Using Ocean Observatory Data to Motivate Hurricane Ocean and Atmospheric Model
Sensitivity Studies in the Mid-Atlantic
poster 1904
Scott Glenn (glenn@marine.rutgers.edu), O. Schofield, J. Kohut, H. Roarty, Y. Xu, L. Palamara, T. Miles & G. Seroka + MARACOOS
Rutgers University, Coastal Ocean Observation Lab’s Hurricane Tuesday Research Group, New Brunswick, New Jersey 08901 USA
Mid-Atlantic’s IOOS Regional Association
The Mid-Atlantic Regional Association Coastal Ocean Observing
System (MARACOOS) extends the 1000 km distance between Cape
Hatteras and Cape Cod. Its densely populated coast includes 10 states,
¼ of the U.S. population, 2 of the largest U.S. ports, the world’s
largest Naval base, large commercial fisheries and a developing
offshore wind energy industry. The wide continental shelf experiences
hydrographic conditions with a strong seasonality. The extreme
summer stratification between the warm surface layer and the bottom
Cold Pool is not resolved by global models. Transitional steps to wellmixed winter conditions are forced by atmospheric cooling and
accelerated by fall mixing storms.
10 States - 76 Million People
CT
MIDDLE
1000 km Cape to Cape
ATLANTIC
NY
REGIONAL
PA
ASSOCIATION
NJ
COASTAL
OCEAN
DE
MD
OBSERVING
SYSTEM
U..
RI
MA
Irene Hydrographic Respone
Ocean Model Results & Interpretation
A new regional scale satellite Sea Surface Temperature (SST) product
was compared to the operational global SST product before (top row)
and after (bottom row) Irene. Before Irene the Mid-Atlantic shelf
SST is 22-24C in both products, with more small-scale variability like
coastal upwelling observed in the regional scale. After Irene, the
regional product indicates the surface cooled to 14-18C, while shelf
waters in the global product remain much warmer (20-24C).
Sept 26
Coldest
Dark
Pixel
Sept 26
Real Time
Global
Sept 26
SST
Difference
Before
Cape
Cod
SST
Before Irene
Sept 31
Coldest
Dark
Pixel
Sept 31
SST
Difference
Sept 31
Real Time
Global
After
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Ocean Forecast
Ensemble
Fig. 2: MARACOOS spatial
observations include satellite
surface temperature and
color,
CODAR
surface
current maps, Slocum Glider
subsurface transects, and
drifter tracks. An ensemble
of
weather
forecasts
validated with an industryenhanced
mesonet
are
available forcing for an
ensemble
of
data
assimilative ocean models.
Hurricanes Irene and Sandy
Irene propagated rapidly along the coast over a highly stratified shelf.
Sandy was larger and approached slowly from offshore over a cooler,
nearly mixed shelf after the fall transition was nearly complete.
Hurricane Irene
August 26, 2011
NOAA/NHC Damage:
#8 with >$15 Billion.
Track Accurate;
Intensity and Storm
Surge Over-predicted.
Hurricane Sandy
October 29, 2012
NOAA/NHC Damage:
#2 with >$60 Billion.
Track Accurate;
Acceleration & Storm
Surge Under-predicted.
Hurricane Irene Glider Sampling
Hurricane Irene
0
26
Temperature
14
55
0
RU16
• Deployed for EPA
• Map bottom DO
• Provided data
during mixing storm
33
55
0
55
Salinity
29
105
% Saturation
(DO)
Aug 12
60
Sep 07
Fig. 5: Track of
the EPA Glider
(yellow line) and
the
complete
time series of
vertical
temperature,
salinity
and
dissolved oxygen
(DO)
profiles.
The black bottom
depth
mask
shows the glider
zigzagging
between deeper
and
shallower
water.
Zooming in on the 2+ days associated with Irene’s passage, much of
the change occurs during the 6-12 hours preceding the eye under the
high velocity winds of the leading edge. The surface layer deepens
from 10 m to 25 m, cools from 24C to 18C, and DO levels drop from
supersaturated to 81%. Bottom temperatures and DO levels remain
relatively constant near 10 C and 60% during the event.
Fig. 6: Glider time
series of temperature
(top
row)
and
dissolved
oxygen
(bottom row) with
profiles (left column)
and surface/bottom
values
(right
column)
during
Irene. Black dashed
line indicates the
time of eye passage
over the glider.
The Regional Ocean Modeling System (ROMS) accurately
reproduces the observed deepening and cooling of the surface layer
during Irene. Before the eye passage, the onshore wind stress is
balanced by an offshore pressure gradient, resulting in a strong
onshore surface layer flow that is compensated by an offshore bottom
layer flow. Mixing coefficients grow in each layer and interact when
the vertical shear across the interface is large. The mixing terms
dominate the heat equation, with cooling in the surface layer and
warming of the interface as it moves down in the water column.
Vertical advection plays a much smaller role in the heat equation, with
other terms even smaller.
Wind Stress =
Pressure Grad.
-------------------Pressure Grad.
= Coriolis Force
On
Population Density
Drifters
Fig. 7: Left column:
Tuckerton
wind
direction & speed (top),
air pressure minimum
(red dotted line), glider
temperature
profiles
(middle), and crossshore velocity (bottom)
from the surface layer
(CODAR),
depth
average (glider) and
bottom layer (inferred).
Right column: CODAR
surface current maps
before & after the eye.
Off
Gliders
depth
HF-Radar
A Slocum Glider deployed on the New Jersey Coast for an EPA
Dissolved Oxygen (DO) monitoring mission before Irene was
purposely not recovered but was instead flown offshore to the 40 m
isobath as Irene approached. Strong summer stratification is observed
before Irene. During the storm, the interface between the surface
layer and the Cold Pool deepens and the surface layer is mixed with
the water below.
depth
Based on user-prioritized needs, MARACOOS operates a sustained
real-time regional-scale observatory that enhances the national
backbone with the spatial data required for assimilation in an
ensemble of regional-scale ocean forecast models.
Fig. 4: Satellite SST maps derived from the new MARACOOS regional coldest
dark pixel composite (1st column), the standard U.S. global product (2nd column),
and the difference (3rd column). Row 1 is the last clear image before Irene and
Row 2 is the first clear image after the storm.
depth
Cape
Hatteras
Cold Pool
Fig. 1: The wide continental shelf of the Mid-Atlantic. Inset
illustrates a typical
summer cross-shelf glider temperature transect (top) with an intense thermocline
above the Cold Pool, and the corresponding RTOFS forecast (bottom).
Satellites
Offshore
SST
After Irene
VA
NC
Before the eye passage near 0900 indicated by the Tuckerton air
pressure, wind is from the east and CODAR surface currents are
strongly onshore, peaking above 50 cm/sec in the onshore direction.
Yet the observed depth-averaged glider velocity remains near zero,
implying a compensating offshore flow in the bottom layer that peaks
as the eye passes. After eye passage, the wind is from the west and
surface currents rotate and accelerate offshore. Since the depth
average glider velocity remains steady near zero, the bottom layer
again must compensate and accelerate onshore.
Vertical
Diffusion
> Vertical
Advection >
Other Terms
Fig.
8:
ROMS
model time series
results
at
the
location
of the
glider during Irene.
Row 1 is the crossshelf force balance.
Row 2 is temp &
cross-shelf velocity.
Row 3 is vertical
velocity & vertical
mixing coefficient.
Row 4 includes the
two dominant terms
in the heat equation,
vertical advection &
vertical diffusion.
Since the Mid-Atlantic Cold Pool covers the full extent of the region,
downwelling-induced shear across the interface is expected to
produce mixing and cooling across the entire region. The SST
difference map below indicates that significant cooling of 6-8 C can
occur over much of the shelf. NOAA weather buoy observations
indicate that much of the cooling occurs during the strong winds of
the leading edge of the hurricane before the eye arrives.
Fig. 3: Hurricanes Irene and Sandy approaching the MARACOOS HF Radar network.
2014 Ocean Sciences Meeting
23-28 February 2014
Honolulu, HI
Fig. 9: Left Column: Satellite
regional
SST
product
difference before and after
Irene. The inset time series
shows the value of the warm
& cold SST under the eye as
it propagates north. Right
Column: NOAA weather
buoy surface temperature
(blue), wind speed (green)
and timing of the pressure
minimum (red) with the
northern buoy at the top.
Atmospheric Model Results & Interpretation
Sensitivities with the advanced Weather Research Forecast (WRF)
model indicate the forecast intensity of Irene is strongly dependent on
SST in the Mid-Atlantic. Specifically, the much colder temperatures
observed in the regional product after the storm reduced intensities by
over 5 m/s, bringing them in line with the best fit post-analysis.
Warm
SST
Cold
SST
Fig. 10: WRF model winds
when the eye is over the
glider generated using the
regional satellite SST product
for fixed surface boundary
conditions assuming the warm
pre-storm conditions (left)
and the cold post-storm
conditions (right).
A WRF model sensitivity matrix is being explored with over 100 runs
to date. The two largest sensitivities are the range of SST boundary
conditions followed by the range of surface layer flux coefficients.
Eye over
Mid-Atlantic
Top Sensitivities:
1) SST
2) Flux Coefficients Warm
3) Flux Coefficients Cold
Fig. 11: Top: Time
series of wind speed
illustrating the Top
Sensitivities to 2 SSTs
(thick red line versus
thick
blue
line)
compared to best fit
(thick black line) and
actual forecast (thick
green line). Bottom:
Time series of wind
speed difference from
the best fit. The vertical
thin black lines indicate
when the eye was over
the Mid-Atlantic.
While Irene’s eye was over the Mid-Atlantic, error statistics for each
model run compared to the best fit intensity are summarized in a box
and whisker plot (below). The official forecast (green) averaged 10 m/s
too high, even though the GFS model (pink) with low resolution physics
and the overly warm SST did well. The warm SST runs (red)
consistently overpredicted the intensity, with the smallest overprediction
resulting from the least advanced flux formulation (warm, 0). Turning
on WRF’s 1-D ocean mixed layer model (aqua) overpredicts the
intensity when realistic mixed layer depths are used (OML,1) and
requires the mixed layer to be reduced to the minimum 10 m even in the
Gulf Stream to match the best fit (OML,2). The cold SST runs (dark
blue) all cluster around the best fit, with the most advanced flux relation
(Cold, 2) providing the best result.
Fig. 12: WRF model
sensitivities
compared with the
best fit intensity over
the
Mid-Atlantic.
Error
is
the
maximum
wind
speed difference.
Conclusions
• Ocean response to Irene includes a 2 layer downwelling that reduces
storm surge & generates shear across the intense summer pycnocline.
• Mixing across the pycnocline causes the surface layer to deepen/cool.
• A cooler surface results in a more accurate intensity forecast for Irene.
Acknowledgements
Financial support provided by U.S. IOOS, NOAA,
EPA, DHS, NJBPU, NJDEP, Teledyne Webb Research,
Nortek & WeatherFlow. The Hurricane Tuesday
working group gratefully acknowledges the support &
scientific expertise of the MARACOOS community.