurate way to compute transport. If you perform any kind of interpolation the truncature error on velocities may lead to quite big errors on transport. cdf_xtrac_brokenline + cdftransport have been used for quite a long time now, and I never encounter big problems or any overestimation. So I am a bit puzzled by your finding. May be you can send me more details on the command you are using for cdf_xtrac_brokenline and cdftransport. (using my personal mail). I will be glad to help, if I can ! Cheers, Jean-Marc

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On 05/03/2019 17:59, phand wrote: Dear everybody, I am working with the output of a high resolution model and am currently computing transports along a non-zonal/ non-meridional section. There are the two possibilities to extract the sections either with cdfsections or cdf_xtrac_brokenline. cdf sections already turned the velocities perpendicular and parallel to the section - here a great number of points on each part of the section is necessary. Here the grid is then derived via interpolation of the original e3u and e3v onto the section points. On the other hand cdf_xtrac_brokenline extracts the closest u or v point closest to the point along the chosen section - it also extracts the grids in horizontal and vertical direction. I did both methods and get very unexpected peaks due to overestimation of the velocities at breaks of the subsections./ overestimation of transports due to too great grid in the vertical - How do you usually tackle this issue? How do you usually derive transports along a section in a model? Because I do model - observations comparison I need to stick to the given array-section. I would be very happy about any input concerning this issue since I have already spend a lot of time on this. Thank you already!! — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#29>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AVJVzKMwNO0tr5k3PRGqS4vKQonbz0Khks5vTqJZgaJpZM4bfETs>.

-- Jean-Marc Molines Modélisation de l'océan / Ingénieur de recherche IGE (UMR5001 / UR 252) /CNRS, Grenoble INP, IRD, et UGA/ Institut des Géosciences de l'Environnement MEOM/IGE UGA CS 40 700 38058 Grenoble cedex 9 Tél. : 04 76 82 50 62 Web :http://www.ige-grenoble.fr/ Twitter : @IGE_Grenoble