This document summarizes a study examining semidiurnal perturbations ("Perts") to storm surges along the US Eastern Seaboard. The researchers analyzed tidal gauge and meteorological data from 1996-2014. They found that 22 out of 85 tropical storms produced "Perts" over 0.15m at Fort Pulaski, GA. Storms that generated "Perts" fell into three categories based on their trajectory. Wind stress, rather than atmospheric pressure, appeared to be the main driver of "Perts". Thresholds for meteorological conditions that produce "Perts" will be further investigated through numerical modeling.

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Storm induced Semidiurnal Perturbations to Surges in the US Eastern Seaboard
Xi Feng1,* Maitane Olabarrieta1,a Arnoldo Valle-Levinson1,a Jessy Van Horn2
*feng@coastal.ufl.edu a P.I. maitane.olabarrieta@essie.ufl.edu, arnoldo@coastal.ufl.edu
1. University of Florida, Engineering School of Sustainable Infrastructure and Environment, Department of Civil and Coastal Engineering, 575K Weil Hall, Gainesville, FL, 32611-6580, USA
2. Department of Geography, Planning, and Environment, East Carolina University, Greenville, NC, 27858-4353, USA
Background and Objectives
INTRODUCTION
Semidiurnal perturbations “Perts” to storm surges have been found in
the South Atlantic Bight, especially at Fort Pulaski, Georgia. These
perturbations are caused by tide-storm interactions and modify the timing
and amplitude of peak water levels. North Atlantic Tropical Storms have
been examined during 1996-2014 to explore the reasons of this
phenomenon. A total of 22 out of 85 hurricanes produced semidiurnal
perturbations. Common conditions associated with the perturbations were
synoptic history, meteorological statistics and tidal conditions.
OBJECTIVES
1. Determine what tropical storms and nor’easters produced semidiurnal
perturbations to the storm surge.
2. Establish thresholds in atmospheric and tidal forcing for which
storms generate perturbations.
3. Evaluate the relative importance of a) atmospheric forcing, b)
‘shallow water effects,’ and c) bottom friction on the tide-surge
interaction and consequent generation of semidiurnal residuals.
Subsequent Work
1. Tides and
Atmos. Data
• Obtained from
20 tidal gauges
from NOAA
Tides & Currents
Center
2. Tropical Storm
Synthetic
History
• Best Track
Archive for
Climate
Stewardship
(Knapp et al.
2010).
• Daily weather
maps from the
National Centers
for
Environmental
Prediction
Results 3
Results 1
1. Subtract tidal
prediction from
observed water levels
for each tidal gauge.
2. Obtain the time
series of the
amplitude of the M2
wavelet in surge
(“Perts”).
3. Select events with
“Perts” amplitudes
>0.15 m and longer
than 1 day.
4. Obtain low-
frequency storm
surge with a low-pass
filter.
5. Construct the “Perts”
amplitude
Hovmöller
diagrams and
analyze correlations
with wind and
pressure fields.
2. Tropical Storm Induced “Perts” Events
From 85 tropical storms that affected the US east coast since 1996, 22
caused >0.15-meter “Perts” at Fort Pulaski, Georgia and nearby.
Figure 6. Regression
Analysis on the
relationship between
“Perts” amplitude with
meteorological forcing.
Methodology Results 2
Figure 2. Tidal Gauge Data during Hurricane Floyd in 1999: (a)
observed level, predicted level and surge; (b) Hovmöller diagram
of the surge; (c) wavelet analysis on the surge at Fort Pulaski, GA
station; (d) Hovmöller diagram of “Perts” amplitude; (e) Time
series of different components of surge; (f) Hovmöller diagrams
of wind and pressure fields.
Figure 1. Stations along the US Eastern Coast selected for data analysis.
Figure 4. A summary of the North Atlantic annually tropical storms vs. storms produced “Perts”.
Figure 3. “Perts”
amplitude Hovmöller
diagrams. Yellow
squares show the
“Perts” events during
the period between
years 1996-2014.
Impetus for “Perts”
--Tropical Storm
--Cold Front
--Precipitation
--Low Pressure Trough
Figure 4. Trajectories of the tropical storms that produced the most distinct “Perts” by types.
Tropical Storms that produced “Perts” can be categorized into three types:
• Type 1: Parallel to the SAB coast
• Type 2: Landfall on FL from Atlantic
• Type 3: Landfall on FL from Gulf of Mexico
Type 1 Type 2 Type 3
Storm Name
“Perts” and Tidal Characteristics
1999 H
Floyd
2001 H
Gabrielle
2004 H
Jeanne
2005 H
Ophelia
2005 TS
Tammy
2012 H
Sandy
“Perts” amplitude (m) 0.28 0.26 0.25 0.25 0.27 0.28
Maximum surge (m) 0.59 0.57 0.34 0.54 0.91 0.43
Exceeding period of “Perts” (hrs) 62.10 40.80 67.60 166.40 49.40 54.20
Reduction in tidal amplitude (m) 0.15 0.13 0.17 0.13 0.17 0.15
Tidal phase change, delay (mins) 74.51 38.68 33.95 44.95 62.52 40.63
Table 1. Tidal & “Perts” Characteristics associated with the six representative tropical storms.
• The major disturbance in tidal propagation during a storm event with
“Perts” was wind stress rather than atmospheric pressure deficit.
• The “Perts” primarily occurred during parallel-to-shore winds on the
coastal region of SAB, but could also be related to winds perpendicular to
shore associated with hurricanes’ landfalls.
• Extreme “Perts” events are mostly associated with major hurricanes (Table
1). In addition, a cold-front caused Perts” with 0.30 meters amplitude in
Oct. 2010.
• Thresholds of meteorological forcing, geometric boundary conditions, and
shallow water effects will be investigated through numerical simulation
based on COAWST modeling system.
• Dynamic mechanisms associated with the cold fronts that produced “Perts”
will be studied in the future.
Figure 5. Correlation
between “Perts”
amplitude with phase
difference and tidal
amplitude attenuation.
Conclusions
3. “Perts” occurred mostly when:
• Verified tide lagged behind the
harmonic tidal prediction
• Attenuation in the verified tidal
amplitude compared with the
predicted tide
1. Determination of “Perts” Events
We detected 71 events with Pert amplitude higher than 0.15 m and
the durations longer than 24 hours.
Acknowledgement
This research was supported by NSF grant Award ID: 00088712.
References
[1] Grinsted, A., J. C. Moore, S. Jevrejeva (2004). Application of the cross wavelet transform and wavelet coherence to geophysical time series, Nonlin. Process. Geophys., 11,
561566
[2] Horsburgh, K. J., & Wilson, C. (2007). Tide‐surge interaction and its role in the distribution of surge residuals in the North Sea. Journal of Geophysical Research: Oceans
(1978–2012), 112(C8).
[3] Knapp, K. R., Kruk, M. C., Levinson, D. H., Diamond, H. J., & Neumann, C. J. (2010). The international best track archive for climate stewardship (IBTrACS) unifying
tropical cyclone data. Bulletin of the American Meteorological Society, 91(3), 363-376.
[4] Pawlowicz, R., Beardsley, B., & Lentz, S. (2002). Classical tidal harmonic analysis including error estimates in MATLAB using T_TIDE. Computers & Geosciences, 28(8),
929-937.
[5] Rosenfeld, (1983). Low-pass filters the time series x using the PL64 filtered described in WHOI technical report 85-35, pg.21
Data Collection