[MITgcm-devel] heff_max...more sea ice issues

[Jinlun Zhang]

Matthew Mazloff wrote:

> Thanks for the help...but I am a bit confused.  Two things
>
> 1) Re model efficiency and time stepping...I see there are 3  
> parameters.  I am guessing  SEAICE_deltaTtherm should be the ocean  
> dynamics time-step as the forcing comes from this.  The other time  
> stepping parameters are  SEAICE_deltaTdyn  and SEAICE_deltaTevp which  
> I assume are the timesteps for each dynamic solver (LSR and EVP)  
> respectively.  And as I understand it LSR can use the "large"  
> timestep, but the EVP should use the "small" timestep...is this  
> correct?  And I am not using both at the same time obviously, but you  
> are saying I should try both independently because it is not obvious  
> which is faster.

Correct.

>
> 2)More important than efficiency (right now anyway) is stability.   
> Jinlun, your first email seemed to suggest I try LSR with a half day  
> time step and LSR_ERROR=1e-4, or try EVP with "small" timestep.  Are  
> either of these methods likely to be more stable?

Although we may use half day time step for LSR, but it is better to use 
the same ocean dynamics time step for LSR for consistency, and 
particularly when the code blows up. And using 1e-4. I would think, from 
the heff_max figure, that the problem is most likely due to the surface 
ocean stress calculation that causes instability. However, you might 
also want to try EVP. I don't have much experience with EVP, but people 
have been telling me that very small time steps should be used for 
stability and for getting rid of unphysical elastic waves. I read one 
paper about high-res. (~10km) Hudson Bay simulation, the time step is as 
small as a few seconds.

Jinlun