A question with coordinate transformation

[linzhenhua]

I just read that now ROMS include more options for the vertical coordinate,congratulations.

But this also remind me of one old question I ever have,I present it below,hopefully some experts could shed some light on this.

To my knowledge,now sigma coordinate is used extensively in the ocean models,such as ROMS,also others.At every time step,the coordinate is transformed from z to sigma coordinate,therefore simplify boundary condition treatment.The computations are performed all in the sigma coordinate.

For one variable,V. The following is computed:

V(new_time,sigma)=V(old_time,sigma)+Other Terms.

My question is,the total water depths at the two time step are different,so V(new_time,sigma) and V(old_time,sigma)should correspond to the physical point at different position.Is this right? I could not find any other terms that explain this effect. Am I missing something?


Reference page:https://www.myroms.org/wiki/index.php/T ... sformation