wind energy_meteorology_unit_8_-_offshore_wind_energy_meteorology

Carl von Ossietzky Universität Oldenburg
Institute of Physics • Energy Meteorology Group
Dr. Detlev Heinemann

Offshore Wind Energy
Meteorology

Detlev Heinemann

Carl von Ossietzky Universität Oldenburg
Institute of Physics/Energy Meteorology Group
ForWind

Mittwoch, 15. Juni 2011

OFFSHORE WIND ENERGY METEOROLOGY

CONTENTS
‣ Introduction: Offshore-specific situation

‣ Characteristics of marine boundary layers
‣ Air-sea interaction (wind & waves)
‣ Momentum & Heat fluxes
‣ Vertical wind profiles over sea
‣ Turbulence

‣ Implications for wind power applications
‣ Consequences for modeling offshore wind speed profiles
‣ „OffshoreGrid“: Mescoscale calculation of offshore wind power

‣ Outlook & future research



A GENERAL REMARK

Basic Physics

Assumptions on Description of
Parameterizations Models
atmospheric ﬂow MBL ﬂow

Measurements



A GENERAL REMARK

Basic Physics


Measurements

mostly proven for non-complex
onshore wind ﬂow



A GENERAL REMARK

Basic Physics


Measurements

mostly proven for non-complex
onshore wind ﬂow

Finite knowledge of offshore wind conditions limits our modeling success!



ONSHORE (INLAND) vs. OFFSHORE (MARINE)
Marine winds are fundamentally different from inland winds in four principal ways:

Still vs. Moving Surface Atmospheric Stability
‣ Water surface moves in three dimensions ‣ Numerical models handle both high
under the influence of wind forcing stability and high instability poorly due to
their boundary layer parameterizations
‣ It has momentum from tides, ocean
currents, and wind-driven currents ‣ Frictional turning: Highly variable over
water (from near-geostrophic flow to highly
‣ Momentum transfer from wind is also ageostrophic flow); linked to wave height
principal energy source of wave generation and stability

Isallobaric Winds Land-Water Interface
‣ Local wind effect due to time-varying ‣ Varying coastal wind effects
pressure fields
‣ Greater significance for marine wind field ‣ Regionally important
due to decreased friction > larger cross- ‣ Include: Terrain-forced ageostrophic flows,
isobar direction of the total wind vector onshore frictional (cyclonic) turning of the
wind, atmospheric wave generation, cold
‣ Potential source of large wind forecast air damming, flow reversals and stalls
errors



MARINE BOUNDARY LAYER CONDITIONS
Height extension

‣ Higher moisture content of marine air masses
>> Lower lifting condensation level (LCL) than continental air masses
>> Marine boundary layer (MBL) is rather shallow compared to continental
air masses
‣ However, convective marine and stratocumulus topped boundary layers have
active mixing processes. Both often provide moisture to the atmosphere from
evaporation and simultaneously deepen by entrainment and mixing.



Frictional Inﬂuences

Lower frictional interactions impact wind direction

‣ For the same geostrophic forcing, winds over the water will have a different
direction than those same winds found over land.
‣ Frictional effects result in the over-water winds not having as much of a
cross-isobar direction as those over land.
‣ It is more likely that winds will be geostrophic and of a greater speed than a
comparable setting over land (exception: high seas conditions).



Sea Surface Temperatures (SST)

Relatively unchanging sea surface temperature on diurnal time scales impacts
lapse rates

‣ SSTs varies much less in space and time than temperatures over land.
‣ The ocean is also a nearly unlimited energy source and energy sink.
‣ Combined, these two attributes can quickly serve to modify the temperature
and relative humidity of the MBL when air masses move off-shore.
‣ Over open water, the same properties generally create a much smaller
diurnal oscillation in air temperature.
‣ Negative lapse rates (i.e., an inversion) due to nocturnal radiation cooling
that are often seen over land are seldom experienced over the ocean.



Momentum Transfer

... and the dynamics involved in creating waves

‣ Winds over the ocean are in contact with a moving surface into which they
impart momentum resulting in wave generation.
‣ In turn, the sea surface roughness inﬂuences the surface wind.
‣ Marine winds are highly subject to boundary-layer processes such as
mechanical and convective turbulence as well as stratiﬁcation, and these
processes control the momentum transfer into wave energy.



MBL Issues in NWP
Most of current NWP models have high vertical resolution in the boundary layer
(up to 15 levels)

But: Main errors in model BL proﬁles of wind, temperature, and dewpoint can be
attributed to turbulence and convective parameterizations

Further limitations:
‣ Simple algorithms for wind-wave coupling
‣ Lack of real-time data for model initialization (> data assimilation, > remote
sensing)

poor resolution of near-surface variables
forecast winds (and waves) are often erroneous during both stability
extremes in the MBL



MBL affects surface wind speed and direction differently
than over land

‣ Friction at the ocean surface is the direct result of the transfer of turbulent
kinetic energy (hence related to speed squared) from the wind ﬁeld to the sea
surface and wave ﬁeld
‣ Magnitude of friction is dependent on stability and whether mechanical
turbulence and/or convective turbulent processes are involved
‣ Stability is crucial in determining the wind-wave frictional coupling



MBL affects surface wind speed and direction differently
than over land
Friction shifts the wind away from
geostrophic flow creating cross-contour
flow toward lower pressure.
The effects of the frictional force
diminish with height until the wind is in
a non-frictional dynamic state
> geostrophic flow at the top of the
boundary layer
With less friction over sea, winds at the
surface are stronger, more consistent
Balance of boundary layer flow and more geostrophic in both speed
The pressure gradient force Fp,h is balanced by the sum and direction as over land given the
of the Coriolis force Fc,h and the friction force Ffr. same atmospheric conditions.



Turbulence (I)

Mechanical Turbulence
‣ through interaction of the wind and surface air mass with sea surface
waves
‣ Resulting eddies formed by the rising and falling sea surface can extend
vertically for tens of meters.
‣ Extent of eddies based on wave height and vertical near-surface wind
shear



Turbulence (II)

Thermal (convective) Turbulence
‣ Due to rising plumes of warm air and compensating downdrafts
‣ Range from 100 to more than 1000 m height
‣ Stability is the main factor for the depth of frictional coupling in the MBL
due to convective turbulence
‣ Stratiﬁed lower atmosphere: Mixing is minimal and the surface air mass
will be decoupled from the winds aloft
‣ Unstratiﬁed (i.e. unstable) atmosphere: Mixing couples winds aloft to the
surface
‣ Temperature difference between rather constant SST and temperature of
overlying air mass is primary factor impacting stability over water



DIFFERENCES BETWEEN MARINE AND
CONTINENTAL ATMOSPHERIC BOUNDARY
LAYERS (I)
‣ Near-surface air is always moist, relative humidity typically ~ 75–100%
‣ Weak diurnal cycle, since surface energy ﬂuxes distribute over a large depth
(10–100+ m) of water (large heat capacity!)
‣ Small air-sea temperature differences, except near coasts. Air is typically 0–2
K cooler than the water due to radiative cooling and advection, except for
strong winds or large sea-surface temperature (SST) gradients.
‣ The MBL air is usually radiatively cooling at 1–2 K/day, and some of this heat
is supplied by sensible heat ﬂuxes off the ocean surface. If the air is much
colder than the SST, vigorous convection will quickly reduce the temperature
difference.



DIFFERENCES BETWEEN MARINE AND
CONTINENTAL ATMOSPHERIC BOUNDARY
LAYERS (II)
‣ Small ‘Bowen ratio’ of sensible to latent heat flux due to the small air-sea
temperature difference:
latent heat fluxes: ~ 50–200 Wm-2, sensible heat fluxes: ~ 0–30 Wm -2
‣ Most of marine boundary layers include clouds.
Excepting near coasts, when warm, dry continental air advects over a colder
ocean, tending to produce a more stable shear-driven BL which does not
deepen to the LCL of surface air.
Clouds can greatly affect MBL dynamics. It also affects the surface and top-
of-atmosphere energy balance and the SST.



AIR-SEA MOMENTUM TRANSFER
Basics
‣ When mean flow momentum ρu varies in the vertical, fluctuating vertical eddy motions
of velocity w′ transport faster fluid from the momentum-rich region, which locally
appears as excess velocity, positive u′.
‣ Averaged, the effects of these eddy motions add up to eddy transport of momentum,
ρu′w′, (Reynolds flux of momentum or Reynolds stress)
‣ From the similarity principle of turbulence, the Reynolds stresses should be proportional
to density times the square of the velocity scale: −ρu′w′ = const.ρu′2
(with velocity scale ( (u′2) ) 1/2)
‣ Other reasonable choice for the velocity scale is the “friction velocity” u* = (-u′w′) 1/2
‣ Particularly useful for low level airflow where the Reynolds stress is nearly constant and
differs little from τi , the effective shear force on the interface, e.g., the momentum flux
from air to water: u* = (τi/ρ) 1/2



AIR-SEA MOMENTUM TRANSFER
Basics
‣ When mean flow momentum ρu varies in the vertical, fluctuating vertical eddy motions
of velocity w′ transport faster fluid from the momentum-rich region, which locally
appears as excess velocity, positive u′.
‣ Averaged, the effects of these eddy motions add up to eddy transport of momentum,
ρu′w′, (Reynolds flux of momentum or Reynolds stress)
‣ From the similarity principle of turbulence, the Reynolds stresses should be proportional
to density times the square of the velocity scale: −ρu′w′ = const.ρu′2
(with velocity scale ( (u′2) ) 1/2)
‣ Other reasonable choice for the velocity scale is the “friction velocity” u* = (-u′w′) 1/2
‣ Particularly useful for low level airflow where the Reynolds stress is nearly constant and
differs little from τi , the effective shear force on the interface, e.g., the momentum flux
from air to water: u* = (τi/ρ) 1/2

‣ ...finally, these considerations lead us to the well-known logarithmic wind profile.



CHARNOCK‘s RELATION FOR WAVE INFLUENCE
H. Charnock: Wind stress on a water surface. Quart. J. Roy. Meteorol. Soc. 639–
640, 1955.

‣ Charnock suggested that the roughness length of airﬂow over ocean waves
should depend on acceleration of gravity g and surface stress/friction
velocity u* ~ τ1/2
‣ Dimensional considerations then gave rise to the Charnock relation for the
roughness length
‣ Assumption: Sea state is completely determined by the local friction velocity
u∗
u2
z 0 = Ac ∗
g
‣ Ac is a „constant“.
But is actually not constant and depends on local state of sea. For the open
ocean usually a value of Ac = 0.011 is generally used. In the coastal sea
around Denmark a value of Ac = 0.018 has been found (Johnson et al., 1998).



CHARNOCK‘s RELATION
Dependence on Fetch (Johnson et al., 1998)
‣ z0 shows a dependence on wave age (the ratio between the phase speed of the
dominant wave (c) and u*)

‣ The dependence on (inverse) wave age is often formulated as

Ac = α(u∗ /c)β
with α=1.89 and β=1.59.

‣ A fetch-dependent variation of c is:
2π xg 1/3
c/u∗ = ( )
3.5 U10
with the fetch x and U10, the wind speed at 10 m height.

‣ A modiﬁcation for all fetches leads to
(u∗ /c)β
with γ=69. Ac = α
1 + γ(u∗ /c)β+2




Charnock constant Ac as a function of
wave age c/u*

‣ Measurements taken from literature
‣ Fits from Johnson et al. (1998)

H.P. Frank et al. (2000)




Sea surface roughness as a function of
wind speed at 10 m a.g.l. at different
distances x from the coast using
Charnock‘s relation with constant Ac =
0.018 and the wave age (i.e. fetch-
1 mm
dependent) Charnock constant.
u < 2.5 ms-1: water surface is
approximately aerodynamically smooth
>>viscous formula for z0 applies
intermediate wind speeds: ﬂow is aero-
dynamically smooth over some parts of
the water surface but rough around and
in the lee of the breaking whitecaps
basic graph: H.P. Frank et al. (2000) u > 10 ms-1: fully rough ﬂow




Sea surface roughness as a function of
wind speed at 10 m a.g.l. at different
distances x from the coast using
Charnock‘s relation with constant Ac =
0.018 and the wave age (i.e. fetch-
1 mm
dependent) Charnock constant.
u < 2.5 ms-1: water surface is
rough flow > Charnock relation approximately aerodynamically smooth
>>viscous formula for z0 applies
intermediate wind speeds: flow is aero-
dynamically smooth over some parts of
the water surface but rough around and
in the lee of the breaking whitecaps
basic graph: H.P. Frank et al. (2000) u > 10 ms-1: fully rough flow



‣ Charnock‘s formula is reasonably accurate for 10 m wind speeds of 4–50
ms-1.
‣ For 10m wind speeds of 5–10 ms-1, this gives roughness lengths of 0.1–1
mm, much less than almost any land surface. Even the heavy seas under in a
tropical storm have a roughness length less than mown grass!
‣ This is because (a) the large waves move along with the wind, and (b) drag
seems to mainly be due to the vertical displacements involved directly in
breaking, rather than by the much larger amplitude longs well.
‣ The result is that near-surface wind speeds tend to be much higher over the
ocean, while surface drag tends to be smaller over the ocean than over land
surfaces.



SENSIBLE AND LATENT HEAT TRANSFER AT
THE AIR-SEA INTERFACE
‣ Usually heat transfer proceeds from the ocean to the atmosphere (only under rare
conditions in reverse) as “sensible” and latent heat transfer via the molecular
processes of conduction and diffusion
‣ Sensible heat raises or lowers air temperature
‣ Bulk of heat transfer from the sea to the atmosphere occurs via evaporation and the
attendant transfer of latent heat
‣ In turbulent flow, Reynolds fluxes of temperature w′θ′ and of humidity w′q′ are the
main vehicles of heat and vapor transport to or from the interface on the air side
‣ But: Different from momentum transfer molecular conduction or diffusion has to
perform the transfer at the interface
>> conductive or diffusive boundary layers confined by eddy motion
‣ This results in a complex interplay among molecular conduction and diffusion, wind
waves, and the eddies of the turbulent air flow



‣ In the constant stress layer, Reynolds fluxes of heat and humidity, w′θ′ and
w′q′, are (very nearly) equal to the mean interface fluxes Qi/ρacpa and E/ρa
(also a constant flux layer)
‣ Gradients of mean temperature then depend only on the temperature flux and
the two scales of turbulence:
dθ/dz = f(w′θ′,u∗,z)
‣ Heat transfer law, connecting interface flux (through the temperature scale
θ∗=−w′θ′/u∗)
to the temperature excess/deficiency θ(h)−θs:

(θ(h) − θs) / θ∗ = κ−1 ln(h/zt)
with zt analogue to z0 for the momentum transfer
‣ Same for humidity...



‣ If buoyancy effects are signiﬁcant, then temperature gradient in the constant
ﬂux layer depends on L, as well as on θ∗ and u∗
(again analogue to momentum, same for humidity)
dθ θ∗ z
= φt ( )
dz κz L



VERTICAL WIND PROFILE OVER SEA

‣ Generally, Monin-Obukhov theory has been found to be applicable over open
sea
(although developed over land...)
‣ We need: homogeneous and stationary ﬂow conditions
u∗ z z
u(z) = [ln( ) − Ψm ( )]
κ z0 L

‣ Coastal areas show strong inhomogeneities due to
– roughness change at coastline
– heat ﬂux change through different surfaces
‣ Common example: Warm air advection over cold water (> well-mixed layer
below an inversion)
‣ Systematic deviations from Monin-Obukhov theory at offshore sites expected!



Example: Ratio of wind speeds at 50m and 10m as a function of stability
parameter 10/L for different estimation methods for L

(1) Sonic method
Directly from sonic anemometer measurements of friction velocity and heat ﬂux:

u3∗,s
L=− g
κ T w¯ s
T

with the covariance of temperature and vertical wind speed ﬂuctuation w‘T‘ at
the surface




But how to determine L?

(1) Sonic method
Directly from sonic anemometer measurements of friction velocity and heat ﬂux:

u3∗,s
L=− g
κ T w¯ s
T

with the covariance of temperature and vertical wind speed ﬂuctuation w‘T‘ at
the surface





(2) Gradient method
Differences of temperature and wind speed measurements at 10m and 50m are
used to calculate the gradient Richardson number RiΔ and converting it to L

with virtual temperature difference ΔTv and the height z‘=(z1-z2)/ln(z1/z2)





(3) Bulk method
Use of air and sea surface temperature measurements and the 10m wind
speed as input to approximation method



Data: Rødsand, Baltic sea, 50m, 1998-1999; solid line: MO theory

unstable

stable

sonic method gradient method bulk method

‣ Evidence of larger deviations from MO for stable stratiﬁcation (despite poor
data quality :-( )
‣ Results depend on „measurement“ of L
Lange (2002)




Results show:

‣ Established theories may fail when basic assumptions are no longer valid
‣ Availability of (more) high quality measurement data is essential
‣ Results may depend on speciﬁc techniques and data for analysis
(usually indicator for non-optimal solution...)



TURBULENCE INTENSITY
Measured turbulence intensity depending on wind speed in different heights
and comparison with IEC 61400-3

FINO 1, January 2004 – November
2006

‣ TI strongly depends on wind speed
‣ Inﬂuence of surface roughness
decreases with height
‣ IEC standard for single turbines
adequate

90-percent quantiles of measured
turbulence intensity (black lines) depending
on wind speed in different heights
M. Türk, S. Emeis (2007) compared to turbulence intensity given by
IEC 61400-3 (grey lines)



TURBULENCE INTENSITY
Measured turbulence intensity depending on wind speed in different heights
and comparison with IEC 61400-3

FINO 1, January 2004 – November
2006

‣ TI strongly depends on wind speed
‣ Inﬂuence of surface roughness
decreases with height
‣ IEC standard for single turbines
convective
turbulence
mechanical turbulence adequate

90-percent quantiles of measured
turbulence intensity (black lines) depending
on wind speed in different heights
M. Türk, S. Emeis (2007) compared to turbulence intensity given by
IEC 61400-3 (grey lines)



APPLICATION I: CONSEQUENCES FOR
MODELING OFFSHORE WIND SPEED PROFILES
‣ Vertical Offshore Wind Speed Profiles
‣ Profiles and Thermal Stratification at FINO1
‣ 1-D Profile Models (Peña/Gryning, MO-ICWP)
‣ Offshore Performance of Meso-Scale Models

thanks to Jens Tambke / ForWind



FINO1 MEASUREMENT TOWER
‣ Height: 103 m
‣ Water depth: ~30m
‣ Location: German beight, 45km north of Borkum
(54° 0.86' N, 6° 35.26' E)
‣ Wind speed measurements: Cup anemometer at 8
heights (33-103m),
sonic anemometer
at 40/60/80m
‣ Problem: short
booms
error by ﬂow
around mast
Corrections!
FINO1



FINO1: THERMAL STRATIFICATION
Binned Wind Speed Ratios

unstable stable



COMPARISON: MEAN PROFILES AT FINO1
Model Input: Wind speed time series at 33m height

Observation Wind directions:
z0=0.2m IEC-3 190° – 250°
m WAsP

Monin-ICWP



MO PROFILES AND BOUNDARY LAYER
HEIGHT zi
Mixing Length Approach from Peña Gryning [BLM 2008]:

Unstable

Neutral

Stable

Boundary Layer
Height
Rossby, Montgomery
(1935)



Speed Ratio (u90/u33) vs. Stability (z/L)

stable

unstable
u(90m) 1.05

u(33m)

Stability: 40m/L
(Sonic@40m)



Speed Ratio (u90/u33) vs. Stability (z/L)

Peña/Gryning 2008

Stability: 40m/L
(Sonic@40m)



Speed Ratio vs. Stability (z/L)

u(90m)/u
(33m)

u(70m)/u
(33m)

u(50m)/u
(33m)



Speed Ratio vs. Stability (z/L)

u(90m)/u
(33m)
Peña/Gryning
u(70m)/u
(2008)
(33m)
u(50m)/u
(33m)



MESO-SCALE MODELS AT FINO1

Bias RMSE
m/s m/s
ECMWF -0.4 1.6
Op. Analysis FINO1,
DWD -0. 1.4 alpha
Op. Analysis 1 ventus
MM5 -0. 2.3
with NCEP 1 (2004)
MM5 -0. 1.5
with ECMWF 1
WRF -0. 1.8
with NCEP 1 (2006)
Mean wind speeds at 100m: ~10m/s
Mean potential power production: 50% of installed
capacity



WRF: MELLOR-YAMADA-JANJIC PBL-SCHEME

Mellor and Yamada [1974, 1982]; Janjic [2002]



WRF: IMPROVED MELLOR-YAMADA PBL-SCHEME

Suselj et al, BLM (2009)



FINO1: MEAN WIND PROFILES AT 0-200m

WRF

DWD-LME

Wind
directions:
190° – 250°
Observation



FINO1: SPEED RATIO (u90/u33) vs. 10m/L
Monin-Obukhov

Observation

WRF



MEAN WRF-PROFILES AND STABILITY

unstable -0.6 10m/L stable 10m/L
+0.6 -0.6

-0.3

0

+0.3

+0.6



FINDINGS
‣ Thermal Stratification has a crucial Impact on offshore wind profiles
‣ Observed Wind Profiles show good agreement with Monin-Obukhov profiles
for stabilities with 10m/L 0.12
‣ Meso-scale Models WRF and LME (Cosmo-EU)
‣ perform very well with MYJ-Scheme
‣ capture low, stable boundary layer heights



APPLICATION II:
EU PROJECT OFFSHOREGRID

Objective (WP 3.2):
Time series calculation of offshore wind speed future wind power
for the North and Baltic Sea

‣ Input: 6-hourly global analysis data (FNL), 100 x 100 km2
‣ Output (WRF): 1-hourly, 9 x 9 km2 or 27 x 27 km2
‣ Pre-processing of input data and boundary conditions
‣ Model setup, supervision of model runs, restarts and review
‣ Validation with selected available data



WRF MODEL DOMAINS

WRF setup:
2-domain, nested
formulation

Resolution:
1st domain: 27x27 km2
2nd domain: 9x9 km2



EXAMPLE: WIND SPEEDS IN STORM „FRANZ“

11 January
2007
WRF-wind
speeds
at 90m height
(in m/s)

2007_01_11_05_00



EXAMPLE: WIND SPEEDS IN STORM „KIRYLL“
18 January
2007
WRF-wind
speeds
at 90m height
(in m/s)



ANNUAL MEAN WIND SPEEDS AT 50m
2007 averages
of WRF wind
speeds
at 90m height
(in m/s)



ANNUAL MEAN WIND SPEEDS AT 90m
2007 averages
of WRF wind
speeds
at 90m height
(in m/s)



MEAN WIND PROFILES AT FINO1

Wind
WRF directions:
190° – 250°
DWD-LME

Observation



STORMS AT FINO1 AND IN WRF



RAMPS AT FINO1 AND IN WRF



VALIDATION OF RESULTS

FINO 1 WRF
Parameter
observation simulation

Mean annual wind speed at 100m height 10.3 m/s 10.1 m/s
Wind speed bias -0.2 m/s
Wind speed dispersion error ~ RMSE 1.8 m/s

Annually averaged potential power
production of a single turbine, GH power 60.6 % 59.1 %
curve (Capacity Factor)

Annually averaged potential power
production of a wind farm, GH power 53.4 % 52.0 %
curve with wake losses (Capacity Factor)

Overall power losses due to wakes 11.9 % 12.0 %



OUTLOOK RESEARCH NEEDS (I)
‣ Measurements for optimization of micro- and meso-scale meteorological
models
‣ Measurements for optimization of micro- and meso-scale meteorological
models
incl. satellite remote sensing (vertical structure?!)

‣ Improved wake modeling (multiple wakes, wind farm wakes, LES)

‣ Future Studies for Offshore Wind Resource Assessment:
‣ Wake Effects and Climate Impacts of Offshore Wind Farms
‣ Wakes from large wind farms
‣ Impact of Wakes on the local to regional climate: boundary layer height,
low level jets, boundary layer clouds
‣ Future climates and wind resources
‣ Validate new mesoscale parameterization for offshore conditions



OUTLOOK RESEARCH NEEDS (II)
‣ Surface waves and turbulent boundary layers and their mutual relationships:
‣ complex wave surfaces in ABL and OBL LES
‣ coupled wind-wave and wave-current models
‣ OBL and ABL mixing parameterizations with wave effects;
‣ wave and turbulence mechanics in high winds (e.g., hurricanes)
‣ wave-breaking structure and statistical distributions
‣ disequilibrium, mis-aligned wind-wave conditions


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