velocity at bottom

[Rodrigo Duran]

Hi,

I would like to get velocity at ocean bottom, I recall seeing variables called something like u_bottom, bottom_u or something along those lines, but I can't find them anymore. 

Does anyone know how to find it?

Thanks much.

Rodrigo.

[Alan Wallcraft]

These fields are available for GOFS 3.1 analysis and reanalysis.

for example https://ncss.hycom.org/thredds/ncss/grid/GLBv0.08/expt_53.X/data/2015/dataset.html and https://ncss.hycom.org/thredds/ncss/grid/GLBy0.08/expt_93.0/uv3z/dataset.html

These are 10m above the bottom, so it is most useful in deep water.  For shallow water you can get a complete profile by downloading the top few z levels, which are at most 5m apart down to 50m.

Alan.

[kevin...@uri.edu]

Are tracers (water_temp_bottom and salinity_bottom) 10 m above the bottom as well?

[Alan Wallcraft]

Yes, all the _bottom fields are 10m from the bottom.

Alan.

[James Kelly]

Alan or HYCOM, greetings, I am working on the  GLBv0.08expt_93.0 data set, for the time period, 22Jan2017 - 31Dec2017, for the area just off the coast of Vancouver, BC. I did not know what the bottom values meant until I read this forum. Before I knew this information I did some experimenting of my own to try to understand the meaning of the bottom values. Attached is what I found, and let me explain. I have attached two plots, one is spatial and the other (2) are time series. The red dots on the spatial plot represent the HYCOM locations where the bottom values are NOT the same as the last non-NaN value of each profile, or to say it another way, the black dots represent where the last profile value matches the bottom value at that location. And the time series plots compare the difference between the last non-NaN value of the profile to the bottom value (the x-axis is Julian day, but just know it represents the span:  22Jan2017 - 31Dec2017). 

So I have 3 questions:

1) What I have learned from my analysis is that you have set some type of threshold (especially for deep water) that determines when a bottom value is assigned to be different than the last value of the profile, meaning, the bottom value cant/isn't always 10 meters from the bottom as described in the above communications. Also note, this area is deep, the intervals at this depth are 500m, ranging from 1500 to 3000m; I have attached a HYCOM bathy image for your reference if it is helpful. Please clarify/describe the threshold that is used to determine when a bottom value is assigned to be different than the last value of the profile at deep depths?

1a) We are creating a hydrographic model of this region and it would be nice to have depth spacings less than 500m, is it possible to get values separated at say 100m at deep depths after 1500m?

2) The time series represent a difference between the last non-NaN depth and the bottom value at the same location. So a velocity of 0 would correspond to the bottom value being the same as the last non-NaN value of the profile. We would expect the entire time series to be either always different or always the same (constant zero). In a few occasions, this does not happen. Please advise on the discrepancies in the 2 time series locations shown (one in each plot) that aren't consistent (and these 2 instances are not the only ones)? 

Thank you

[Alan Wallcraft]

The way the "3z" vertical interpolation program works is:

a) Build a continuous profile consistent with the model's layer-averaged quantities

b) Sample the profile at fixed depths, including 10m from the bottom.

It is often the case that 100's of meters from the bottom are all represented by one HYCOM layer and if so the velocity (say) across this depth range will be constant.  Since HYCOM layers can move in the vertical, a fixed z level could be in different layers at different times.  If the lowest z value and the bottom value sometimes differ, it is possible that the z value is then from a layer that is not the deepest non-zero one.

The GLBv0.08 bathymetry is at ftp://ftp.hycom.org/datasets/GLBv0.08/expt_93.0/topo/depth_GLBv0.08_09m11.nc the lowest fixed "z" depth value could be above or below 10m from the bottom but their locations depend only on the bathymetry and do not change with time.

There are daily mean native HYCOM archive files from 93.0 at ftp://ftp.hycom.org/datasets/GLBb0.08/expt_93.0/data/archm/ and the depth and grid files are at ftp://ftp.hycom.org/datasets/GLBb0.08/expt_93.0/topo/

These start near the end of 2018 but you can use the archv2ncdf3z program from HYCOM-tools/archive to sample them at whatever depths you like and for any rectangular subregion.  However, note that the grid is curvilinear north of 47N.  The velocities very near the bottom in mean archive files can be large because if a mismatch with p-grid mean layer thickness.  There is an example script at ncdf3z_mn_153_one.csh. Use [ij]orign and [ij]dmp to select a subregion.

Alan.