[MITgcm-support] initial W caused by bathymetry?

[David Hebert]

Hi Martin,

The scale of W is about 10% of U. So I specify U=0.1m/s, and those 
stripes are +- 0.0137m/s.  The value of 0.0137m/s agrees with continuity 
computations when comparing with my grid...

dx = 14.5m
dz = 1m
u=0.1m/s

Thus, du/dx would be 0.1/14.5 = 0.0069. Using hfac's, dx could be half 
near bathymetry, which would yield 0.1/7.25 = 0.0138.

I also wonder if this were a simulation with variable dz, and the bottom 
dz was larger (say 100m), would that manifest the magnitude of w since 
we now are integrating over a large z?

Anyway, the stripes persist for quite some time in the simulation. 
Admittedly, if I were to start this simulation with u,v from larger, 
mesoscale simulation then W might not stretch through the domain.  Often 
here we will start with a large mesoscale simulation and interpolate to 
simulate sub-mesoscale with higher resolution bathymetry. It would seem 
we need to be carefull that non-divergent free horizontal motions are 
created.

Thanks,

David

Martin Losch wrote:
> David,
>
> what is the scale of the w-values? The only thing I can think of is 
> that this is the round-off error showing up. This would not be visible 
> with a non-uniform u-field, because other ("real") effects would 
> dominate.
>
> Martin
> On 7 Jan 2009, at 03:12, David Hebert wrote:
>
>> Hi Everyone,
>>
>> I would like to conduct a 3D, nonhydrostatic simulation with a 
>> barotropic flow over a seamount. In the past I have typically started 
>> these simulation with a U velocity throughout the domian. In this 
>> case, U=10cm/s. Since U=10cm/s everywhere (and V=0) at t=0, I would 
>> expect continuity to be satisified and W=0. However, I get several 
>> vertical "stripes" for W at t=0 (see attached vertical cross-sections 
>> for U,W at t=0).  These "stripes" seem to be caused because the model 
>> is using areas where there is bathymetry to compute continuity. i.e., 
>> where there is land, U=0, and the adjacent point U=10cm/s, resulting 
>> in a nonzero value of du/dx. Furthermore, since du/dx is zero above 
>> the bathymetry, the induced W is manifested vertically.
>>
>> My question is has anyone come across this issue where bathymetry 
>> seems to cause velocity gradients? How do I get around it? I should 
>> note I see similar stripes in W when I initialize U and V from a 
>> larger domain simulation.
>>
>> Thanks for your help,
>>
>> David
>>
>> <Ut0.png>
>> <Wt0.png>
>> <Ut0.png><Wt0.png>
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