SST is lower when change the resolution of the grid and expand the study area

[HONGWANG]

Hi everyone!

Thank you for reading this post.

I simulated an tropical cyclone event using ROMS in Northwest Pacific region. I found the SST on the west side of Hainan Island, China, is lower compared with the observations (There is always a cold water inexplicably appearing there). I have modified many vertical parameters (i.e., 30 levels to 16 levels vertically and smooth the bathymetry..) and found that this phenomenon has not changed. I further found that areas with poor SST simulation may match areas with larger grid file RX1 (see the Figure).

Here is my ROMS grid with horizontal resolution of 12 km:

Here is the comparisons with the observations from NOAA. The red box represents the area of SST simulation anomalies, and the red arrow represents the direction of cold water spread:

Actually, I have obtained good simulation results using a ROMS grid with horizontal resolution of 4 km and 16 levels vertically in a relatively smaller study area (see figure below).

snipaste_20230704_182329.png (82.65 KiB) Viewed 2617 times

I have to expand the scope of the grid due to research needs. Does anyone know why this is? Can I do anything to improve this situation? I would greatly appreciate it if you could provide a response. Thank you very much!

Hong

[rduran]

If the cold water originates at the boundary as it seems that it may, perhaps your BCs are the problem. I saw very intense upwelling when using monthly BC and that was resolved when using daily BC, it had to do with the continuity equation, the model wants outflow and the BC is still stuck at inflow because it changes too slowly.

[HONGWANG]

Thank you for your reply, rduran.

Actually, I simulated the typhoon event and used the daily BCs from HYCOM data. This is my inflow boundaty condition settings (I also try to change the type and the results are almost the same):
LBC(isFsur) == Clo Cha Cha Cha ! free-surface
LBC(isUbar) == Clo Fla Fla Fla ! 2D U-momentum
LBC(isVbar) == Clo Fla Fla Fla ! 2D V-momentum
LBC(isUvel) == Clo RadNud RadNud RadNud ! 3D U-momentum
LBC(isVvel) == Clo RadNud RadNud RadNud ! 3D V-momentum
LBC(isMtke) == Clo Gra Gra Gra ! mixing TKE
LBC(isTvar) == Clo Cla Cla Cla \ ! temperature
Clo Cla Cla Cla ! salinity
For the input lateral boundary conditions file, I used the daily BCs from HYCOM including the sea level, 2d/3d current velocity, temparature, and the salinity. But for the outflow, I don't understand how to resolve the outflow in ROMS.

Actually, I used WRF-ROMS coupled model and I find there is an unusual decrease in SST at the boundary (see Figure below):

The left is the NOAA data and the right is my simulation results.