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Validation of decadal simulations of mesoscale structures in the North Sea and Skagerrak. Jon Albretsen and Lars Petter Røed. Outline. Background and motivation Models and configuration Validation results Application to ecosystem Conclusions. Feistein Lighthouse. - PowerPoint PPT Presentation
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Validation of decadal simulations of mesoscale
structures in the North Sea and Skagerrak
Jon Albretsen and Lars Petter Røed
Norwegian Meteorological Institute met.no
#2
Outline
• Background and motivation• Models and configuration• Validation results• Application to ecosystem• Conclusions
Feistein Lighthouse
Norwegian Meteorological Institute met.no
#3
Background and Motivation
• Switching to ROMS– To become our new NOWP model (decision based on earlier
model-model comparison and validation results, LaCasce et. al, 2007)
Old model: MIPOM - old code, yesterdays numerics
New model:ROMS – modern code, sophisticated numerics, e.g., better conservation properties, able to run with higher vertical resolution
• Applications to cod fish eggs/larvae drift from the North Sea to Skagerrak– What is the chance of the spawned North Sea cod fish eggs to
enter the Skagerrak?
Norwegian Meteorological Institute met.no
#4
Background and Motivation
• Goal: – investigate the skill of the various models
with respect to its ability to reproduce the statistical properties
• To be presented– Results from 27 year long hindcast
simulations of the North Sea/Skagerrak area on eddy-permitting (4km) and eddy-resolving (1.5km) grids (period is 1981-2007) using MI-POM and ROMS
• Validation tools– Mainly probability distributions (PDF’s), but
also time series and vertical sections
Norwegian Meteorological Institute met.no
#5
Computational Domains
• Atmospheric forcing: – ERA40 and ECMWF OA
• OBC:– 4 km: SODA reanalysis
+ climatology 2005-2007
– 1.5 km nested to 4 km
• Tides included• Rivers:
– Climatology– Baltic S=12 psu
• No data-assimilation
1.5 km4 km
Norwegian Meteorological Institute met.no
#6
Item MIPOM ROMSResolution 4 km 1.5 km 4 km 1.5 km
# of vertical levels 26 21 32 32
Long (internal) time step
150 s 60 s 120 s 90 s
Ratio of internal to external time step
30 40 30 30
Horizontal dissipation Smagorinsky No explicit diffusion
Vertical mixingMellor-Yamada
2.5 levelGLS mixing scheme
Horizontal advection scheme
2nd order centered 3rd order upwind
Surface fluxes MI-IMStandard ROMS bulk
fluxes (analytic)
Model facts
Norwegian Meteorological Institute met.no
#7
Circulation pattern in the area of interest
1981-2007 average
ROMS 4kmsurface currents
daily mean (2007-3-9)
Norwegian Meteorological Institute met.no
#8
Observations for validation
• Institute of Marine Research: – Current
measurements (one location, valid from 27.10.1992-4.4.1993)
– Monthly data from the Hirtshals – Torungen section (12 stations, all years)
Norwegian Meteorological Institute met.no
#9
Average current speed
Observation period: Nov 1992 – Mar 1993Model values from the exact same period (daily means)58.37N,8.51E: Measured total depth: 120m
Equil. depth: 233m (4km) and 163m (1.5km)
Standard deviation
Validation of current speed
Norwegian Meteorological Institute met.no
#10
Average current speed Standard deviation
Validation of current speed
Observation period: Nov 1992 – Mar 1993Model values from Nov-Mar all winters from 1981-2007
(daily means)
Norwegian Meteorological Institute met.no
#11
Validation of current speed
Obs. period: Nov'92-Mar'93, Model: same period 26 winters:
Obs. period: Nov'92-Mar'93, Model: same period:
13m
75m
13m
75m
Statistical skill: the models' abilities to reproduce the statistical properties of the observed currents
Norwegian Meteorological Institute met.no
#12
Validation of current direction
Obs. period: Nov'92-Mar'93, Model: same period:
Obs. period: Nov'92-Mar'93, Model: same period 26 winters:
Currents from the NE parallel to the:- coast: 238 deg- local isobaths: 225 deg
13m
13m
75m
75m
Norwegian Meteorological Institute met.no
#13
Observation period: Nov 1992 - Mar 1993 Model values from the exact same period (daily
means)
13m depth 75m depth
Validation of current speed
Useful to denote forecast skill
Norwegian Meteorological Institute met.no
#14
M4.0
M1.5
R4.0
R1.5
Obs.
Validation of hydrography
Average density:
Norwegian Meteorological Institute met.no
#15
M4.0
M1.5
R4.0
R1.5
Obs.
Validation of geostrophic velocities
Average velocity:
Norwegian Meteorological Institute met.no
#16
Applications
Simulate drift of cod eggs/larvae from the North Sea to Skagerrak
Example from one location based on:Currents fromROMS 4km,10m depth, 22.2.–1.5. 2006
Probability for a particle to enter Skagerrak: 92%
Norwegian Meteorological Institute met.no
#17
Results – particle drift
Probabilities for particles entering Skagerrak from locations in the North Sea between 1981 and 2007 at 10m depth
1981-2007-average
Annual-variability between 1981 and 2007
Norwegian Meteorological Institute met.no
#18
• Eddy resolution is crucial to get the mesoscale statistics of the circulation correct, and in particular the strength of the current jets
• This is brought about by the much better resolved topography when employing the 1.5 km mesh in combination with eddy resolution (particularly important regarding circulation in areas exhibiting prominent topographic features as f. ex. the Norwegian Trench cutting into the heart of the North Sea/Skagerrak area)
Conclusions
Norwegian Meteorological Institute met.no
#19
Conclusions
• MI-POM reproduces temperature and salinity well on average, but with the largest, positive salinity bias along the Norwegian coast in Skagerrak (the Baltic outflow challenge)
• The analytical expressions in ROMS for surface heat and salinity fluxes creates positive biases in both temperature and salinity (~1oC warm-bias in the Skagerrak and slightly saltier than MI-POM)
• Applying similar surface heat and salinity flux algorithms in ROMS as in MI-POM will hopefully improve the modelled hydrography without downgrading the quality of the currents
• The model simulations form a valuable basis for analysis of statistical properties of the pathways important for the migration, growth and recruitment of fish stocks