[MITgcm-support] How to force river runoff data in a regional domain

[Stefano Querin]

Hi Kunal,

> On 29 Jul 2021, at 14:20:40, kunal madkaiker <kunal.madkaiker02 at gmail.com> wrote:
> 
> Thank you all for your suggestions.
> 
> Incase I go with option 3, my question is, do I have to select grid points for runoff in X or Y (1D) direction, or can they be 2D (XZ or YZ)?

If you choose option 3 with OBCS, you simply have to produce 2D OBCS "slices". In your specific case, if you use northern OB, set a 2D (XZ) slice with all zeros, except in the x-coords where you have the sources of your rivers. If your riverbeds are (for example) 4 cells deep, set in your northern OBC file: V(x-coord,1:4)=V_prescribed (the same for T and S). U=0.
V_prescribed (entering from the north, hence negative) [m/s] times delX(x-coord) [m] times sum(delZ(1:4)) [m] is your prescribed flow rate.

> If 2D, then how can I force them through data.obcs.

River runoff file (series of different slices, if they vary in time) must be treated the same way as the other OBCS files (W, S, E, in your case).
In data.obcs you have to specify (OB_Jnorth) the x and y coordinates of your river sources (upstream end of each riverbed).
More details here: https://mitgcm.readthedocs.io/en/latest/phys_pkgs/obcs.html#tab-phys-pkg-obcs-runtime-flags <https://mitgcm.readthedocs.io/en/latest/phys_pkgs/obcs.html#tab-phys-pkg-obcs-runtime-flags>

Anyway, as correctly suggested by Dimitris and Stanislav, it is better to start in a simple way first, and then try more complex approaches, if things don't seem to work properly...

Kind regards,

SQ


> Regards,
> Kunal
> 
> On Thu, Jul 29, 2021 at 10:25 AM Stanislav Martyanov <martyanov.sd at gmail.com <mailto:martyanov.sd at gmail.com>> wrote:
> Hello, Kunal,
> 
> I use option #1 (#define ALLOW_RUNOFF, and corresponding settings in data.ext) for the Kara Sea regional domain, and it works fine. The spatial resolution is also rather high: O(1 km) in horizontal and 2m in vertical. Simulation of thermohaline structure near river estuaries and getting realistic river plumes (at least as much as the resolution allows) also sometimes requires knowing the temperature of riverine waters, which often is not available. As I remember, by default MITgcm takes the local water temperature as the input river water's temperature, but in reality it is not always the case.
> 
> Regards,
> Stanislav Martyanov
> 
> ср, 28 июл. 2021 г. в 13:10, kunal madkaiker <kunal.madkaiker02 at gmail.com <mailto:kunal.madkaiker02 at gmail.com>>:
> Dear MITgcm users,
> 
> I am doing regional simulations in the Northern Indian Ocean. Stratification in Bay of Bengal is controlled by salinity and freshwater flux since many rivers like Ganges, Brahmaputra empty into this basin.
> 
> I am not able to properly simulate my temp, salinity and currents into the Head bay region of BoB as I am unable to produce an equatorward flow in my domain. Freshwater increases in this region during month of JJAS due to melting of Himalayan glaciers.
> 
> I have gone through previously asked similar questions about river runoff and came to know the following possibilities:
> 
> 1. To feed at surface through data.exf
> 
> 2. Feeding a 3-D run-off input using addMassFile
> 
> 3. Through OBCS/ RBCS.
> 
> My domain is regional, with small dz and high spatial resolution.
> Please let me know which option is the best suited and reliable, since I not only want to improve my temp/ salinity but also to rectify the circulation in head bay region. I would be using Dai et al. (2009) river runoff data.
> All suggestions are welcome. Attaching figure of my model domain.
> 
> Thanks and Regards,
> Kunal
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