ROMS: Real Ocean Modeling Stories or Reasons Ocean Modeling Sucks

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ROMS Real Ocean Modeling StoriesorReasons
Ocean Modeling Sucks

• Sally Warner
• tOAStER, Friday Harbor Labs
• January 25, 2008

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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?

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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.

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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)

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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.
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Analytic bathymetry
Nonlinear/analytical.f elif defined TTP1
Xsize20.00E03_r8 Esize 10.00E03_r8
depth200.0_r8 f00.0_r8
beta0.0_r8
Nonlinear/analytical.f elif defined TTP1
DO jJstrR,JendR DO iIstrR,IendR
val (xr(i,j)-(Xsize/2.0_r8))/1000.0_r8
h(i,j)depth-5.0_r8EXP(-(valval))
END DO END DO
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Complicated bathymetry
Any land (except straight side-walls) must have a
land mask.
Real bathymetry must be smoothed before
sending it to ROMS.
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PROBLEM! Leaky land mask
psu
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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)
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Time stepping
CFL (Courant, Friedricks, Lewy) condition
determines the size of the time step
One M2 tidal cycle 44712 s 20 tidal cycle
run 900,000 time steps Luckily, if you break
CFL conditions, you will know
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Forcing and boundary conditions
elif defined TTP1 IF (WESTERN_EDGE) THEN
facTANH((tdays(ng)-dstart)/1.0_r8)
omega2.0_r8pitime(ng)/(12.42_r83600.0_
r8) val1.0_r8 phase0.0_r8deg2rad
DO jJstrR,JendR
BOUNDARY(ng)zeta_west(j)facvalSIN(omega-phase)
END DO END IF
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PROBLEM Incorrect boundary conditions
Test 44
channel entrance
channel exit
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PROBLEM Incorrect BCs
Test 45
Test 46
Test 51
Test 49
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Nudging
Nudging forces velocity near edges to a
prescribed value.
Test 61 - with nudging Time averaged velocity at
3 depths
Test 44 - without nudging Time averaged velocity
at 3 depth
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Advantages to modeling

• Know values of all parameters (velocity,
  salinity, temperature, etc.) at every single grid
  point.
• Dont need a ship or fancy instrumentation less
  money.
• Easily test what ifs (e.g. Ive got my model
  running accurately, what if the tides were twice
  as fast?)
• Modeling can fill in holes in measurements.

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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 doesnt need
  patience in grad school.)

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Whoa! Its like a Grateful Dead concert!