Clarification Sought on Depth Levels of water_u_bottom and water_v_bottom Variables - expt_93.0

[Joseph Spain]

Dear HYCOM Team,

I hope this message finds you well. I am currently working with the ocean current velocity data from 'HYCOM GLBy0.08', specifically expt_93.0, and have encountered an observation that requires clarification.

Context:

I am focusing on a grid point with coordinates approximately 3.04 deg N and 6.72 deg E, considering a date range from 2023-03-08 to 2023-09-29. Based on the bathymetry data retrieved from the provided URL, the seafloor at this grid point is at 1527.2 m. URL: https://data.hycom.org/datasets/GLBb0.08/expt_93.0/topo/

Observation:

I have observed that the water_u and water_v variables are available down to a depth of 1500 m. Interestingly, the values of water_u_bottom and water_v_bottom variables are identical to the water_u and water_v data recorded at 1500 m.

Concern:

Given the depth of the seafloor at the aforementioned grid point, I would expect the water_u_bottom and water_v_bottom data to represent values at the 1527.2 m level, rather than at 1500 m. It raises a concern whether the bottom variables are correctly represented at the bathymetry bottom level for this specific grid point or if this observation is consistent with the model's normal behavior.

Inquiry:

I would appreciate your insights or clarifications on whether this similarity in values between the bottom variables and the 1500 m level is to be expected, or if there might be discrepancies in the depth representation of these variables. Additionally, is this phenomenon observed consistently across other depth levels and grid points, or is it peculiar to the mentioned coordinates?

I understand that model representations may involve approximations and limitations, and any elucidation on the model's behavior regarding depth levels and their correspondence with bathymetry data would be extremely valuable.

Thank you very much for your time and consideration. I look forward to hearing back from you.

Kind regards,
Joseph

[Alan Wallcraft]

The GLBy0.08 values are obtained as follows:

a) The native GLBb0.08 41-layer archive file is interpolated to GLBy0.08

b) The GLBy0.08 41-layer archive file is SAMPLED at fixed depths (including 1500 m an d 10m above the GLBy0.08 bottom) using a specified profile type, the 41 layer velocities are first averaged from the cell edges to the cell center (where T & S is defined).

The GLBy0.08 bottom is at 1524.5187 m and so _bottom values are at 1514.5187  m:

narwhal11 451> pwd
/app/projects/hycom/GLBy0.08/topo
narwhal11 452> hycom_lonlat2ij 6.72 3.04
 2335 2089
narwhal11 453> hycom_ij2lonlat 2335 2089
   6.720E   3.040N
narwhal11 455> hycom_range depth_GLBy0.08_09m11.a 4500 4263 2335 2089
min, max =    4.9999995       7196.8779        (2335,2089) =    1524.5187   

The samples at 1500 m and at 1514.5187  m are the same because both depths are in the same layer of the 41-layer archive.  The profile isn't always constant in the vertical across a layer, but it is often constant across layers near the bottom.

I don't have any GLBy0.08 archives on-line, but I do have the 2010_001_00 native GLBb0.08 archive from the 3.1 Reanalysis and I attach the profile at your location.  The deep layers are:

#  k    utot    vtot  p.temp    saln  p.dens    thkns      dpth

  32   -8.15   -5.38  4.1093 34.8842 36.7700  152.919  1366.421
  33   -6.17   -3.78  3.9720 34.9336 36.8300   51.447  1468.604
  34   -4.80   -5.29  3.6741 34.9515 36.8900   32.920  1510.787
  35    0.00    0.00  3.6741 34.9515 36.8900    0.000  1527.247

In this case 1500m and 10m from the bottom would both be in layer 34.

Alan.

[Joseph Spain]

Alan,

Thank you for your detailed reply. That's exactly the information I needed to know.

Kind regards,

Joseph