Transcript Progress on modifying offshore boundary conditions for full

Progress on the Modification of Offshore
Boundary Conditions for Full-Bay ROMS
Simulations
• Have shown previously that ROMS simulations
capture tides well, but that subtidal variability is not
well simulated.
• Hypothesis in CHRP-II proposal: Poor subtidal
model skill results from the fact that the shelf
dynamics in the model is poorly simulated.
• Proposed to extend “large-domain” model farther
out on the continental shelf and to force this model
with output of regional models.
• Before building a new model, have examined the
model and observations with an eye towards a
better understanding of the dynamics.
Providence: sea level
Blue: “large-domain” model
Red: “fullbay” model
East Passage
Channel: currents
Newport: sea level
Large-domain model open
boundary forcing:
•Tides from ADCIRC.
•T, S from seasonal
climatology.
North/South Velocity (EP Channel)
North
South
North
South
Depth-Averaged North/South Velocity (EP Channel)
Model skill (Wilmott) = 0.75
Approximate depth-averaged momentum balance
(north/south direction)
A
B
¶v
¶h g ¶
= -g ò
¶t
¶y H ¶y -H
0
C
D
E
bottom
æ r - r0 ö
t
ty
ò çè r ÷ødzdz + H - H
0
z
0
wind
y
A. Local acceleration (time change of velocity):
estimate from ADCP obs. (E. Passage channel).
B. Barotropic pressure gradient (due to surface slope):
estimate from sea level obs. (Prov. and Newport).
C. Baroclinic pressure gradient (due to density gradient):
estimate by difference.
D. Wind stress ( t ywind = CD windy windy):
estimate from wind obs. (Quonset).
E. Bottom stress ( t ybottom = CD v bottom vbottom ):
estimate from ADCP obs. (near-bottom bin).
Depth-Averaged Momentum Balance (EP channel)
Up-estuary
Down-estuary
Depth-Averaged Momentum Balance (EP channel)
Up-estuary
Down-estuary
Momentum Balance Obs. vs Model
Forcing from FVCOM model
Model results for
2006 obtained from
C. Chen, UMASS
Dartmouth (for
OSAMP project).
Model output sampled
along large-domain
model open boundary
and used as forcing.
Force Large-Domain Model:
•Low-pass filtered sea level
and velocity from FVCOM
simulation applied along open
boundaries of large-domain
model (in addition to tides and
T/S climatology as in standard
run).
Open boundaries
Momentum Balance Obs. vs Models
Sea Level, Obs. vs Models
Remaining Questions/Next Steps
• Big unanswered question: Why are model pressure gradient
fluctuations in Bay so severely underestimated?
1. Open boundary forcing still not right?
2. Bottom drag coefficient in model needs adjustment?
3. Wind stress too weak?
• Next Steps:
1. Obtain updated version of Chen model. Check its fidelity with
Newport sea level compared to present version. If improved, then
extract forcing from it and re-run large-domain and fullbay models.
2. Run existing models with changes to drag coefficient and/or
increased wind stress. Should provide useful insight.
3. Create new large-domain model to simulate shelf circulation and
sea level setup in vicinity of Bay mouth. Use this to force fullbay
model.
Sea Level and Wind
Sea level/wind correlations