bottom friction coefficient

[azhang]

I am applying ROMS3.0 (minimum water depth 10cm with wet-dry option) in Tampa Bay tidal simulation. It ran well for 30 day simulation, but the elevation amplitude is too larger than the tidal prediction in the upper bay. Therefore, I tried to increase bottom friction by changing RDRG2 since I use quadratic bottom friction option. The original RDRG2=0.003 is fine, but I increased RDRG2=0.005, then ROMS blew up after a few days run. I tried LOGDRAG as well with Z0B=0.02, ROMS blew up after a few hour simulation. I wonder why bottom friction coefficient is so sensitive to ROMS, and what value of z0b I should use for LOGDAG option. Can someone give me some advice on how to reduce tidal range in shallow waters for ROMS application, in other words, which parameters should I adjust in ROMS *.in control file.
Thanks very much in advance

AJ

[jcwarner]

for tidal applications I typically use
#define UV_LOGDRAG
I often work in the ranges of
zob=0.002, dt ~ 60 sec, ndtfast ~20 to 30

-j

[azhang]

Hi, John,
Thanks a lot. I tried LOGDRAG option with z0b=0.02 and 0.002, The model blew up at almost same time (about 3 hour simulation). By the way, I use dt=30s, and Ndtfast=30. Will it be helpful to increase vertical layer (currently I use 11 vertical layers) or reduce dt?

AJ

[jcwarner]

when the model "blows up" , and I want to really find out why, then i follow a procedure to
1) run model til it blows up
2) restart model, let it run til it blows again
3) repeat step 2, until it blows up in only a few steps.
4) then save every time step and see exactly where/why it is happening.
Is it blowing up at a wet/dry interface, at the open boundary, at some weird grid location, ???
it is difficult to tell these things until you see why and where the instability is developing.

-j