ROMS: Real Ocean Modeling

Storiesor

Reasons Ocean Modeling Sucks

Sally WarnertOAStER, Friday Harbor Labs

January 25, 2008

Motivation

• PO students and faculty have discussed the need for a modeling class, but no class has been taught yet.

• I have been approached by numerous grad students with the question, “How hard would it be for me to model region X?”

What is ROMS?

• ROMS: Regional Ocean Modeling System

• Free code is maintained by oceanographers/computer scientists at Rutgers University

• ROMS is one of many ocean models (POM, HIM, etc.), but probably the most advanced and widely used.

How does ROMS work?ROMS is simply thousands of lines of Fortran code

1. Edit or add code that tell ROMS what you want it to do (days to weeks)

2. Compile the code (minutes, need a Fortran compiler)

3. Run the code (hours to days, need a fast computer or lots of time)

4. Analyze the output which come as NetCDF files (months, need MATLAB)

5. Debug and repeat (months to years)

Setting up the code

• Bathymetry• Grid • Time stepping• Stratification• Forcing• Boundary

Conditions• Nudging

All of these parameters can be defined analytically in ROMS or you can read in data from external files.

Analytic bathymetryNonlinear/analytical.f

# elif defined TTP1 Xsize=20.00E+03_r8 Esize= 10.00E+03_r8 depth=200.0_r8 f0=0.0_r8 beta=0.0_r8

Nonlinear/analytical.f

# elif defined TTP1 DO j=JstrR,JendR DO i=IstrR,IendR val = (xr(i,j)-(Xsize/2.0_r8))/1000.0_r8 h(i,j)=depth-5.0_r8*EXP(-(val*val)) END DO END DO

Complicated bathymetry

Any land (exceptstraight side-walls)must have a land mask.

Real bathymetry must besmoothed before sendingit to ROMS.

PROBLEM! Leaky land mask

psu

Grid

• Terrain-following vertical grid (a.k.a. S-coordinate grid)

• rho-grid: temp and salinity (5x3)

• u-grid: x-direction velocity (6x3)

• v-grid: y-direction velocity (5x4)

Time steppingCFL (Courant, Friedricks, Lewy) condition determines the size of the time step:

Δt ≤Δx

c=Δx

gH=

1000m

10 ms2∗200m

= 22.4s

>> One M2 tidal cycle = 44712 s>> 20 tidal cycle run = 900,000 time steps

* Luckily, if you break CFL conditions, you will know

Forcing and boundary conditions

# elif defined TTP1

IF (WESTERN_EDGE) THEN

fac=TANH((tdays(ng)-dstart)/1.0_r8)

omega=2.0_r8*pi*time(ng)/(12.42_r8*3600.0_r8)

val=1.0_r8

phase=0.0_r8*deg2rad

DO j=JstrR,JendR

BOUNDARY(ng)%zeta_west(j)=fac*val*SIN(omega-phase)

END DO

END IF

PROBLEM: Incorrect boundary conditions

channelentrance

channelexit

Test #44

PROBLEM: Incorrect BCsTest #45 Test #46

Test #51Test #49

Nudging

Test #44 - without nudgingTime averaged velocity at 3 depth

Test #61 - with nudgingTime averaged velocity at 3 depths

Nudging forces velocity near edges to a prescribed value.

Advantages to modeling

• Know values of all parameters (velocity, salinity, temperature, etc.) at every single grid point.

• Don’t need a ship or fancy instrumentation = less money.

• Easily test “what ifs” (e.g. I’ve got my model running accurately, what if the tides were twice as fast?)

• Modeling can fill in holes in measurements.

Disadvantages to modeling

• The answer can seem so right, but is it?• Need a powerful computer with lots of

storage space.• A modeler must not be scared of

computer code, MATLAB or computers.• Must have patience. (But who doesn’t

need patience in grad school.)

“Whoa! It’s like a Grateful Dead concert!”