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Extratropical Storm Surges in the Chesapeake Bay
323
EXTRATXOPICAL STORM SURGES IN THE CHESAPEAKE BAY DONG-PING WANG Chesapeake Bay Institute, The Johns Hopkins University, Baltimore, MD (U.S.A.)
ABSTRACT
Two major extratropical storm (cyclone) surges in the Chesapeake Bay, in 19741975 are examined. The subtidal sea level was the dominant surge component, and it was induced by the local wind set-up and the nonlocal coupling with coastal sea level. The study suggests that the observational study is essential to the improvement of storm surge forecast. INTRODUCTION Extratropical storms (cyclones) over the U.S. Atlantic coast can cause severe damage.
For example, the coastal storm of early March 1962 caused damage over
$200 million.
While storms causing this much damage are rare, storms of lesser
damage potential do occur several times each winter. Accurate forecasts of flooding and beach erosion caused by these storms are important. There are basically two different approachs to storm surge forecast. The empirical method relates the storm surge to meteorological data from a regression analysis.
The theoretical method determines the storm surge from numerical inte-
gration of the equations of motion and continuity, with appropriate boundary conditions. In the empirical method, physical reasoning is essential in selecting the proper predictors. The theoretical method has less uncertainty in selecting meteorological forcing. However, the numerical model is designed for limited area forecast, and therefore, the choice of model domain and boundary conditions can be critical. A better understanding of the nature of storm surge is thus vital to the improvement of forecast skill. With the advancing of computer technology, the three-dimensional model for semi-enclosed sea, lake and estuary, has been developed (Heaps and Jones (1975), Leenderste et al. (19731, Simons (1973)).
In particular, Heaps has applied the
numerical model to operational surge forecast in the North Sea.
In contrast,
there have been few studies on the storm surge from direct observations. Lack of solid observational evidence, makes it difficult to evaluate model performance.
324
Fig. 1. Map of t h e Chesapeake Bay and its t r i b u t a r i e s ( s e a l e v e l and meteorological s t a t i o n s a r e marked).
325
Recently, Wang (1978a) has examined the subtidal sea level in Chesapeake Bay (Fig. 1) and its relations to atmospheric forcing, from a year-long record. His results indicated that the Bay water response depends on the time scale of atmospheric forcing.
At time scales longer than 7 days, sea levels in the Bay were
driven nonlocally by coastal sea level.
Between 4 and 7 days, both coastal sea
level and local forcing (particularly,lateral wind) were important. At shorter time scales (1 to 3 days), the Bay water response was local, driven by the longitudinal wind.
Wang (1978a) also constructed a response model (empirical method)
which accounts for over 90% of the total subtidal variance. The success in explaining the observed sea level suggests that subtidal sea level is closely related to large-scale atmospheric forcing.
In contrast, super-
tidal sea level was strongly affected by inhomogeneous topography, shoreline and small-scale atmospheric disturbances.
It would be interesting to know if sub-
tidal sea level is the major component of storm surge.
In other words, can the
storm surge be adequately determined from subtidal sea level alone, which is relatively well-understood? We will examine the two major storm surge events in the period of our subtidal sea level study (July 1974 to June 1975). We will describe the atmospheric forcing (extratropical cyclone), the Bay water response, and the relation between subtidal sea level and storm surge. STORM SURGE A.
Event I (December 1 to 4, 1974) On December 1, 1974, a low pressure disturbance (cyclone) was centered around
35'N,85OW
(Fig. 2a).
Winds were southwestward along the Mid-Atlantic coast (Cape
Cod to Cape Hatteras), which generated an onshore Ekman transport. Consequently, sea levels increased over the entire Bight.
In particular, the sea level rise
was about 70 cm at the mouth of Chesapeake Bay (Kiptopeake B.) (Fig. 3).
Assoc-
iated with coastal sea level change, sea levels also increased throughout the Bay. The cyclone propagated to the northeast, and its center passed over the Bay area on 0600 December 2 (Fig. 2b), which resulted in a local northward wind (Fig. 3).
The northward wind set-up w a s quite pronounced; this explains the high
sea level at the Bay head (Havre de Grace). The cyclone continued moving northeastward, and it was centered around Nova Scotia on December 3 (Fig. 212).
The intensity of the cyclone also had signifi-
cantly increased; the central pressure on December 3 was 982 m b , compared to 1004 m b on December 1. Winds were northeastward along the Mid-Atlantic coast, which generated an offshore Ekman transport. Consequently, sea levels decreased
326
Fig. 2 .
Surface weather (atmospheric pressure) map on: (a) 1200 December 1, (b) K3XJ December 2 , and (c) 1200 December 3, 1974.
327
9AnA
I
Kiptopeoke B
.
v
I dyne/cm2
D e c e m b e r , 1974
Fig. 3.
The o r i g i n a l ( s o l i d l i n e s ) and lowpass (dashed l i n e s ) s e a l e v e l s , and t h e lowpass windstress a t P a t u e n t . Kiptopeahe
5
I
Fig. 4.
December.
1974
The highpass s e a l e v e l s .
B
over the entire Bight.
The additional sea level drop at Havre de Grace was due
to the local wind set-down (Fig. 3 ) . The storm surge was dominated by subtidal sea level.
In fact, the response
model (Wang, 1978a) which was developed for subtidal sea level, gives a satisfactory account of the surge event.
The Bay and coastal sea levels responded to
the E-W windstress at time scales of 4 to 7 days; the rise/fall of sea level was associated with the westward/eastward windstress.
In addition, the N-S windstress
drove.loca1set-up/down at time scales of 1 to 3 days. The supertidal component was small. Fig. 4 shows the highpass records (difference between the original and subtidal sea levels):
the semidiurnal tide was
dominant, and the diurnal tide was also clearly reflected by the "diurnal inequalities." There were indications of storm influence in the upper Bay (Annapolis and Havre de Grace).
However, they were too small compared to the subtidal com-
ponent, to have practical significance. B.
Event I1
(April 3 to 6 , 1975)
On April 3 , 1975, a low pressure disturbance was centered around 45"N,80°W (Fig. 5a).
Winds were westward along the New England coast, however, they were
northward over the southern Bight and Chesapeake Bay.
Coastal sea levels did
not respond to the northward wind, apparently due to the lack of large-scale (coherent) forcing. On the other hand, significant set-up in the Bay was induced by the local wind (Fig. 6). The cyclone propagated to the east, and it was centered around the Gulf of Maine on April 4 (Fig. 5b), which resulted in a southeastward wind along the MidAtlantic coast. As the cyclone continued moving eastward (Fig. Sc), winds became southward over the Chesapeake Bay. large:
The local southward wind set-down was
the sea level difference was over 100 cm between Kiptopeake B. and Havre
de Grace (Fig. 6).
Coastal sea level also dropped slightly on April 4.
The storm surge was dominated by subtidal sea level. The rise/fall of sea level was mainly due to the northward/southward wind set-up/down.
The eastward
wind was partly responsible for the sea level decrease on April 4. The supertidal component was also significant in the upper Bay (Fig.7).
The regular tidal
oscillation was suppressed during the storm period. DISCUSSION Our analysis of two strong extratropical storm surges in the Chesapeake Bay suggests that subtidal sea level is the dominant surge component. Our results and Wang (1978a) also indicate that surqes can he induced by local wind set-up,
329
.
d
9
F i g . 5.
Surface weather (atmospheric p r e s s u r e ) map on: (b) 1200 April 4, and (c) 1200 April 5, 1975.
(a)
1200 April 3,
330
I
0
Klptopeoke B
0
u 0
ovre de Grace
v)
\
:
:
:
:
:
:
5
I
6.
:
:
I
9
April,
Fig.
:
1975
The o r i g i n a l ( s o l i d l i n e s ) and lowpass (dashed l i n e s ) sea l e v e l s , and t h e lowpass windstress a t Patuxent.
I
Kiotooeoke 8
-
U
-
U
m
H o v r e de G r o c e
Fig. 7 .
April
1975
The highpass s e a l e v e l s .
331 and nonlocal coastal sea level effect. The nonlocal effect (coastal surge) can be very important under favorable large-scale forcing conditions. For example, the maximum surge height (at Havre de Grace) was comparable between the two events, despite the fact that the local longitudinal windstress was about twice the magnitude in event 11. The compensation was due to the large coastal surge in event I. The local wind set-up is well-known; Wang (1978a) found high coherence between longitudinal windstress and surface slope over a year-long period. The wind set-up can be easily adopted and calibrated in the storm surge model. local effect however, is less well-known.
The non-
In the estuary surge model, the
coastal effect is usually modeled as "observed" surface elevations at the open ocean boundary. Wang (1978a) indicated that the Bay and coastal water response to E-W wind forcing is coupled, Thus, it may not be appropriate to treat the two syqems separately. The present modeling of "open ocean" surge is also rather poor. Wang (1978b) indicated that coastal sea levels along the Mid-Atlantic Bight are driven by:
(a) the local Ekman transport, (b) the local alongshore wind set-up,
and (c) the nonlocal shelf waves.
The "open ocean" surge model however, mainly
considers the effect of cross-shore wind set-up (Pagenkopf and Pearce, 1975). It seems unlikely that the "open ocean" surge model is applicable to extratropical storm surges. In conclusion, our study on the storm surge in Chesapeake Bay suggests that observational study should be emphasized. Recognizing that the model validation procedure is usually rather arbitrary, governing processes must be examined from observations. Only if these processes are clearly identified, can the regional storm surge model be formulated and tested properly.
A continuous feedback be-
tween model prediction and field verification is the only lead to a verified model for surge forecast. ACKNOWLEDGEMENTS We thank Mr. Jose Fernandez-Partagas who kindly made the weather charts available to us.
This study was supported by the National Science Foundation, under
Grant WE74-08463 and OCE77-20254. REFERENCES Heaps, N.S. and Jones, J.E., 1975. Storm surge computations for the Irish sea using a three-dimensional numerical model. Mgmoires Societe Royale des Sciences de Ligge, 6e s6rie;"tome VII, 289-333. Leenderste, J.J., Alexander, R.C. and Lin, S.K., 1973. A three-dimensional model for estuaries and coastal sea. The RAND'Corp., R-1417-OWRR, 57 pp.
332 Pagenkopf, J.R. and Pearce, B.R., 1975. Evaluation of techniques for numerical calculation of storm surges. R.M. Parsons Laboratory, MIT, Report No. 199, 120 pp. Simons, T.J., 1973. Development of three-dimensional numerical models of the Great Lakes. Canada Centre for Inland Waters, Scientific Series No. 12, 26 pp. Wang, D.P., 1978a. Subtidal sea level variations in the Chesapeake Bay and relations to atmospheric forcing. To appear in J. Phys. Oceanogr. Wang, D.P., 197833. Low-frequency sea level variability on the Middle Atlantic Bight. Submitted to J. Mar. Res.
EXTRATXOPICAL STORM SURGES IN THE CHESAPEAKE BAY DONG-PING WANG Chesapeake Bay Institute, The Johns Hopkins University, Baltimore, MD (U.S.A.)
ABSTRACT
Two major extratropical storm (cyclone) surges in the Chesapeake Bay, in 19741975 are examined. The subtidal sea level was the dominant surge component, and it was induced by the local wind set-up and the nonlocal coupling with coastal sea level. The study suggests that the observational study is essential to the improvement of storm surge forecast. INTRODUCTION Extratropical storms (cyclones) over the U.S. Atlantic coast can cause severe damage.
For example, the coastal storm of early March 1962 caused damage over
$200 million.
While storms causing this much damage are rare, storms of lesser
damage potential do occur several times each winter. Accurate forecasts of flooding and beach erosion caused by these storms are important. There are basically two different approachs to storm surge forecast. The empirical method relates the storm surge to meteorological data from a regression analysis.
The theoretical method determines the storm surge from numerical inte-
gration of the equations of motion and continuity, with appropriate boundary conditions. In the empirical method, physical reasoning is essential in selecting the proper predictors. The theoretical method has less uncertainty in selecting meteorological forcing. However, the numerical model is designed for limited area forecast, and therefore, the choice of model domain and boundary conditions can be critical. A better understanding of the nature of storm surge is thus vital to the improvement of forecast skill. With the advancing of computer technology, the three-dimensional model for semi-enclosed sea, lake and estuary, has been developed (Heaps and Jones (1975), Leenderste et al. (19731, Simons (1973)).
In particular, Heaps has applied the
numerical model to operational surge forecast in the North Sea.
In contrast,
there have been few studies on the storm surge from direct observations. Lack of solid observational evidence, makes it difficult to evaluate model performance.
324
Fig. 1. Map of t h e Chesapeake Bay and its t r i b u t a r i e s ( s e a l e v e l and meteorological s t a t i o n s a r e marked).
325
Recently, Wang (1978a) has examined the subtidal sea level in Chesapeake Bay (Fig. 1) and its relations to atmospheric forcing, from a year-long record. His results indicated that the Bay water response depends on the time scale of atmospheric forcing.
At time scales longer than 7 days, sea levels in the Bay were
driven nonlocally by coastal sea level.
Between 4 and 7 days, both coastal sea
level and local forcing (particularly,lateral wind) were important. At shorter time scales (1 to 3 days), the Bay water response was local, driven by the longitudinal wind.
Wang (1978a) also constructed a response model (empirical method)
which accounts for over 90% of the total subtidal variance. The success in explaining the observed sea level suggests that subtidal sea level is closely related to large-scale atmospheric forcing.
In contrast, super-
tidal sea level was strongly affected by inhomogeneous topography, shoreline and small-scale atmospheric disturbances.
It would be interesting to know if sub-
tidal sea level is the major component of storm surge.
In other words, can the
storm surge be adequately determined from subtidal sea level alone, which is relatively well-understood? We will examine the two major storm surge events in the period of our subtidal sea level study (July 1974 to June 1975). We will describe the atmospheric forcing (extratropical cyclone), the Bay water response, and the relation between subtidal sea level and storm surge. STORM SURGE A.
Event I (December 1 to 4, 1974) On December 1, 1974, a low pressure disturbance (cyclone) was centered around
35'N,85OW
(Fig. 2a).
Winds were southwestward along the Mid-Atlantic coast (Cape
Cod to Cape Hatteras), which generated an onshore Ekman transport. Consequently, sea levels increased over the entire Bight.
In particular, the sea level rise
was about 70 cm at the mouth of Chesapeake Bay (Kiptopeake B.) (Fig. 3).
Assoc-
iated with coastal sea level change, sea levels also increased throughout the Bay. The cyclone propagated to the northeast, and its center passed over the Bay area on 0600 December 2 (Fig. 2b), which resulted in a local northward wind (Fig. 3).
The northward wind set-up w a s quite pronounced; this explains the high
sea level at the Bay head (Havre de Grace). The cyclone continued moving northeastward, and it was centered around Nova Scotia on December 3 (Fig. 212).
The intensity of the cyclone also had signifi-
cantly increased; the central pressure on December 3 was 982 m b , compared to 1004 m b on December 1. Winds were northeastward along the Mid-Atlantic coast, which generated an offshore Ekman transport. Consequently, sea levels decreased
326
Fig. 2 .
Surface weather (atmospheric pressure) map on: (a) 1200 December 1, (b) K3XJ December 2 , and (c) 1200 December 3, 1974.
327
9AnA
I
Kiptopeoke B
.
v
I dyne/cm2
D e c e m b e r , 1974
Fig. 3.
The o r i g i n a l ( s o l i d l i n e s ) and lowpass (dashed l i n e s ) s e a l e v e l s , and t h e lowpass windstress a t P a t u e n t . Kiptopeahe
5
I
Fig. 4.
December.
1974
The highpass s e a l e v e l s .
B
over the entire Bight.
The additional sea level drop at Havre de Grace was due
to the local wind set-down (Fig. 3 ) . The storm surge was dominated by subtidal sea level.
In fact, the response
model (Wang, 1978a) which was developed for subtidal sea level, gives a satisfactory account of the surge event.
The Bay and coastal sea levels responded to
the E-W windstress at time scales of 4 to 7 days; the rise/fall of sea level was associated with the westward/eastward windstress.
In addition, the N-S windstress
drove.loca1set-up/down at time scales of 1 to 3 days. The supertidal component was small. Fig. 4 shows the highpass records (difference between the original and subtidal sea levels):
the semidiurnal tide was
dominant, and the diurnal tide was also clearly reflected by the "diurnal inequalities." There were indications of storm influence in the upper Bay (Annapolis and Havre de Grace).
However, they were too small compared to the subtidal com-
ponent, to have practical significance. B.
Event I1
(April 3 to 6 , 1975)
On April 3 , 1975, a low pressure disturbance was centered around 45"N,80°W (Fig. 5a).
Winds were westward along the New England coast, however, they were
northward over the southern Bight and Chesapeake Bay.
Coastal sea levels did
not respond to the northward wind, apparently due to the lack of large-scale (coherent) forcing. On the other hand, significant set-up in the Bay was induced by the local wind (Fig. 6). The cyclone propagated to the east, and it was centered around the Gulf of Maine on April 4 (Fig. 5b), which resulted in a southeastward wind along the MidAtlantic coast. As the cyclone continued moving eastward (Fig. Sc), winds became southward over the Chesapeake Bay. large:
The local southward wind set-down was
the sea level difference was over 100 cm between Kiptopeake B. and Havre
de Grace (Fig. 6).
Coastal sea level also dropped slightly on April 4.
The storm surge was dominated by subtidal sea level. The rise/fall of sea level was mainly due to the northward/southward wind set-up/down.
The eastward
wind was partly responsible for the sea level decrease on April 4. The supertidal component was also significant in the upper Bay (Fig.7).
The regular tidal
oscillation was suppressed during the storm period. DISCUSSION Our analysis of two strong extratropical storm surges in the Chesapeake Bay suggests that subtidal sea level is the dominant surge component. Our results and Wang (1978a) also indicate that surqes can he induced by local wind set-up,
329
.
d
9
F i g . 5.
Surface weather (atmospheric p r e s s u r e ) map on: (b) 1200 April 4, and (c) 1200 April 5, 1975.
(a)
1200 April 3,
330
I
0
Klptopeoke B
0
u 0
ovre de Grace
v)
\
:
:
:
:
:
:
5
I
6.
:
:
I
9
April,
Fig.
:
1975
The o r i g i n a l ( s o l i d l i n e s ) and lowpass (dashed l i n e s ) sea l e v e l s , and t h e lowpass windstress a t Patuxent.
I
Kiotooeoke 8
-
U
-
U
m
H o v r e de G r o c e
Fig. 7 .
April
1975
The highpass s e a l e v e l s .
331 and nonlocal coastal sea level effect. The nonlocal effect (coastal surge) can be very important under favorable large-scale forcing conditions. For example, the maximum surge height (at Havre de Grace) was comparable between the two events, despite the fact that the local longitudinal windstress was about twice the magnitude in event 11. The compensation was due to the large coastal surge in event I. The local wind set-up is well-known; Wang (1978a) found high coherence between longitudinal windstress and surface slope over a year-long period. The wind set-up can be easily adopted and calibrated in the storm surge model. local effect however, is less well-known.
The non-
In the estuary surge model, the
coastal effect is usually modeled as "observed" surface elevations at the open ocean boundary. Wang (1978a) indicated that the Bay and coastal water response to E-W wind forcing is coupled, Thus, it may not be appropriate to treat the two syqems separately. The present modeling of "open ocean" surge is also rather poor. Wang (1978b) indicated that coastal sea levels along the Mid-Atlantic Bight are driven by:
(a) the local Ekman transport, (b) the local alongshore wind set-up,
and (c) the nonlocal shelf waves.
The "open ocean" surge model however, mainly
considers the effect of cross-shore wind set-up (Pagenkopf and Pearce, 1975). It seems unlikely that the "open ocean" surge model is applicable to extratropical storm surges. In conclusion, our study on the storm surge in Chesapeake Bay suggests that observational study should be emphasized. Recognizing that the model validation procedure is usually rather arbitrary, governing processes must be examined from observations. Only if these processes are clearly identified, can the regional storm surge model be formulated and tested properly.
A continuous feedback be-
tween model prediction and field verification is the only lead to a verified model for surge forecast. ACKNOWLEDGEMENTS We thank Mr. Jose Fernandez-Partagas who kindly made the weather charts available to us.
This study was supported by the National Science Foundation, under
Grant WE74-08463 and OCE77-20254. REFERENCES Heaps, N.S. and Jones, J.E., 1975. Storm surge computations for the Irish sea using a three-dimensional numerical model. Mgmoires Societe Royale des Sciences de Ligge, 6e s6rie;"tome VII, 289-333. Leenderste, J.J., Alexander, R.C. and Lin, S.K., 1973. A three-dimensional model for estuaries and coastal sea. The RAND'Corp., R-1417-OWRR, 57 pp.
332 Pagenkopf, J.R. and Pearce, B.R., 1975. Evaluation of techniques for numerical calculation of storm surges. R.M. Parsons Laboratory, MIT, Report No. 199, 120 pp. Simons, T.J., 1973. Development of three-dimensional numerical models of the Great Lakes. Canada Centre for Inland Waters, Scientific Series No. 12, 26 pp. Wang, D.P., 1978a. Subtidal sea level variations in the Chesapeake Bay and relations to atmospheric forcing. To appear in J. Phys. Oceanogr. Wang, D.P., 197833. Low-frequency sea level variability on the Middle Atlantic Bight. Submitted to J. Mar. Res.