Checkerboard in SST

[Johan Söderkvist]

Dear GETM community,

 

I have been running a series of experiments with focus on deep water inflow to the Baltic Sea and maintaining the halocline structure, as well as baroclinic currents in the Danish Straits (e.g., Great Belt).

Ther have been several changes in both the GETM and GOTM configurations, but also updated bathymetry. We now have a setup that maintains the salinity structure over several years.

However, a checkerboard pattern in Sea Surface Temperature (SST) appeared during spring and summer. At some occasions it presents more like a seesaw pattern along the longitude (i-direction). The pattern appears in several areas of the domain, with largest amplitude in the Baltic Sea and somewhat weaker in Kattegat. The pattern disappears during nighttime.

After several experiments, we identified two parameter settings that generate such patterns in our setup:

• smooth_bvf_hor = .false. (in getm.inp)
• ubc_type = 2 (in gotm.nml)

Setting smooth_bvf_hor = .true. and ubc_type = 1 removes the checkerboard pattern in daytime SST.

 

I am wondering if others have observed similar checkerboard patterns in SST in their GETM setups. If so, have you identified other parameter choices that may generate or suppress such patterns?

I do not think the pattern is related to bathymetry gradients, as it also appears in areas with relatively flat bottom (e.g., Southern Baltic, ~60m depth).

 

best regards

JOHAN sÖDERQVIST

Oceanographer, PhD

 

JOINT GEOMETOC SUPPORT CENTRE

DANISH MINISTRY OF DEFENCE ACQUISITION AND LOGISTICS ORGANISATION

Lautrupbjerg 1-5, DK-2750 Ballerup

Denmark

Phone: +45 4078 7321
E-mail: j...@fcoo.dk

www.fmi.dk

 

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[Knut]

Dear Johan,

earlier we also observed SST checkerboards, but only(?) in shallow waters. Hans once explained this with the uncoupled turbulence between water columns and introduced the -DSMOOTH_BVF_HORI define which was turned into a nml parameter in the iow branch. I never tried out changing ubc_type, but craig-banner tke injection does not sound that bad either. Also advection of tke and eps could help to better couple the turbulence from neighbouring water columns.

@Hans: do you remember the explanation for the checkerboard set up?

@Ulf: any corrections?

Cheers, Knut

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[Adolf Stips]

Hi,

The checkerboard is the classical instability of the Boussinesq equation. 

( It occurs when your pressure and velocity variables are poorly coupled, manifesting as a high-frequency alternating grid pattern)

For smoothing this checkerboard, the famous AN smoothing solution was introduced by Hans (2002?), on recommendation from Jean-marie Beckers.

(Now this became AN_method, AN_constant and AN_field)

Much later, I proposed and introduced SMOOTH_BVF, after investigating and learning from the MOM code (at least in my memory).

So checking and likely adapting what AN type is used, should be the first step for problem solution.

Ciao Adolf

  

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[Knut]

Hi Adolf, I think An is for checkerboards in ssh.

To view this discussion visit https://groups.google.com/d/msgid/getm-users/CAEXw-aY63wD5eb25E8hQVj9e8o91eH4LHQ4UTCO8EyCV3s7nug%40mail.gmail.com.

[Adolf Stips]

Yes, of course Knut, but do you then assume that they have nothing to do with the 3D variables? 

I'm not really sure,..

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[Hans Burchard]

My eperience is indeed that checkerboard in elevation is related to
checkerboard in surface density (temperature & salinity). So, increasing
An most cases cures both.

Best regards, Hans.

>> * smooth_bvf_hor = .false. (in getm.inp)
>> * ubc_type = 2 (in gotm.nml)

>>
>> Setting smooth_bvf_hor = .true. and ubc_type = 1 removes the
>> checkerboard pattern in daytime SST.
>>
>> I am wondering if others have observed similar checkerboard patterns
>> in SST in their GETM setups. If so, have you identified other
>> parameter choices that may generate or suppress such patterns?
>>
>> I do not think the pattern is related to bathymetry gradients, as it
>> also appears in areas with relatively flat bottom (e.g., Southern
>> Baltic, ~60m depth).
>>
>> best regards
>>
>> JOHAN sÖDERQVIST
>>
>> Oceanographer, PhD
>>
>> JOINT GEOMETOC SUPPORT CENTRE
>>
>> DANISH MINISTRY OF DEFENCE ACQUISITION AND LOGISTICS ORGANISATION
>>
>> Lautrupbjerg 1-5, DK-2750 Ballerup
>>
>> Denmark
>>
>> Phone: +45 4078 7321
>> E-mail: j...@fcoo.dk
>>

>> www.fmi.dk [1]
>>
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Hans Burchard, Prof. Dr.

fon: +49-381-5197-140
fax: +49-381-5197-114
e-mail: hans.b...@iow.de
website: https://www.io-warnemuende.de/hans-burchard-en.html

Leibniz Institute for Baltic Sea Research Warnemünde
Physical Oceanography and Instrumentation
Seestraße 15
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Germany

[Johan van der Molen]

Hi Johan,
If it disappears at night, and it's only there in spring and summer, that suggests that it may have something to do with solar heating?
So the place to look is in the routines/processes that handle that?
Or maybe the heating is so fast that the numerics can't keep up? Does it disappear if you reduce the time step?
Best wishes,
Johan

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Hans Burchard, Prof. Dr.

fon: +49-381-5197-140
fax: +49-381-5197-114
e-mail: hans.b...@iow.de
website: https://www.io-warnemuende.de/hans-burchard-en.html

Leibniz Institute for Baltic Sea Research Warnemünde Physical Oceanography and Instrumentation Seestraße 15
D-18119 Rostock
Germany

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[Hans Burchard]

Dear Johan,

my experience shows that it is not the solar heating directly, but the
stratification induced. So, strong near-surface stratification over deep
water is the challenge. We had some extreme checkboard cases when we
simulated circulation in shelf-ice cavities (certainly without solar
radiation), where only the upper 10m were stratified, and the water
depth was more than 500m. We solved it with increasing An, see
https://doi.org/10.1029/2023MS003721, in last para of sec. 2.1.

Cheers, Hans.

[Johan Söderkvist]

No checkerboard in elevation identified in this case
Previously, we have had checkerboard pattern in elevation. Added An field for this particular region solved the problem.
There is a benefit with this regional field when the checkerboard only occurs in specific areas
I don't recall that we observed any issues in sea surface temperature at that time

Here, the SST checkerboard occurs in different areas at different times. Så An-field will not solve this.
I am wondering if global An is preferred over SMOOTH_BVF.

Best regards
Johan Söderqvist



-----Oprindelig meddelelse-----
Fra: getm-...@googlegroups.com <getm-...@googlegroups.com> På vegne af Hans Burchard
Sendt: Wednesday, 20 May 2026 14.45
Til: getm-...@googlegroups.com
Cc: Adolf Stips <adolf...@gmail.com>
Emne: Re: [getm-users: 3456] Checkerboard in SST

To view this discussion visit https://groups.google.com/d/msgid/getm-users/ba4667c25fdda5757523806061cacf34%40io-warnemuende.de.

________________________

[Johan Söderkvist]

I have tested different configurations for solar heating (jerlov parameterisation), and it did not generate any checkerboard pattern.
There are occasions where the mixed layer is greater than 15 meter, exhibits a checkerboard pattern.
I have not tested with shorter time step. Current setup use 6 seconds, and split factor = 15. Could this be a problem when the mixed layer is rather deep?

Best regards
Johan Söderqvist

-----Oprindelig meddelelse-----
Fra: getm-...@googlegroups.com <getm-...@googlegroups.com> På vegne af Hans Burchard

Sendt: Thursday, 21 May 2026 09.14
Til: getm-...@googlegroups.com
Cc: Johan van der Molen <johan.van...@nioz.nl>
Emne: Re: [getm-users: 3458] Checkerboard in SST

To view this discussion visit https://groups.google.com/d/msgid/getm-users/543591ddb884e1cd7961340cd0b618f6%40io-warnemuende.de.

________________________