Passage Of Storm Research Articles

Circulations Associated with a Mature-to-Decaying Midlatitude Mesoscale Convective System. Part I: Surface Features—Heat Bursts and Mesolow Development

Abstract This study examines surface features associated with a mature-to-dissipating midlatitude mesoscale convective system that occurred on 23–24 June 1985 during the Oklahoma-Kansas Preliminary Regional Experiment for STORM-Central. The primary data sources include a 400 × 500 km surface mesonetwork on a 50 km grid, rawinsonde observations from 12 supplementary sites in Kansas and Oklahoma and radar measurements from conventional as well as dual-Doppler networks. The mesoscale convective system under investigation developed in an environment with weak vertical shear and had a lifetime of 9–12 h. It consisted in its mature stage of a southward-moving arc-shaped line of deep convective cells with a trailing stratiform precipitation region to the north. Thirty-three percent of the surface rain in the portion of the mesonetwork experiencing storm passage was from the stratiform region. An intense mesoscale downdraft developed beneath the stratiform cloud with a strong mesohigh at the surface. A wake low w...

Baltimore Canyon as a modern conduit of sediment to the deep sea

An integrated study using moored current meters, transmissometers and sediment traps revealed that sediment in Baltimore Canyon was resuspended regularly when energy from internal tides was focused along the canyon axis. Such focusing occurred primarily during late winter and early spring. During resuspension events particle concentrations at 275 m in the canyon axis typically increased from a background level of 100 μg l −1 to over 1000 μg l −1 within a 5-min period and often exceeded 5000 μg l −1. Net transport of both water and sediment 5 m above bottom (mab) at 275 m in the canyon axis was consistently downcanyon, while 5 mab at 600 m in the canyon axis net transport was upcanyon, implying convergence between the two sites. The distribution of suspended sediment was determined using a CTD/transmissometer during hydrographic transects along the canyon axis and across the slope adjacent to either side of the canyon. Resuspension on the shelf, canyon walls, and along the slope was minimal, but in the canyon interleaving tongues of turbid water were always present between 200 and 800 m. Although concentrations of resuspended sediment were high, they were not sufficient to generate downcanyon flows. Turbid plumes moved seaward along density surfaces at the convergence between 275 and 600 m and other regions of the canyon. The passage of atmospheric storms and Gulf Stream rings may have changed the pattern of currents of resuspension in portions of the canyon sometimes, but not always. Fluxes of particles collected in sediment traps along the canyon axis increased exponentially toward the buttom, but the size of particles collected in traps decreased with distance from the canyon head, suggesting that resuspended sediment was advected, but depleted in larger, denser particles away from the canyon. An exponential increase with depth in trap fluxes at 1000 m, despite fairly uniform concentrations of suspended particles measured by transmissometers, suggests that many particles are advected as large aggregates (marine snow) that are not detected quantitatively by transmissometers. Currents outside the canyon walls quickly dilute and dissipate sediment moving out of the canyon. Subamrine canyons can act as conduits of sediment to the deep ocean not only by channeling turbidity currents, but also by focusing internal tides that resuspended sediment, after which it is advected to deeper water.

Atmospheric conditions during the spring and fall transitions in the coastal ocean off western United States

We examine large‐scale atmospheric behavior around the time of the spring and fall transitions in the coastal ocean off the west coast of North America. Records of adjusted sea level (ASL), coastal wind stress, sea level atmospheric pressure (SLP), and 500‐mbar heights for the years 1971–1975 and 1980–1983 are analyzed. The records cover periods of 91 days, centered on the dates of the spring and fall transitions as determined from coastal adjusted sea level data. Empirical orthogonal functions are obtained from the ASL, coastal wind stress, and SLP records. The two dominant modes of the ASL and coastal wind stress are similar around the times of both the spring and fall transitions, and the time series for these modes are highly correlated with one another. Transitionlike behavior is evident in the time series of the first modes in both spring and fall, but the spring transition is more pronounced. Principal estimator patterns, formed from the dominant empirical orthogonal functions show the spatial patterns of SLP which force the ASL and coastal wind stress during the transitions. The SLP pattern which coincides with the spring transition is the formation of a high‐pressure system centered at 45°N and 140°W along with the development of a low‐pressure cell over the southwest continental United States. Inspection of the 500‐mbar height composites for 91 days surrounding the spring transition for the 9 years reveals the formation of a ridging pattern and diffluent flow over the western United States at the time of the transition; following the transition, the ridging relaxes but the diffluent flow over the continent remains for the duration of the 45 days examined here. The fall transition is characterized by a rise in ASL, particularly north of 40°N, and a change from southward to northward wind stress. The SLP pattern which coincides with the fall transition involves the appearance of a low‐pressure system off western North America centered at 50°N and 140°W, representing the passage of synoptic storms through the region. Prior to the fall transition, the 500‐mbar heights are somewhat diffluent and show a trough over the southwestern United States; after the transition the 500‐mbar flow over the northeast Pacific and North America is nearly zonal.

Atmospheric Boundary Layer and Oceanic Mixed Layer Observations in Hurricane Josephine Obtained from Air-Deployed Drifting Buoys and Research Aircraft

Abstract Three drifting buoys were successfully air-dropped ahead of Hurricane Josephine. This deployment resulted in detailed simultaneous measurements of surface wind speed, surface pressure and subsurface ocean temperature during and subsequent to storm passage. This represents the first time that such a self-consistent data set of surface conditions within a tropical cyclone has been collected. Subsequent NOAA research overflights of the buoys, as part of a hurricane planetary boundary-layer experiment, showed that aircraft wind speeds, extrapolated to the 20 m level, agreed to within ±2 m s−1, pressures agreed to within ±1 mb, and sea-surface temperatures agreed to within ±0.8°C of the buoy values. Ratios of buoy peak 1 min wind (sustained wind) to one-half h mean wind > 1.3 were found to coincide with eyewall and principal rainband features. Buoy trajectories and subsurface temperature measurements revealed the existence of a series of mesoscale eddies in the subtropical front. Buoy data revealed st...

The Characteristics and Evolution of Supercooled Water in Wintertime Storms over the Sierra Nevada: A Summary of Microwave Radiometric Measurements Taken during the Sierra Cooperative Pilot Project

A comprehensive examination of the evolution and vertical distribution of supercooled liquid water in Sierra Nevada winter storms was completed as part of the Sierra Cooperative Pilot Project. Measurements of supercooled water in 63 storms, which affected the area from the 1983/84 through the 1986/87 winter field seasons, were made with a dual-channel microwave radiometer located near the Sierra Nevada crestline. The analyses were carried out in two parts. In the first part, winter storms were grouped into two general categories according to the prevailing flow and resultant storm trajectory. The two storm types consisted of those propagating in zonal flow and those in meridional flow. A complete description of the storm typing is provided. Storms in zonal flow were either developing, occluding, or dissipating during the time they affected the Sierra Nevada. The stage of evolution of the storm dictated the characteristics of supercooled water observed during the storm's passage over the Sierra Nevada. The presence of supercooled water in meridional storms was influenced mainly by the trajectory of the storm. Results indicated that zonal storms had the most sustained periods of supecooled water in the postfrontal region, while sustained supercooled water measurements in meridional storms were usually greatest within the prefrontal portions of the storm. These periods of supercooled water were associated primarily with orographic clouds. Studies of the vertical supercooled water distribution showed that saturation often existed within 1 km of the local terrain when supercooled water was measured by the radiometer. Fifty percent of the rawinsondes launched through supercooled water-bearing clouds measured water-saturated layers deeper than 0.5 km. Saturated layers were found to exist most often between −8° and and −10°C, although they were often present over a much wider temperature range. Conditions favorable for ice crystal growth were common, which supports the pursuit of precipitation enhancement as a means of supplementing regional water supplies.

Vertical column abundance measurements of atmospheric hydroxyl From 26°, 40°, and 65°N

Hydroxyl (OH) vertical column abundances measured with Pepsios spectrometers from Fritz Peak, Colorado (40°N), Boca Raton, Florida (26.4°N), and Poker Flat, Alaska (65°N), are presented. The Colorado normalized abundances have remained nearly constant since 1980 at levels about 1.7 times the 1977 values. Theoretical models agree well with the pre‐1980 abundances, but recent data appear to require a significant change in the atmospheric photochemistry. The Colorado seasonal diurnal asymmetry and its correlation with total ozone are shown to persist. The Florida data show little seasonal variation and have diurnal asymmetry characteristics which differ considerably from those of the Colorado asymmetry. Large Florida OH abundance excursions with respect to Colorado levels are seen in the wintertime of 1980, 1984, and 1986, which suggests the possibility of a relationship with the quasi‐biennial oscillation in tropical stratospheric winds. Responses of the Florida OH column to the passage of a tropical storm (July 1985) and to a partial solar eclipse (October 1986) are also documented. An anomalous drop of 80% in OH column abundances on March 18, 1986, is discussed. Hydroxyl abundance values at Poker Flat (June–July 1983) are 40% higher than corresponding Colorado or Florida values.

Physical and biological responses to the passage of a winter storm in the coastal and inner shelf waters of the northern Gulf of Mexico

Hydrographic and biological properties in coastal and inner shelf waters of the northern Gulf of Mexico were monitored daily over 3 days in February 1984, immediately after the passage of a meteorological front. Strong northerly and westerly components of the wind resulted in upwelling of high salinity inner shelf water containing low nitrate and chlorophyll concentrations. Low salinity coastal water, with associated high concentrations of chlorophyll and nitrate, was transported offshore at the surface. Isopleths sloping from the surface to the bottom over a distance of approximately 10 km characterized an oceanographic front that separated the two water types of day 1. The frontal boundary became more compact and decreased in slope on day 2, and isopleths were essentially horizontal by day 3, with the coastal water overlying the inner shelf water. The passage of such winter storms at 3–8 day intervals could result in a periodic offshore transport of nutrient- and phytoplankton-rich water from the coastal boundary layer to the inner shelf regions. Laboratory experiments with the neritic copepods, Acartia tonsa and Centropages furcatus, showed that egg production increases dramatically within a few hours after an increase in food availability such as might occur in inner shelf waters during upwelling events. It is possible that winter storms and the associated redistribution of nutrients and phytoplankton have significant direct biological consequences in the inner shelf waters of the northern Gulf of Mexico.

The impact of Hurricane Diana on the north Carolina continental shelf

Intense storms, including hurricanes, are believed to have an important impact upon the biota and sediment cover on the continental shelf. Hurricane Diana crossed Onslow Bay, North Carolina, during September 1984 and provided an opportunity to examine its effect on the sea floor. An area in the bay transited by the hurricane had been surveyed previously by side-scan sonar, 3.5 kHz echosounding, and underwater television. We re-surveyed the area in November 1984 with the same techniques and collected box cores for sedimentary structure analysis. The post-hurricane survey yielded no unequivocal evidence of hurricane impact on the sea floor and, surprisingly, showed an abundance of seemingly delicate benthic organisms thriving on the impacted area. Sedimentary structures in the box cores included rip-up clasts and cross-laminations that are indicative of a dynamic sedimentary environment, but they could have formed during non-hurricane storm conditions. The lack of a dramatic signature on the sea floor from Hurricane Diana is attributed to the low sediment influx and frequent occurrence of intense storms on the North Carolina shelf. These factors result in an “armored” depositional environment that shows little response to the passage of each successive storm.

Minor storm impact on the beachface of a sheltered sandy beach

During a squall at Sorrento, Western Australia, winds exceeded 50 km h −1 for 6.5 h and gusted up to 80 km h −1 at the storm peak. Profiles of adjacent cusp-horn and cusp-embayment locations were surveyed half hourly, for over 40 h spanning onset and passage of the storm, in conjunction with measurement of water table fluctuation and swash ranging. The sequence of beach changes followed the storm structure, although with lag effects related to tide, groundwater and swash interaction with the local morphology. Four phases of beachface adjustment were apparent. These were: (1) a period of adjustment to rising tide and energy conditions, during which the cusp-horn was eroded and sediment was deposited in the embayment; (2) beachface erosion as storm, tide and watertable approached their peak; (3) a period of adjustment to declining energy conditions, falling tide and watertable levels that began immediately after the storm peak on the cusp-horn and at mid-ebb tide in the embayment; and (4) steady beachface accretion through mid-to-low tide conditions in the latter part of storm passage. Beach responses on the cusp-horn and embayment hence were dissimilar, with net erosion resulting on one and net deposition on the other.

Hummocky cross‐stratification, tropical hurricanes, and intense winter storms

ABSTRACTMost previous workers have inferred a storm origin for hummocky cross‐stratification, which typically occurs in shallow‐marine deposits. On the modern Earth, the only storms capable of profoundly affecting shallow‐marine depositional environments are severe tropical cyclones (hurricanes) and mid‐latitude winter wave cyclones (intense winter storms).This paper examines the palaeogeographic distribution (including palaeolatitude and palaeogeographic setting) of 107 occurrences of hummocky cross‐stratification, ranging in age from the Proterozoic to Recent. In each of these stratigraphic units, both palaeolatitude and palaeogeography are consistent with a direct storm influence (associated with the passage of hurricanes or winter storms directly over the site of deposition). This palaeogeographic evidence lends support to the inferred storm origin for hummocky cross‐stratification; further, the distribution of the structure suggests that most occurrences (73%) were generated by tropical hurricanes, the remaining 27% being generated by intense mid‐latitude winter storms. The preferential generation of hummocky cross‐stratification by hurricanes is consistent with: (1) the known differences in the nature of the bottom flows generated by the two major storm types, and (2) the inferred nature of the flows which form hummocky cross‐stratification. Hurricanes couple less effectively with the water column than do intense winter storms. Due to this ineffective coupling, hurricane‐generated bottom flows tend to be oscillatory‐or multidirectional‐dominant, with only minor unidirectional components of motion. In contrast, intense winter storms generally do couple effectively with the water column, generating bottom flows which possess a dominant or significant unidirectional component. Most previous workers have suggested that hummocky cross‐stratification forms under oscillatory‐ or multidirectional‐dominant flow; thus, it is conceptually reasonable that the vast majority of ancient occurrences of hummocky cross‐stratification were probably hurricane‐generated, as suggested by the aforementioned palaeogeographic distribution.The Proterozoic, Palaeozoic, Neogene, and Quaternary were times when global climate was similar to that of today. The distribution of hummocky cross‐stratification deposited during these times suggests that both hurricanes and intense winter storms occupied latitudinal belts during these times which were essentially identical to those occupied by their modern counterparts.The Mesozoic and Palaeogene were non‐glacial times when global climate was much warmer than that of today. The distribution of hummocky cross‐stratification deposited during this interval suggests that hurricanes occurred more frequently at higher latitudes during non‐glacial times than they do at present. The possibility of a broadened hurricane belt during the Mesozoic and Palaeogene is consistent with climatic considerations. A limited number of Mesozoic and Palaeogene rock units containing hummocky cross‐stratification were deposited in palaeogeographic settings that preclude a direct hurricane influence; these examples were deposited in the middle latitudes, suggesting that intense winter storms continued to form hummocky cross‐stratification in the middle latitudes during these much warmer times.Some previous workers have suggested that tsunamis may be capable of generating hummocky cross‐stratification. The palaeogeographic distribution of the structure does not support an origin due to tsunamis.Lacustrine examples of hummocky cross‐stratification reported herein are the first known non‐marine occurrences; they suggest that storm effects strongly influence the sedimentary record of some lakes.

An Atmospheric Solitary Gust Observed with a Doppler Radar, a Tall Tower and a Surface Network

Abstract Doppler radar and a 444 m tall instrumented tower provide a detailed view of the kinematic and thermodynamic structure of a solitary gust. A study of the data fields, and comparison with theoretical and laboratory work leads to the conclusion that the gust is an internal solitary wave of permanent form launched by a thunderstorm outflow onto an inversion layer formed by the passage of an earlier storm. Comparisons with results from fluid experiments and numerical models are made and the similarities are striking. Both observations show turbulent breakdown at the rear of the wave. The ease with which solitary waves can be generated in experiments gives reason to believe that these nonlinear waves might be of considerable interest in the context of geophysical fluid dynamics.

Nimbus 7 SMMR observations of the Bering Sea ice cover during March 1979

A method of deriving sea ice concentrations from the 0.81‐cm wavelength radiances of the Nimbus 7 scanning multichannel microwave radiometer (SMMR) is described and used in a comparative study of the Bering Sea ice cover during March 1979. Case studies during 2 days in March demonstrate the utility of the SMMR for studying marginal ice zone processes. The first day provides an example of ice divergent conditions with winds from the northeast advecting the ice southwest toward the ice edge. Comparison of the SMMR‐derived ice concentration maps with surface, aircraft, and Tiros‐derived ice charts shows that the microwave data give the ice edge position to within 30 km, corresponding to the highest spatial resolution of SMMR. The, SMMR ice concentrations also give an accurate description of the general morphology of the ice cover including the presence of lee shore polynyas. The second day illustrates the effect of a storm passage on the ice cover. The SMMR ice map for this day correctly describes the retreat of the ice cover and gives the positions of local regions of open water and of high ice concentration within the pack. The two examples show that for the Bering Sea the SMMR algorithm provides accurate large‐scale to mesoscale data on both the ice edge position and the location of low and high ice concentration regions.

Day‐to‐day variations in the spring‐summer photosynthetic parameters of coastal marine phytoplankton

The day‐to‐day variations over a 70‐day period in the parameters describing the light‐saturation curve of natural assemblages of coastal marine phytoplankton were examined and related to changing environmental conditions. The initial slope, αB, and the light‐saturated rate, PmB, both showed a threefold variation. Correlation and regression analysis attributed most of the variation in αB and PmB to factors relating to community structure. Changes in community structure resulting from the normal progression of spring‐summer conditions were interrupted periodically by transient physical phenomena (passage of storms, periods of up‐welling, etc.). The importance of physical transients to the short term dynamics of the phytoplankton production system places severe constraints on the development of predictive models of phytoplankton production.

The Wake of Hurricane Allen in the Western Gulf of Mexico

Abstract In August 1980, Hurricane Allen passed over a moored array of instruments recording current, temperature and conductivity in the western Gulf of Mexico. An alongshore surge occurred during the storm passage, with the horizontal current speed reaching 91 cm s−1 in the thermocline (200 m) and diminishing to 15 cm s−1 32 m above the bottom (732 m). A wake of near-inertial frequency internal waves commenced after the storm passed the array. The alongshore current oscillation reached a maximum range of 50 cm s−1 within 3 days and decayed thereafter with a time scale of about 5 days. The current oscillations were clockwise-polarized and slightly elliptical, with a period of 22–23 h or about 85% of the local inertial period. Near-uniform upwelling of ±20 m occurred in the thermocline region (200–300 m) during the most intense part of the wake. Depth-leading phases of the horizontal current and temperature oscillations indicated downward radiation of wake energy. The wake oscillations were highly coheren...

Large-Scale Response of the Pacific Ocean Subarctic Front to Momentum Transfer: A Numerical Study

A numerical four-layer hydrodynamical model is implemented to investigate the role the atmosphere exerts on the dynamics of the subarctic front. While the effect of the westerly winds causes a southward shift of the front, the effect of a negative curl shows convergence at the front. This convergence strongly favors frontogenesis. The salient features at the wake of a strong cyclone can be summarized as follows: (i) upwelling of 20 m of the four interfaces occurs; (ii) the water remains upwelled for several weeks; (iii) the e-folding time scale of the vertical oscillations induced by the storm's passage is of the order of 10 days; (iv) a region of relative maximum convergence is observed in the lower layers below a region of relative maximum upwelling in the upper layer, and (v) the along- and cross-front velocity fields are in quadrature in time, with the cross-front velocity lagging the along-front velocity. With the exception of the cyclone case, it is established that the effects of the atmospheric wind forcing are confined to the upper layer. These results are supported by oceanic observations. Finally, it is also proved that the response of the front to the atmospheric momentum transfer is not sensitive to the initial form of the subarctic front.

Environmental conditions and structure of the West African and Eastern Tropical Atlantic squall lines

The environmental conditions in advance of three West African disturbance lines and three Eastern Atlantic squall lines are examined as is the modification of the environment during the passage of such storms over a site. A comparison with other tropical squall lines observed over Venezuela is made. A model of the relative airflow within a West African disturbance line is presented and some aspects of storm movement are considered. The hydraulic jump analogy and a density current model are applied to the convective systems and the results obtained are compared with observations. The hydraulic jump analogy gives reasonable predictions for the storms' speeds and the density current model results show that for the West African disturbance lines, as well as the Venezuelan storms, a close relationship exists between the storm speed and the speed of the density current. As this relationship is not observed in the Eastern Atlantic squall lines, it appears that the density current model is not applicable to convective systems occurring over the ocean.

Contrasting Depositional Processes of Sub-Clarksville and Woodbine Reservoir Sandstones: ABSTRACT

ABSTRACT The Sub-Clarksville and Woodbine sandstones are newly discovered gas reservoirs in Grimes County. Although the sandstones accumulated in a dominantly shallow marine environment, depositional processes and transport mechanisms were different for each sand body. The Sub-Clarksville represents rapid deposition by storm-generated bottom currents. Sand accumulations are lens-shaped and were restricted to lows by shelf topography. Sedimentary structures grade from laminated to massive sandstone above a sharp basal contact, to churned shaly sandstone at the top overlain by Austin micrite. Mean grain size decreases upward with an average composition of 83 percent quartz, 6 percent rock fragments, 9 percent matrix, and abundant carbonate cement. Quartz content decreases and matrix increases upward. Bedding sequence and textural gradation suggest that sand was eroded from preexisting Woodbine sediments to the east, and transported to the shelf during passage of a major storm. In contrast, Woodbine sandstones represent a prograding deltaic environment. The sandstones are highly quartzose (80-94 percent) and show an increasing grain size upward. The C sandstone at Hill field is directly overlain by the Austin and grades from bioturbated mudstone at the base to massive or faintly laminated sandstone near the top. Woodbine sandstones C and D thicken significantly eastward as an extension of a giant prograding deltaic system to the northeast. Conversely, Woodbine A and B sandstones were deposited as offshore bars farther west at Kurten (Bell, 1980; Turner and Conger, 1981). Distribution of both the Sub-Clarksville and Woodbine sandstones was affected by deep-seated salt movement at Hill field and the unconformity associated with this local high. Future exploration will depend on a detailed knowledge of the depositional framework in these downdip areas.

Cyclic and Seasonal Beach Response: A Comparison of Oceanside and Bayside Beaches

This study compares the cyclic and seasonal geomorphic responses of oceanside and bayside beaches on Sandy Hook Spit, New Jersey. It was hypothesized that the different nature of onshore wind regimes at bayside and oceanside beaches would cause different types of beach change. On the east-facing ocean shoreline, rates of beach change are related to weather patterns associated with the passage of mid-latitude cyclonic storms. Bayside beach change is related to the influence of the prevailing westerlies. Storm erosion and post-storm deposition is more rapid on oceanside beaches. Swell waves occurring between storms rapidly reinstate pre-storm equilibrium conditions. Lower bay-side wave energies occurring between storms have little effect on profile development, and foreshore slopes inherited from previous storms undergo little change. Bayside beaches therefore appear to be more in equilibrium with storm conditions than oceanside beaches. Changes in foreshore slope, beach volume, and beach position associate...

Sands and Sand Transport on Palimpsest Carbonate Shelf: ABSTRACT

The West Florida Shelf east of Cape San Blas, an End_Page 700------------------------------ area of about 250,000 sq km, is the only major shelf of the continental United States which is presently dominated by carbonate sedimentation. The veneer of sediments which comprises the present surface of the shelf is called the West Florida Sand Sheet. It is composed of greater than 75% carbonate and is the latest expression of a 5 km thick accumulation of carbonate rocks and evaporites of Mesozoic and Tertiary ages which has been cut off from major clastic provenance since Jurassic time. The West Florida Sand Sheet differs from many great carbonate banks such as those of the Bahamas, the Persian Gulf, and the Great Barrier reef in that it extends as far north as 29°30^prime and is composed mostly of residual carbonate, specifically of patches of molluscan shell hash, foraminiferal, algal, and even oolitic sands. Only a few patch reefs and one relatively large deep-water (>20 m) tropical reef, called the Florida Middle Ground, are present. Sediments resemble more closely those of the shelf of the southeastern Atlantic United States, with the clastic components removed, than those in other carbonate banks. Inshore of the carbonate sands and separated from them by a transition zone of mixed composition lies a mature fine quartz sand, which also comprises the beaches of southwest Florida. The quartz sand appears to have been deposited at lower sea-level stands and then to have been moved up and down the peninsula in a seasonally changing longshore current system. Side-scan and seismic surveys of the West Florida shelf show that far from being a featureless plain beneath the relatively low-energy gulf, the sand sheet has a full suite of bed forms from giant sand waves to small-scale ripples. These suggest that the seafloor is undergoing major redistribution and reworking of sediments, probably primarily as the result of passage of major storms. End_of_Article - Last_Page 701------------

Computer model of wind, waves, and longshore currents during a coastal storm

A mathematical model has been developed to forecast or hindcast wind, waves, and longshore currents during the passage of a coastal storm. Storm intensity is a function of the barometric pressure gradient which is modeled by rotating an inverted normal curve around the center of an ellipse. The length and orientation of the major and minor axes of the ellipse control the size and shape of the storm. The path of the storm is determined by a sequence of storm positions for the hindcast mode, and by interpolated positions assuming constant speed and direction for the forecast mode. The site location, shoreline orientation, and nearshore bottom slope provide input data for the shore position. The geostrophic wind speed and direction at the shore site are computed from the latitude and barometric pressure gradient. The geostrophic wind is converted into surface wind speed and direction by applying corrections for frictional effects over land and sea. The surface wind speed and direction, effective fetch, and wind duration are used to compute wave period, breaker height, and breaker angle at the shore site. The longshore current velocity is computed as a function of wave period, breaker height and angle, and nearshore slope. The model was tested by comparing observed data for several coastal locations with predicted values for wind speed, wave period and height, and longshore current velocity. Forecasts were made for actual storms and for hypothetical circular and elliptical storms.