[MITgcm-support] Simple problems about the output of salinityand eta

[Terry Chen]

Hi David,
  Thank you so much for answering !
  Non-hydrostatic process is quite important in my research, so it seems that linear free surface is my only choice now. 
  Do you think it is applicable to use z* coordinate with linear surface and non-hydrostatic mode ?  


Best,
Terry  


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Thank you for your reading！
Terry Chen



Physical OceanographySchool Of Marine Science

Sun Yat-sen University

  
  
  


 




------------------ Original ------------------
From:  "David Ferreira";<dfer at mit.edu>;
Date:  Jan 8, 2018
To:  "mitgcm-support at mitgcm.org"<mitgcm-support at mitgcm.org>; 

Subject:  Re: [MITgcm-support] Simple problems about the output of salinityand	eta



 Hi Terry,
 There is not wetting and drying implemented in the MITgcm, i.e. a "wet" column will remain wet with all 28 levels wet.
 With a linear free surface approximation, you will find that the free surface can be lower than the top level or even the full depth (which is an unphysical regime obviously). There is the possibility to use a non-linear free surface with the z* coordinate,  in this case there is a rescaling of the z level thicknesses with the movements of the free surface. But there is a limit to this. It won't get dry and the scaled depth might not go lower than 10-20% of the initial depth without blowing up.
 cheers,
 david
 
 
  
 From: MITgcm-support [mitgcm-support-bounces at mitgcm.org] on behalf of Terry Chen [505883357 at qq.com]
 Sent: Monday, January 08, 2018 3:11 AM
 To: mitgcm-support at mitgcm.org
 Subject: [MITgcm-support] Simple problems about the output of salinity and eta
 
 
 
  Hello everyone,
   Since I am a beginner of the z-coordinate model, these questions form me may be a little bit silly.
   I am using MITgcm in a coastal region, and I find some confusing problems about the eta.
   I have divided 28 layers vertically, and start my modeling with the force of tide. From the manual I learn that k=1 is for the surface layer, and k=28 is for the bottom. Then I find that during the tide period, all of the salinity and velocity outputs  was for all 28 layers, while actually the total depth at that time was less than the sum thickness of the 28 layers. In other words, I can't see any changes resulted from the decline of water level in the outputs of the salinity and velocity.  
   At the ebb tide, some eta output near the land is even lower than my model depth, which may mean this point is dry or with a depth set by the hFacMin, but I can still find that the output here is more than 1 layer.
   Is that mean the z-coordinate is actually moving up and down or some extrapolation exist here ?  How should I understand the eta can be lower than my depth?
 
 
 Best,
 Terry
   
   
 
 
  ------------------
  Thank you for your reading！
 Terry Chen
 
 
 
 Physical Oceanography School Of Marine Science
 
 Sun Yat-sen University
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