The NJ sea breeze occurs as a
result of differential heating between relatively dry warm air over land and
moist cool air over water. This
thermodynamic process produces a strong temperature gradient with a resultant
intense onshore airflow that will cause sea breeze advection to extend from the
coast toseveral kilometers inland.
The following diagram depicts the sea breeze circulation.
Sea Breeze Circulation
As the Atlantic Sea
Breeze propagates inland, the onshore flow will be opposed by the prevailing
synoptic flow causing the speed of inland penetration to lessen and eventually
be curtailed. At this transition
location, which is defined as the sea breeze front, winds converge with a
significant vertical velocity component.
At some upper level, winds above the ÒfrontÓ will travel toward the sea
completing the circulation creating the sea breeze cell. The following Doppler radar images show
the propagation of the sea breeze as defined by the concentrated red
backscatter that delineates the sea breeze front.
Doppler Radar showing the Sea
Breeze Front
Sea breeze structure and
dynamics are dependent on the following physical properties:
-
Thermodynamics of the
terrestrial and adjacent marine boundary layers.
- Temperature gradient between the land and water
interface.
- The occurrence or non-occurrence of coastal
upwelling (colder water near the bottom of the ocean is brought to the surface
along the coast).
-
Coastline
configurations and inland terrain features.
- The images shown below illustrate the cause and effect relationships associated with sea breeze development:
MODIS Visible Satellite | Infrared Satellite Image |
Referring to the
preceding visible satellite image, convex coastlines (e.g., central and south
Jersey coastlines) will strengthened sea breeze development and concave
coastlines (e.g., the Raritan Bay area) will tend to weaken sea breeze
development. Inland surface
features will also affect sea breeze development. Airflow over the forest canopy associated with the extensive
Pinelands area located in central and south Jersey is relatively undisturbed
allowing the sea breeze to intensify and propagate well inland. Urban features along with a prominent
concave shoreline relevant to the Raritan Bay area will not only weaken sea
breeze development but could also negate the existence of the sea breeze.
Coastal upwelling will
tend to intensify the sea breeze circulation. However, sea breeze depth and penetration inland will not be
as great when compared to a well-developed sea breeze not affected by
upwelling. Upwelling cases tend to
be associated with relatively strong south to southwesterly synoptic flow,
which will dampen inland sea breeze propagation. The following sea breeze simulations illustrate the
influence of upwelling on sea breeze development: The upwelling case shows
limited inland sea breeze penetration with strong winds along the coast and
immediately offshore ( 8 to 12m/s).
The non-upwelling case shows that the sea breeze has moved farther
inland with lesser wind intensities along the coast (7 to 9 m/s).
10m Wind Speed (knts)
Non-Upwelling Case
10m Wind Speed (m/s)
Local sea breeze
circulations can occur during any month of the year when the thermal
characteristics of the marine air and adjacent terrestrial air provide the
conditions necessary for sea breeze development. As stated previously, these conditions include
significant temperature gradients from colder coastal waters to warmer inland
locations and minimal wind intensities associated with the prevailing synoptic
flow. Although, sea breezes can
occur throughout the year, well-developed sea breezes occur most frequently
during the spring and summer seasons.
Sea breeze frequencies are displayed in the following chart:
Sea Breeze Simulation
Modeling
Coastal meteorological
monitoring data, ocean wave, surface current, and wind data derived from CODAR
remote sensing along with actual sea surface temperatures (SSTs) were
incorporated into the Weather Research Forecast (WRF) model to develop
representative high-resolution 3-D wind field simulations. The Rutgers IMCS Coastal Laboratory for
Applied Meteorology (CLAM) configured WRF to produce both synoptic (macro) scale
and regional (meso) scale simulations that enabled the model to resolve local
wind fields. These local wind
fields (e.g., the sea breeze and/or DE Bay breeze circulations) can then be
embedded in the larger scale wind flow pattern to produce a representative
simulation of the actual wind resource.
This ÒnestingÓ feature with grid resolutions ranging from 500 m to 1 km
and from 2 km to 4 km enabled us to provide accurate and precise
offshore/coastal wind resource analyses.
The resultant model, RU-WRF, can be utilized for both diagnostic and
predictive applications necessary for cost-effective policy and decision-making
protocols associated with offshore/coastal wind energy development.
RU-WRF Model Atlantic Sea Breeze and DE Bay Breeze Simulation
The following wind map
displays a composite RU-WRF model simulation of a ÒtypicalÓ Atlantic sea breeze
during its initial development.
The simulation, which is centered on Central and Southern NJ coastal
regions, indicates that wind intensities decrease from the coast to offshore
areas out to ~30nm and then increase farther offshore from ~35nm to >50nm. Wind intensities decrease from the
coast to the sea breeze front, which is located ~5 miles inland from the coastal
shoreline. Maximum wind
intensities occur near the coast behind the sea breeze front and ~50nm to 65nm
offshore (orange to red areas).
Minimum wind intensities occur inland at the sea breeze front delineated
by the dark blue line running nearly parallel to the coast. Minimum wind intensities also occur
offshore defined by the dark blue area between 15nm and 30nm offshore (Central
Region); 10nm and 40nm offshore (Southern Region). The black arrows indicate the general wind directions
for respectively the prevailing synoptic flow, the onshore sea breeze, and the
"offshore" sea breeze.
����������������� RU-WRF model Simulation of Both Onshore and Offshore Sea Breeze Components������������������� �
Light winds will occur over a relatively large area offshore as a result of divergent winds that occur between the onshore and offshore sea breeze circulations (refer to the above figure).� The same physical processes occur during DE Bay breeze development except the DE Bay breeze is not as intense and the offshore component is usually undetectable.� As the sea breeze intensifies during the day, onshore wind speeds will increase by nearly a factor of two.� Also, as the sea breeze intensifies, the ratio of the distances form the coast to the greatest extent of inland penetration and to the greatest extent of offshore development will vary from 1/10 to 1/2.� Since the spatial and temporal dynamics of the sea breeze circulation varies for any given day, coastal and offshore wind resource parameters will be somewhat different over the area of interest for each sea breeze event. A diagram of the onshore and offshore sea breeze components depicting flow vectors and thermal properties (red: warm, blue: cool) is presented in the following figure:
RU-WRF model Simulation of Both Onshore and Offshore Sea Breeze
Components
Light winds will occur over a relatively large area offshore as a result of divergent winds that occur between the onshore and offshore sea breeze circulations (refer to the above figure). The same physical processes occur during DE Bay breeze development except the DE Bay breeze is not as intense and the offshore component is usually undetectable. As the sea breeze intensifies during the day, onshore wind speeds will increase by nearly a factor of two. Also, as the sea breeze intensifies, the ratio of the distances form the coast to the greatest extent of inland penetration and to the greatest extent of offshore development will vary from 1/10 to 1/2. Since the spatial and temporal dynamics of the sea breeze circulation varies for any given day, coastal and offshore wind resource parameters will be somewhat different over the area of interest for each sea breeze event.
A diagram of the onshore and
offshore sea breeze components depicting flow vectors and thermal properties
(red: warm, blue: cool) is presented in the following figure:
Recent RU-WRF model
simulations indicate that the spatial extent of the offshore component of the
sea breeze is far greater than originally expected. Model runs along with visible satellite imagery show that the
offshore component of the sea breeze circulation exceeds the inland component
by a factor of two to possibly an order of magnitude or greater. This finding has significant
implications when analyzing the offshore wind resource to determine the best
locations for offshore wind energy development. The following figure is a visible satellite image with an
overlay of a RU-WRF model wind field simulation that shows a developing sea
breeze off Long Island and the northern/central NJ coasts. The sea breeze forms along the coast
with minimal inland penetration.
However, the offshore sea breeze component extends several miles (~5 to
50nm) from the coast as indicated by the black areas where divergent winds
result in low wind intensities (light winds <5m/s). Consequently, on the basis of these
observations, it can be assumed that the offshore wind resource past 5nm
becomes ineffective for wind power production during certain well-developed sea
breeze events. These observations
validate the previous RU-WRF model simulation and resultant assumptions
presented on page 5.
Sea
Breeze Front DE
Bay Breeze Front Divergent
(light) Winds
RU-WRF model simulations
of representative sea breeze occurrences show that the sea breeze front
location and orientation coincide very closely with the actual sea breeze front
displayed in concurrent Radar images.
A representaive sea breeze event is used to illustrate the close
comparison between observed sea breeze front locations and model
simulations. The first (top) map
depicts digitized Radar data showing the position and progression of the sea
breeze front. The second (bottom)
map is the corresponding RU-WRF model simulation.
Digitized Radar Data Delineating
the Sea Breeze Front and its Inland Progression
RU-WRF Sea Breeze Simulation
Sea Breeze Front @19Z
RU-WRF simulations verified by Radar and
meteorological tower data were selected to represent a ÒtypicalÓ sea breeze
circulation with onshore winds that occur within the surface boundary layer
(sfc to ~100m), 500m winds (winds from the northeasterly sector) that occur
above the TIBL, and the prevailing offshore (westerly) synoptic flow occurring
at the upper boundary (1000m) of the sea breeze cell. Sea breeze wind vectors at 100m, 500m, and 1000m above mean
sea level are shown respectively
in the following simulations:
The Sea Breeze and
Coastal Wind Shear
The preceding RU-WRF model
simulations (Page 8) suggest that there is strong vertical wind shear within
the sea breeze circulation with winds backing from easterly (onshore) sectors
to the northerly and then to the westerly (offshore) sectors respectively from
lower to upper levels of the sea breeze cell. The simulations also give indications that over an area
following the sea breeze front and extending vertically to a level immediately
above the TIBL, higher wind speeds occur producing a low level sea breeze
ÒjetÓ. For example, wind speeds
indicated in the simulations over coastal areas from Forked River south to
Atlantic City range from 8 to 10m/s at 100m, 7 to 9m/s at 500m, and 9 to 11m/s
at 1000m. Therefore, wind speeds increase with height up to levels associated
with TIBL heights along the coast and over adjacent inland areas (respectively,
~20m to >200m). Winds
then decrease above the TIBL to midlevels of the sea breeze cell (~500m). From
the mid-level, wind speeds will increase with height to near the upper boundary
of the sea breeze cell (~1000m).
Winds will then have a tendency to decrease during the transition from
the sea breeze circulation to the prevailing upper airflow.
After analyzing several
refined RU-WRF modeling runs and synthesizing the results, the following
graphic is the resultant simulation of a vertical cross section of a
representative NJ sea breeze. The
sea breeze front is positioned inland at approximately 75W Longitude with the
coastline located at about 74.2W Longitude. Maximum winds (~9 to 10 m/s) within the TIBL occur inland
from the coast near 74.75W Longitude.
This area of maximum wind speeds (i.e., the low level sea breeze ÒjetÓ)
occurs at a height of ~50m and extends to near 250m. The TIBL heights over the sea breeze ÒjetÓ and near the
coastline are estimated to be respectively 225m and <50m. Wind speeds at the coastline and near
offshore to approximately 74W Longitude are ~5 to 7 m/s at heights ranging from
< 50m to ~150m. Winds offshore
from 74W Longitude eastward to approximately 73.5w Longitude decrease from 7
m/s to < 2 m/s at heights ranging from <50m to > 150m. Wind directions within the onshore
component of the sea breeze cell are ESE to SE with winds being from the
northeasterly section within the offshore componet of the sea breeze
circulation. Wind directions ahead
of the sea breeze front are NW to N with offshore winds east of the sea breeze
cell being from the southwesterly sector.
The preceding discussion suggests that there is pronounced vectorial
wind shear associated with the sea breeze circulation.
RU-WRF Model Cross-Sectional
Simulation of the Sea Breeze Circulation
Offshore
Light
Winds Coastline Sea
Breeze Front Sea
Breeze jet
The previous RU-WRF model
simulation and resulting assumptions (Page 9) regarding sea breeze wind shear
are given more validity by comparing them with monitoring data compiled during
sea breeze events. Additional
monitoring data obtained from the Oyster Creek Nuclear Plant meteorological
tower along with a more in-depth analysis confirm most of our initial findings
presented in Phase 2 of the project.
However, our refined analysis suggests that the physical processes that
produce the sea breeze ÒjetÓ and its resulting location are different than were
originally stated. Wind data from
the 10m, 46m, and 116m tower levels were acquired predominately during sea
breeze events. This data
along with model simulations revealed that wind shear within the sea breeze
circulation is significantly different when compared to wind shear over land
areas not affected by the coastal environment. Our findings are summarized as
follows:
-
Observed higher wind speeds
at the Oyster Creek 46m tower level compared to calculated data were the result
of the sea breeze ÒjetÓ.
-
Wind shear effects at
the upper boundary of the sea breeze cell will result in a retardation of wind
intensities at this level. The
upper sea breeze cell boundary height at the tower location, which is adjacent
to the Barnegat Bay, will generally range form ~100 to 200m. This height interval coincides with the
116-meter sensor location. Wind
speeds will increase in the free airflow above this transition layer (upper bounds of the sea breeze
cell). The upper boundary of the
sea breeze cell generally occurs at a height of approximately 1000m. This height could be substantially
greater during the most intense sea breezes or could be less depending on
atmospheric and oceanic conditions.
- Since wind speeds are greater at the 46m level when
compared to the 116m level, it can be assumed that the sea breeze ÒjetÓ at the
Oyster Creek tower site occurs at heights ranging from ~40m to 100m which coincide
with the 46m sensor height and heights within and immediately above the TIBL.
To determine the wind
intensities at heights associated with wind turbines (hub and blade heights)
the power law using 1/7 (0.143) as the wind shear exponent is generally used as
the ÒstandardÓ for calculating wind speeds at altitudes above sensor heights:
v2 =
v1 [z2/z1] a
where,
v2 = the calculated wind speed.
z2 = the height of the calculated wind speed.
v1 = the observed speed
z1 = the wind sensor height
a
= the wind shear exponent.
To test the 1/7th
power law for coastal applications, the meteorological tower at Oyster Creek
(Forked River, NJ; adjacent to the Barnegat Bay) was utilized. Wind sensors on the tower are located
at multiple levels (10m, 46m, and 116m), which enabled us to obtain wind
profiles that are needed to compare calculated with observed data. When comparing actual data from 46m to
the calculated data obtained when applying the 1/7th power law to
the 10m data, the formula provides results that are generally biased too
low. Raising the exponent to a
value closer to 0.3 as compared to 0.143 more accurately estimates actual wind
speeds. This suggests that
free airflow may be approached at lower heights over coastal and offshore areas
when compared to inland areas with similar surface roughness values. However, when comparing the observed
116m tower data to the calculated data, most cases exhibited higher formula
values. Similar results were
obtained from the 90m PSEG tower located in Lower Alloway Twp, NJ near the DE
Bay and the 60m B.L. England tower located in Beesleys Point, NJ near Ocean
City, NJ.
Based on the profile data
acquired from the three referenced meteorological towers, a representative yet
conservative coastal wind shear exponent would fit closely the 1/5th (0.2) power law. However, during Atlantic sea breeze or
DE Bay breeze events, a wind shear exponent that equates to the 1/3rd
(0.34) power law appears to be realistic for heights within the coastal surface
boundary layer (sfc to ~100m + 50m). Coastal TIBL heights range from ~20m to 100m and from ~40m
to 200m over adjacent inland areas.
TIBL heights will depend on coastal topography and thermodynamics of the
lower atmospheric marine and adjacent terrestrial boundary layers. Considering offshore and coastal wind
turbine design and associated hub heights along with typical coastal TIBL
heights, the 1/3rd power law would apply to sea breeze events at
heights of 80m + 40m. Since
onshore wind distributions above ~ 40m to TIBL heights are relatively uniform, wind speeds at
heights ranging from 40m to 120m would be derived as follows:
v2 = v1 [z2/z1]
a
where,
v2 is the calculated wind speed at 80m.
z2 is the focal height (80m) for wind speed calculations for heights ranging from
40m to 120m..
v1 is the observed speed.
z1 is the wind sensor height
a
is the sea breeze wind shear exponent (0.34).
To determine wind speeds
at heights below 80m to 40m, subtract 0.1m/s from the 80m wind speed for each
5m height increment. To determine
wind speeds at heights above 80m to 120m, add 0.1m/s to the 80m wind speed for
each 5m height increment. Since
data was not available for levels >120m, coastal wind speed calculations for
heights above 120m would be skeptical.
The coastal wind shear algorithm applies only to NJ and DE near shore,
coastal, and immediately adjacent inland locations (<10km from the
coastline) influenced by the Atlantic sea breeze or DE Bay breeze.
Base on the fact that TIBL
heights generally increases with increasing distances inland, a different wind
shear algorithm would probably have to be developed as function of distance
from the coast, associated thermodynamic characteristics, and topography. TIBL heights can be estimated using the
following algorithm:
ht = [znrefz3p(1+2B)(n+1)(n+p+1)g(x)Hox+ht,on+p+1]1/n+p+1
________________________________
where,
p(T3-To)cpPaUref
ht
= TIBL height.
ht,o
= initial TIBL height.
Ho
= sfc heat flux.
zref
= height of monitored wind speed
within the TIBL.
n=
wind power-law exponent.
Z3=height of estimated wind speed and temp. at the upper
TIBL boundary.
B=ratio of heat flux at the top of the TIBL.
p=temperature lapse rate power-law exponent.
To = observed sfc
temperature
T3
= temp. at top of TIBL.
cp=specific heat of air.
Pa = air
density.
Uref = monitored wind speed within the TIBL.
The suggested coastal wind
shear algorithm using the 1/3rd power law (0.34 wind shear exponent)
is based on three specific but similar sites and does not cover the entire NJ
coast or DE Bay shoreline. Coastal
locations, such as Atlantic City, with large high-rise buildings and locations
with other irregular surface features will affect both onshore and offshore flow
characteristics. Therefore, the
resultant coastal wind shear algoritm may be significantly different than the
one derived in this study. To
verify and possibly adjust the suggested coastal wind shear shear algorithm,
additional wind profile observations using current and new in-situ
(meteorological towers) and remote (SODAR or RASS) monitoring systems located
at selected sites along the coast should be employed. This enhanced monitoring capability could also be designed
to develop site-specific coastal wind shear algorithms for areas designated as
being favorable for wind energy development.
The pronounced spatial and temporal changes in wind shear, atmospheric stability, turbulence, and resultant airflow associated with the sea breeze circulation have important implications for wind energy development. Additionally, the sea breeze characteristics that influence the offshore, coastal, and adjacent inland wind resources will affect material transport and dispersion with implications that can be applied for effective environmental management. Sea breeze wind flow vectors, the sea breeze ÒjetÓ, the sea breeze front, and TIBL (dashed curve) are depicted in the following diagram:
TIBL
The most intense sea breeze
development occurs during the spring and summer seasons. Well-developed sea breezes occur when
there is a weak synoptic flow along with a large temperature gradient between
the marine air and air over adjacent land areas. These conditions generally result in above normal
temperatures with high humidity over inland areas during afternoon and early
evening hours. Therefore, sea
breeze development during the summer season has a high probability of occurring
concurrently with peak energy demand.
When analyzing the offshore
and coastal wind resource during a ÒtypicalÓ sea breeze event, maximum wind
intensities occur along the coast and decrease with increasing distance
offshore until the influence of the sea breeze becomes minimal. This generally occurs at an offshore
distance >25nm.
Consequently, wind generators located between 5nm and 30nm offshore may
not have adequate wind speeds needed to produce the supplemental power required
during peak demand periods.
Therefore, wind turbines located along the coast or far offshore may
prove to be the most efficient for producing power when sea breeze events
coincide with peak energy demand.
Possibly, turbines could be strategically located along the coast (i.e.,
preferably inland along the coastline or as an alternative, at the coast or
within 3 nm offshore) to take advantage of the wind resource that ranges from
Good to Outstanding during sea breeze occurrences. These ÒcoastalÓ turbines could then be operated as
"peaking" units and could also provide power throughout the year as
needed. Other issues, such as
regulatory policy and public perception, would have to be evaluated prior to
making a decision to locate wind turbines at or adjacent to the coast.
To ensure efficient power
production, very specific placement of the wind turbines within the area where
the optimum wind resource occurs needs to be accomplished since the power
generated by the wind is directly proportional to the wind speed cubed. Therefore, relatively small
fluctuations in wind speed could translate into large power production
variations. Based on this premise,
wind turbines should be located in an area with an adequate wind resource with
minimal variability during both sea breeze occurrences and non-occurrences when
power production may be critical for meeting energy demand. A portion of the study domain including
the Brigantine Shoals/Little Egg Inlet/Great Bay area and adjacent near
offshore areas < 5nm or far offshore >25nm defined in of our wind energy
analysis satisfies these criteria.
The wind power equation is
expressed as follows:
Power = (p / 2) * r * E * R2 * M3
where,
r = air density.
E = turbine
efficiency.
R = the
turbine-blade radius.
M = the wind
speed.
The wind power equation is
expressed as follows:
Power
= (p
/ 2) * r
* E * R2 * M3
where,
r = air density.
E = turbine
efficiency.
R = the
turbine-blade radius.
M = the wind
speed.
Although, the NREL NJ wind
map and RU-WRF model simulation for the annual average wind resource are in
relatively close agreement, the preceding discussion regarding local sea breeze
events suggests the wind resource associated with the sea breeze circulation is
in direct contrast to the wind resource portrayed by both the NRELÕs NJ Wind
Map and the Rutgers annual wind resource simulation. The contrasting NREL annual wind resource analysis and
RU-WRF model sea breeze simulation are respectively presented in the following
wind resource maps:
Wind Speed Units (m/s; mph)
Wind Speed Units (knts; 1knt=0.51 m/s)
An analysis of the local wind
resource @50m associated with the Atlantic sea breeze and DE Bay breeze
circulations revealed the following results:
The results of the local NJ
offshore/coastal wind resource analysis are summarized in Table 3 and Chart 3:
Table 3: Sea Breeze Wind Resource (<1km to ~100km) |
|
|
|
Sea Breeze Wind Field Location |
Wind Speed (m/s) |
Wind Power Class |
Wind Resource Potential |
Coastline and Adjacent Offshore Waters (0 <5nm) |
8.0 to 7.0 m/s |
6 to 4 |
Outstanding to Good |
>5nm to 15nm |
7.0 to 4.0 m/s |
4 to 1 |
Good to Poor |
>15nm to 25nm |
4.0 to 2.0 m/s |
1 |
Poor |
>25nm to 50nm |
2.0 to 7.0 m/s |
1 to 4 |
Poor to Good |
>50nm |
7.0 to >9.0 m/s |
4 to 7 |
Good to Superb |
Chart 3
References
AWS Scientific (1997): Wind Resource Assessment Handbook, National Renewable Energy
Laboratory, NREL Subcontract No. TAT-5-15283-01, Golden, CO.
Bowers, L (2004): The
Effect of Sea Surface Temperature on Sea Breeze Dynamics
Along the Coast of New Jersey, Master of Science Thesis, Institute of Marine and Coastal
Sciences, Rutgers University, New Brunswick, NJ.
Cope, A., Bowers, L., Oman, L., Dunk, R., and Glenn, S. (2003): An Evaluation of the NJ
Sea Breeze and Coastal Upwelling EventsÓ, report prepared for the National Weather Service under a grant provided by NOAAÕs Cooperative Program for Operational Meteorology, Education, and Training (COMET) initiative.
Magnel, Project Coordinator (1976): Environmental Evaluation of the Proposed Offshore
Atlantic Generating StationÓ, report prepared by EG&G Environmental Corp. for PSEG Nuclear.
Petersen, R., R. Dunk, B.
Cochran, and A. Womble (1997): Evaluation of TIBL Growth,
Plume Rise, and Dispersion Under Shoreline Fumigation Conditions; A Wind
Tunnel and Analytical Evaluation, Cermak, Peterka, and Petersen, Inc., Final Report
Submitted to: Jersey Central Power and Light (JCP&L), Morristown, NJ, GPU
Contract No. 412981, CPP Project 95-1244, Ft. Collins, CO.
Raynor, G., Hayes, J., and Ogden, E. (1974): ÒMesoscale Transport and Dispersion of Airborne
Pollens (meteorological/pollen profile monitoring study conducted during
seabreeze events), J. Applied Meteorology, 13, 87-95.
Simpson, J (1994): Sea
Breeze and Local Wind. Cambridge
University Press.
Press Syndicate of the University of Cambridge, New York, NY.