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Filling the Gap:The Structure of Near Coastal
WindsJeroen Molemaker, Francois Colas and Xavier Capet
University of California Los Angeles
Forcing our ocean models: wind
Global atmospheric model (reanalysis) Observed winds (scatterometer,
climatology, stations) Regional atmospheric model (early stage
of coupling)
Wind forcing Global atmospheric models are too course to
resolve any structure near the coast.
Coupling with regional atmospheric models may be the future but is there an intermediate option?
quikscat leaves an 25-50 km gap
Why should we care?
Wind stress curl determines upwelling. Small changes in curl result ‘big’ changes in
upwelling. (Capet, 2004) and, again, this talk.
Near-shore wind drop-off
• Absent in coarse reanalysis wind products• Still questionable in (uncoupled) atmospheric
regional models
Near-shore wind drop-off
• Land-sea changes in surface drag and boundary layer.
• In-situ observations : Very scarce, but give indication of a drop-off
Point Reyes
Summer: /4 within ~ 25km
Winter: /2 within ~ 25km
Dever et al. (2006), Dorman et al. (2006)
A relation between wind and SST gradients(Chelton using QSCAT)
(Chelton et al. 2001, 2007)
Wind / SST gradient Empirical Relation
Wind / SST Empirical Relation implementation in ROMS
“online”: Correct Wind stress during simulation using actual computed SST’s .
Application: wind feedback on eddies and fronts.
“offline”: Correct QuikCOW wind stress climatology before using in ROMS Objective : impact on quasi-equilibrium climatological solutions.
Try to fill wind ‘gap’ using this relation.
Online wind/sst coupling
Test wind/SST relation at climatologic time scalesCurl() Div ()
Crosswind SST gradient Downwind SST gradient
Test wind/SST relation at climatologic time scales
-- crosswind grad(SST)-- curl (t)
Along-shore
Surprisingly good already!!
So what is missing?
• Land-sea changes in surface drag and boundary layer.
Summer: /4 within ~ 25km
Winter: /2 within ~ 25km
Dever et al. (2006), Dorman et al. (2006)
Orographic effects
Orography: zeroth order approach Reduce near coastal wind by 50% corr = [1- 0.5exp(-D) ]
D = distance to coast = 1/(20 km)
Coast wind reduction
Wind / SST Empirical Relation in ROMS
2 ROMS configurations – climatological conditions
Peru/Chile (4 km)(VOCALS region)California (5 km)
(Capet et al. 2008) (Colas et al. 2008)
Near-shore SST sensitivity (CCS)
Difference with Pathfinder Climatology
quikCOWquikCOW plus ‘Orography’
Alongshore current difference
Average structure of undercurrent
quikCOW quikCOW plus ‘Orography’
Lagrangian estimate of undercurrent flow
Garfield et al. (2001)
Surface Eddy Kinetic Energy original quikCOW winds corrected winds altimetry (DUACS)
[cm2.s-2]
Along-shore averaged EKE
-- quikCOW-- With
‘Orography-- Altimetry
original QSCAT winds corrected winds
Near-shore SST sensitivity (PCS)
Difference
Why did we care: Oceanic heat balance in the PCS - role of eddies
Systematic errors in CGCMs in the South East Pacific: Difficult to reproduce the stratus cloud deck and to simulate the upwelling and its
effects.Role of eddies in the transport of heat (VOCALS project).
(Large and Danabasoglu, 2006)
SST warm bias in CGCM
Potential upscaling effects
from EBUS
- Wind/SST empirical coupling in regional model: near-shore wind drop-off, significant changes on upwelling structure and consequent related
eddy activity (heat balance, offshore transport).
- Wind/SST relation and near-shore structure need to be tested using an
coupled atmospheric model
- Possible upscaling effects at regional scale and even larger scale are
important.