Vertical Viscosity (AKv) result is strange

[CBian]

Dear Roms users,

I cannot get a good wind-driven current result near the coast, so I output the vertical viscosity (AKv) from model.

The AKv results are very strange. I check some papers, and the AKv should have a bigger value in mixed coastal area than in deep water. However, when I used MY25_MIXING, the surface and bottom AKv = 0. I got the similar results when I used GLS-MIXING. The depth-averaged AKv near coastal area is about 0.001 N/m2, which is far smaller than it should be.

Can anybody give me some suggestion about this? Thanks!

[LiuZHQ]

Hi Bian,
I remind you to recheck the "frc_uvstress.cdl" file before generating the forcing netcdf files.
I noticed that there is some defination say "svstr:units = "Newton meter-2 * 1000;" in this file, which indicates that you should time the svstr with 1000 after converting from the wind speed V, or you may get the wrong forcing file.
Just a suggestion, but may not be the exact problem.

ZQ

[CBian]

Dear Qiang,

Thanks for your suggestion.

I check the frc_uvstress.cdl:


short sustr(sms_time, eta_u, xi_u) ;
sustr:long_name = "surface u-momentum stress" ;
sustr:units = "Newton meter-2" ;
sustr:time = "sms_time" ;
sustr:scale_factor = 0.0005 ;

short svstr(sms_time, eta_v, xi_v) ;
svstr:long_name = "surface v-momentum stress" ;
svstr:units = "Newton meter-2 * 1000" ;
svstr:time = "sms_time" ;
svstr:scale_factor = 0.0005;


This is very very strange, why 'sustr' use different units with 'svstr'? Do you use this CDL to generate wind stress and it works well? Because I use 'svstr' with units "Newton meter-2", the result seems OK. However, in my ROMS output log file, it is:


GET_2DFLD - surface u-momentum stress, t = 360 00:00:00
(Rec=1440, Index=1, File: byecs-frc.nc)
(Tmin= 0.2500 Tmax= 360.0000)
(Min = -1.60126703E-04 Max = 1.01056013E-04)
GET_2DFLD - surface v-momentum stress, t = 360 00:00:00
(Rec=1440, Index=1, File: byecs-frc.nc)
(Tmin= 0.2500 Tmax= 360.0000)
(Min = -2.41676363E-04 Max = 4.41068446E-06)


The surface stress seems 1000 times smaller than the wind-stress in my forcing data. But the wind-induced current result seems OK. Why this happened? I am confused.

=====================================

And for my first doubt, the vertical viscosity (AKv), it seems have no much relation with wind stress. There must be some model-configuration mistake induce the very small AKv value at the well mixed coastal area. Still waiting for someone to give me some suggestions about this.

[LiuZHQ]

Bian,
plz also chang the "sustr:scale_factor = 0.0005 ;" and "svstr:scale_factor = 0.0005 ;" to "sustr:scale_factor = 1 ;" and "svstr:scale_factor = 1 ;" respectively,

"svstr:units = "Newton meter-2 * 1000" ;" to "svstr:units = "Newton meter-2" ;"
and then generate the new wind forcing files.
good luck,
ZQ

[zhaoqiang]

I did not read the codes of this part, but I think sustr is divided by the density of water in the model, so you got the small values in your log file. If you do not use CDL file to generate your nc file, you can ignore it.
The value of AKv you got is very small. It depends on the stability of the water. And you need not confuse the value of AKv at the surface or bottom, cause what we really care about is the turbulence in interior region.

[CBian]

Dear ZhaoQiang,

Thanks for your suggestion, it's really helpful.

I made a stupid mistake, the vertical viscosity near coast is about 0.01 N/m2 not 0.001 N/m2. This doesn't seem too small.

In my opinion, when there is a very strong northward wind (eg. 10 m/s), the wind-induced current near the coast(depth < 20m) should be also northward. However, in my model results, the wind-induced surface current is northward, but the bottom current is southward. The water is well-mixed. So I wonder the vertical viscosity (0.01 N/m2) is still very small? Or there are other reasons lead to this strange result?