Strong Wind Events Research Articles

Surface current measurements by HF radar in freshwater lakes

HF radar has become an increasingly important tool for mapping surface currents in the coastal ocean. However, the limited range, due to much higher propagation loss and smaller wave heights (relative to the saltwater ocean), has discouraged HF radar use over fresh water, Nevertheless, the potential usefulness of HF radar in measuring circulation patterns in freshwater lakes has stimulated pilot experiments to explore HF radar capabilities over fresh water. The Episodic Events Great Lakes Experiment (EEGLE), which studied the impact of intermittent strong wind events on the resuspension of pollutants from lake-bottom sediments, provided an excellent venue for a pilot experiment. A Multifrequency Coastal HF Radar (MCR) was deployed for 10 days at two sites on the shore of Lake Michigan near St. Joseph, MI. Similarly, a single-frequency CODAR SeaSonde instrument was deployed on the California shore of Lake Tahoe. These two experiments showed that when sufficiently strong surface winds (2 about 7 m/s) exist for an hour or more, a single HE radar can be effective in measuring the radial component of surface currents out to ranges of 10-15 km. We also show the effectiveness of using HF radar in concert with acoustic Doppler current profilers (ADCPs) for measuring a radial component of the current profile to depths as shallow as 50 cm and thus potentially extending the vertical coverage of an ADCP array.

Laboratory Simulation of Wind Vibrations in Roadside Structures

A method of frequency domain analysis in a laboratory is presented for simulating wind vibration affecting roadside transportation structures. The approach is intended to be complementary to or in lieu of a test in a large-scale wind tunnel. A full-size two-pole roadside sign was erected in a laboratory and attached to an electromechanical actuator. Dynamic response of the structure to an impact load from the actuator was converted into the frequency domain to determine a harmonic transfer function. Records of velocities from several actual strong wind events recorded in free-field conditions were combined with drag coefficients to predict the variation of pressure on the structure. Time histories of force that are to be imposed by the actuator on the signposts were developed using frequency response functions and converting them into the frequency domain by fast Fourier transform. Results show that nearly identical structural response of a roadside sign to simulated wind events can be obtained by using an actuator in a laboratory environment.

Observations and simulations of a transient shelfbreak front over the narrow shelf at Sydney, southeastern Australia

Hydrographic and ADCP velocity measurements of a transient shelfbreak front over the narrow New South Wales shelf at Sydney, southeastern Australia, are presented. These measurements represent the first high-resolution data set of observations of such an event over the Sydney shelf. The observations are also significant since few comprehensive measurements of shelfbreak fronts over narrow shelves have been acquired. The particular roles of the processes that led to the formation of the front could not be conclusively determined from the observations alone since both a strong local wind event and an intrusion of a meso-scale East Australian Current (EAC) eddy occurred during the formation period. A numerical model was used to simulate the response of the shelf to an idealised meso-scale EAC eddy, represented by an alongslope baroclinic current, and a local downwelling favourable wind event. The simulations suggest that shelfbreak fronts can be formed under the concurrent action of an alongslope jet and downwelling favourable local winds.

Katabatic winds, hydraulic jumps and wind flow over the Vestfold Hills, East Antarctica

This paper examines two strong or gale-force wind events which began suddenly at Platcha, near the Antarctic ice sheet edge in the Vestfold Hills of East Antarctica, and at a later time were recorded at Davis after progressing slowly across the intervening 25 km of relatively low hills.

Impacts of precipitation in the upper ocean in the western Pacific warm pool during TOGA‐COARE

An ocean mixed‐layer model with a modified Kraus‐Turner parameterization scheme is used to investigate the impacts of precipitation in the upper ocean in the western Pacific warm pool during Tropical Ocean Global Atmosphere‐Coupled Ocean Atmosphere Response Experiment (TOGA‐COARE). Heat and salt budgets calculated in the upper ocean indicate local balance between surface forcing and the ocean response. Thus the mixed‐layer model captures the dominant processes governing heat and salt variability. The model responses are analyzed and compared with the observed upper ocean in three distinctive layers determined by Monin‐Obukhov length scales. In the top layer (the top 5 m), about 90% of the surface buoyancy flux is absorbed, and strong diurnal and intraseasonal variations are excited. The second layer, 5–20 m, contains intraseasonal variability that is characterized by nearly neutral stratification during strong westerly wind events, strong thermal stratification during clear‐sky days, and strong saline stratification during heavy precipitation. The dominant effect of precipitation is to generate a stable stratification and to form a barrier layer. The third layer, 20–50 m, has intraseasonal variations due to mixing during westerly wind events. Heavy precipitation amplifies mixed‐layer temperature fluctuations by a cycle of strong surface cooling and entrainment warming through the following processes. Heavy precipitation causes a shallower mixed layer and a larger cooling rate. Surface temperature drops rapidly, and the upper ocean becomes thermally unstable. The salinity maintains a weak density stability, which causes strong entrainment warming. Surface freshwater flux is the key factor controlling saline structure when advection is excluded. However, experiments without the entrainment process show a significant bias toward a lower salinity, indicating that the entrainment process must be properly treated in the model to prevent a biased trend.

Surface cooled convection simulations with application to ice-covered seas

Some idealized numerical experiments were carried out to study certain basic aspects of convective motions likely to occur during cooling and ice formation in the Nordic seas. The studies were restricted to two-dimensional motions in shallow water up to 300m deep. The freezing point was not assumed to depend on salinity. Cooling rates of 200–500 W m-2 (mass flux rates of (4–10) X 10−5 kgm−2 s−1), and brine discharge rates of (4–8) X 10−5 kgm−2 s−1 were assumed, the latter corresponding roughly to a cm h−1 ice growth. The initial fluid state was assumed to be homogeneous, having been thoroughly stirred by the preceding strong wind events common to those regions. It was assumed that upon reaching the freezing temperature, all further cooling of the surface goes into ice formation, thus losing the buoyancy forcing owing to the cooling-associated mass flux. Under these conditions, the results showed that the occurrence of freezing regions and ice formation can either inhibit or enhance convection in the underlying water masses, depending on the ratio of the cooling-associated mass flux loss (owing to ice formation) to the brine release mass flux gain. Another important dependence was found on diffusivity, which can enhance (or diminish) the heat advected from the ambient ocean below. The augmentation of the upward heat flux by the higher diffusivity delayed both the onset of freezing and the growth of the freezing area, thus diminishing the loss of buoyancy forcing owing to the (higher) cooling-associated mass flux. For a cooling/brine mass flux ratio of five, the kinetic energy of the no-freezing system exceeded that of the freezing system by a factor of three. For cooling rates of 476W m-2 the maximum vertical velocities of the no-freeze simulation steadied at about 10 cm s−1, and for the case of cooling-freezing with a mass flux ratio of about five, at about 5 cm s−1 These values are within the range observed in penetrative ocean convection, or simulated by previous modelers in convection experiments. In the case of insufficient cooling (238 vs. 476 W m-2 the freezing point was reached only briefly, with brine injection lasting for a period of about 3 h. Then the heat transported from below by the convective cells (mainly driven by cooling-associated buoyancy) raised the surface temperature to above freezing conditions.

Impact of westerly wind bursts on the warm pool of the TOGA-COARE domain in an OGCM

A primitive equation model is used to investigate the warm pool equilibrium of the tropical Pacific ocean. Attention is focused on the upper ocean. The oceanic response is described using an isothermal approach applied to warm waters contained in the TOGA-COARE domain. The heat balance shows that all the terms, atmospheric surface fluxes, advection and diffusion, operate in the heat bugdet with different time scales. Over long periods, diffusive heat fluxes transfer heat received from the atmosphere out of the warm pool trough the top of the main thermocline. Over short periods, the impact of westerly wind bursts modifies this balance: atmospheric heating is reversed, diffusion is enhanced and advective heat transports out of the warm pool operate through zonal and vertical contributions. We were able to relate the two latter processes to zonal jets and Ekman pumping, respectively. Conversely, the meridional contribution always represents a source of heat, mainly due to the tropical wind convergence. The modelling results clearly show that except during strong wind events, entrainment cooling is not an important component of the budget. The inability to remove heat is due to the salt stratification which needs to be first reduced or even destroyed by westerly wind bursts to activate heat entrainment into deeper layers. Finally, we suggest that the near zero estimate for the surface heat flux entering the warm pool may be extended to longer periods including seaosnal to interannual time scale.

A numerical simulation of water mass formation in the northern Gulf of California during winter

A numerical simulation was performed using realistic bathymetry, winter hydrography and forcing (wind, surface heat flux and evaporation) in the northern Gulf of California. As a result of cooling and evaporation, water in the shallow regions increases its density and sinks. At depth, the surface water spreads horizontally in a cyclonic sense (shallow water to its right) around the basin. Most of the sinking takes place toward the end of a strong wind, cooling, and evaporation event. In the northern Gulf of California salinity decreases with depth, under normal hydrographic conditions. Therefore, the surface water formed is recognized at depth by its saline and warm signature, relative to the original waters at that depth. The water that sinks reaches depths at which its density is almost equal to the surrounding water, but its salinity and temperature are higher. Sinking to intermediate depths of 75 m occurs mainly along the relatively broader northeastern shelf. This water travels toward the head of the gulf (toward the northwest), where it sinks farther due to the increased bottom slopes in the region. The densest water, arriving at the head of the gulf, sinks to depths of 115 m during weak forcing about 25 days after the strong forcing events have ceased. This water now travels toward the southeast at a speed of about 8 cm s −1 along the southwestern slope. Along this path, water mixes, lowering its salinity and density. High salinity surface water can be traced to depths of 230 m and all the way to the open boundary of the domain. The final state presents strong bottom temperature and salinity fronts, which are in qualitative agreement with the limited available observations. Although the effect of increased salinity on density is about 25% of the increase resulting from temperature, the effect of evaporation is significant in producing saltier, denser waters that reach greater depths.

Surface Buoyancy Forcing and the Mixed Layer of the Western Pacific Warm Pool: Observations and 1D Model Results

The broad, shallow body of warm (>29°C) water found in the western tropical Pacific Ocean plays an important role in the coupled ocean-atmosphere dynamics and thermodynamics associated with the El Niño-Southern Oscillation phenomenon. Thus, it is important to understand the processes that maintain and perturb that warm pool. Measurements from a buoy moored in the center of the warm pool during the TOGA Coupled Ocean-Atmosphere Response Experiment show that the exchange of mass between the ocean and atmosphere is as important as the exchange of heat. Rain forms a shallow, buoyant layer that does not mix with the water below except during infrequent strong wind events. Using a one-dimensional mixed layer model, it is demonstrated that the rate of local precipitation governs the mixed layer depth and can thus alter the rates of change in sea surface temperature during both warming and cooling periods. The observed mixed layer depth in the warm pool is at a depth that allows for maximum warming by capturing a significant portion of the shortwave radiation, but is shallow enough to avoid entrainment cooling. Either increased rain rates or reduced wind forcing yields a shallow mixed layer, which may be associated with cooling of the sea surface temperature in spite of a net positive heat flux at the sea surface because penetrating shortwave radiation is lost across the base of the shallow, fresh mixed layer. Decreased rain rates or increased wind forcing can turn on entrainment cooling of the surface mixed layer.

Inertial currents in the Beaufort Sea: Observations of response to wind and shear

Doppler sonar observations from the Arctic Internal Waves Experiment and Arctic Leads Experiment in the Beaufort Sea are used to examine currents below the ice during spring conditions. Following two strong wind events, the dominant fine‐scale current consists of both downward and upward propagating near‐inertial internal waves. The downward propagating waves are most evident above 100 m depth, while the upward waves are most energetic below this depth. The appearance of these waves coincides in time and depth with a westward subinertial current following the winds with maximum speeds of 0.05 m s−1 at 100 m depth. Simple ray simulations show that the observed subinertial shear can significantly refract near‐inertial waves with length scales similar to those observed. In particular, waves that propagate both in the direction of the current and toward the depth of maximum current are enhanced. These simulations suggest that the observed near‐inertial wave enhancement is due, in part, to wave‐shear effects rather than solely to surface forcing.

Structure and variability of Langmuir circulation during the Surface Waves Processes Program

A cooperative, multiplatform field experiment was conducted in the eastern North Pacific during February and March of 1990 as part of the Surface Waves Processes Program (SWAPP). One of the experimental objectives was to investigate Langmuir circulation so that its role in the evolution of the oceanic surface boundary layer could be better understood. The concurrent use of different observational techniques, ranging from simple surface drifters to complex Doppler sonar systems, resulted in new information about Langmuir circulation structure and variability. Estimates of Langmuir cell spacing indicated that a broad range of scales, from about 2 to 200 m, was excited during periods of strong surface forcing and that the energy containing scales evolved with time. Estimates of cell spacing based on Doppler velocities from a surface‐scanning sonar directed crosswind showed this scale evolution, but estimates based on backscattered intensity did not. This was attributed to the fact that the intensity‐based estimates were only indirectly related to circulation strength. The near‐surface convergent velocities from the sonar were used to form an objective, quantitative measure of the temporal variations in Langmuir circulation strength. As expected, the circulation strength increased dramatically during strong wind events. However, circulation strength and wind stress did not decrease simultaneously, and Langmuir circulation was detectable for up to a day after abrupt reductions in wind stress. Energy from the surface wave field, which decayed more slowly than the wind, was apparently responsible for maintaining the circulation. The variation of circulation strength was found to be better related to (u*Us)½ than to u*, where u* = (τ/ρ)½ is the friction velocity, τ is the wind stress, and Us is the surface wave Stokes drift. This scaling is consistent with wave‐current interaction theories of Langmuir cell generation.

Near-surface recirculation over Georges Bank

Satellite-tracked drifters with drogues centered near-surface (5 m) and below the seasonal thermocline (50 m) were launched during late winter and spring of 1988 and 1989 in the northern Great South Channel in the western Gulf of Maine to investigate the regional circulation as part of the South Channel Ocean Productivity Experiment (SCOPEX). Many of the near-surface drifters became entrained in the clockwise gyre over Georges Bank, and eight drifters made a total of 16 complete circuits around the bank during the stratified season. The average recirculation period of these eight drifters was 48 days, and the average drifter speed around the bank was 12 cm s−1. There is no clear evidence from the drifter data that the strength of the clockwise gyre over the bank increased with time during the stratified season. On average, these drifters (i) followed a relatively narrow path around the bank, except over the eastern end of the bank where three preferred paths were observed, (ii) moved fastest over the northern and southern flanks of the bank, (iii) did not enter a core area of 3500 km2 centered at 41°17′N, 68°00′W, approximately 30 km southwest of the topographic center of the bank, and (iv) stopped circling the bank by the end of November, due in part to strong wind events that appeared to drive drifters off the bank. Curiously, none of the near-surface drifters moved from the southern flank of Georges Bank onto the New England shelf as might be expected from continuity of flow along the outer shelf; instead, the drifters that circled the bank tended to move off the bank along its southern flank. None of the drifters with drogues centered at 50 m appeared to recirculate around Georges Bank.

A pilot study on the interactions between katabatic winds and polynyas at the Adélie Coast, eastern Antarctica

Infrared satellite images of the coastal area off Adélie Land were examined together with two wind data sets, one from the manned French station, Dumont d'Urville, the other one from an Automatic Weather Station (AWS) during the 1986 austral winter. A correlation between the development of open water areas (polynyas) and the appearance of extremely strong offshore winds can be drawn. The wind direction tended to be more perpendicular to the coastline during these extreme ‘events’, suggesting a katabatic origin of the increase in wind strength. In the study area the influence of the katabatic wind on the sea ice extends 20–100 km offshore. Sea ice motion further off the coast seems to be more dominated by synoptic scale weather systems. Broader scale atmospheric influences may create large polynya structures which influence the development of coastal winds, as the temperature contrast between open water and the cold continent generates its own circulation. Strong wind events can have a weakening effect on the coastal sea ice which can lead to a much more sensitive reaction of the sea ice in response to following anomalous wind events.

Sediment Transport Processes in a West-central Florida Open Marine Marsh Tidal Creek; the Role of Tides and Extra-tropical Storms

The extensive open marine marshes on Florida's Gulf of Mexico coast constitute one of the largest continuous coastal marsh systems in the U.S.A. and are characterized by (1) the absence of an apparent modern or relict sediment supply, (2) a thin 1-2 m sediment veneer overlying highly karstified bedrock and (3) both low wave and low tidal energy regimes. More importantly, the Florida open marine marsh system appears to be keeping pace with current rates of sea-level rise in spite of a limited inorganic sediment supply and low tidal energies. Although the magnitudes and directions of suspended solid transport and the processes controlling these transports have been rigorously documented for other U.S.A. marsh systems, they have not been documented in the Florida marsh system. Total suspended solid (TSS) concentrations, current speeds and water levels were monitored in Cedar Creek, Florida, so that the TSS loads could be calculated and the processes exerting control over material exchange could be determined. Both TSS concentration and load are modulated by spring/neap variations and time-velocity asymmetries in the tidal currents. Concentrations at the creek mouth increase by as much as two orders of magnitude during strong wind events due to the presence of waves; however, large net sediment loads appear to be related to the coupled effects of waves and large tidal prisms. Waves initially mobilize sediments in the adjacent embayment but increased tidal prisms, and the associated higher velocities, are requisite for transport of this material further into the creek. Large tidal prisms may be the result of astronomically high tides or meteorologically forced tides. In Cedar Creek, the most important meteorological events affecting sedimentary processes are extra-tropical storms. This is because they occur at much higher frequencies than tropical storms and hurricanes, even though the latter are more potent and potentially could transport greater amounts of material. This study identifies the important processes controlling suspended solid transport in the broad expanses of Juncus roemerianusdominated marsh adjacent to the large arcuate embayments prevalent along the west-central Florida marsh coast as described by Hine and Belknap (1988). The processes exerting control over sediment transport in the Cedar Creek drainage basin are similar to those documented in other marsh systems.

Forecasting the Impacts of Strong Wintertime Post-Cold Front Winds in the Northern Plains

Strong post-cold front wind events in the northern plains of the United States are a difficult problem for operational forecasters. The various atmospheric ingredients that lead to these events are examined from an operational point of view. Indications that are evident in analyses and prognoses are identified and placed in an operational checklist. From the checklist, a numerical rating of the potential strength of a developing situation is derived. This rating can assist the forecaster in deciding what type of watch or warning to issue. Application of the checklist to the strong post-cold front wind event of 10 January 1990 is demonstrated.

Entrainment of Nitrate in the Fraser River Estuary and its Biological Implications. III. Effects of Winds

Two 24-h time series of vertical profiles of velocity, salinity, temperature, fluorescence and NO3were conducted during weak and strong winds (2·2 and 7·3m s−1, respectively) to demonstrate wind effects on entrainment of NO3near the mouth of the Fraser River. The results showed that wind-induced entrainment of NO3was mainly responsible for the increased NO3concentrations in the upper layer because shear velocities between the upper and lower layers were great enough to break down the pycnocline and allow diapycnal mixing to occur. Strong shear was indicated by Richardson numbers less than 0·25 near the depth at which flows in the upper and lower layers moved opposite to each other. As a result of wind-induced entrainment and mixing, the NO3minimum in the water column was gradually eroded and disappeared at the end of the time series during the strong wind event. The amount of entrained NO3under windy conditions (44mmol m−2) was almost three times that (16mmol m−2) under weak winds. The high ratios of the amount of entrained NO3to river-borne NO3(12 under windy conditions and 5·6 under weak winds) indicates that wind-induced entrainment of NO3in summer is particularly important for new production. Because turbulent energy came from winds, mixing started at the surface and moved downward. Thus, phytoplankton cells remained in the surface mixed layer, and responded to the entrained nutrients and grew rapidly. Phytoplankton biomass and primary production in the water column increased at the end of the time series, compared to the beginning.

Observed directional characteristics of the wind, wind stress, and surface waves on the open ocean

Sonic anemometer data were taken during the Surface Waves Processes Program (SWAPP) in March 1990 in the North Pacific. Measurements of the wind stress vector span several strong wind events. Significant angles between the wind stress vector and the mean wind vector are seen. Simultaneous measurements of the directional wave field were made with a surface scanning Doppler sonar. The data suggest that the wind stress direction is influenced by the direction of the surface waves, especially for stronger winds. Overall, the stress vector lies between the mean wind and the mean wave directions. At the higher wind speeds (over 8.6 m/s), there is a nonzero correlation between the variations in wave directions and stress directions as well. Finally, the stress and wave component directions have similar frequency dependence over the frequency band where wave energy is nonnegligible, suggesting a dynamic link.

Interannual variability of sea‐ice cover in Hudson bay, Baffin bay and the Labrador sea

Abstract The spatial and temporal relationships between subarctic Canadian sea‐ice cover and atmospheric forcing are investigated by analysing sea‐ice concentration, sea‐level pressure and surface air temperature data from 1953 to 1988. The sea‐ice anomalies in Hudson Bay, Baffin Bay and the Labrador Sea are found to be related to the North Atlantic Oscillation (NAO) and the Southern Oscillation (SO). Through a spatial Student's i‐test and a Monte Carlo simulation, it is found that sea‐ice cover in both Hudson Bay and the Baffin Bay‐Labrador Sea region responds to a Low/Wet episode of the SO (defined as the period when the SO index becomes negative) mainly in summer. In this case, the sea‐ice cover has a large positive anomaly that starts in summer and continues through to autumn. The ice anomaly is attributed to the negative anomalies in the regional surface air temperature record during the summer and autumn when the Low/Wet episode is developing. During strong winter westerly wind events of the NAO, th...

Focusing Patterns of Transition Metals in Lake Sediments — Indicators for the Reconstruction of Deep-Water Mixing in the Past?

Transition metals in lake sediments show characteristic focusing patterns according to their mobility, which is strongly influenced by the redox conditions in the deep water. Oxygen concentrations in deep lakes depend on two factors: The mineralization of organic matter and deep-water mixing. Under conditions of constant nutrient input the wind-induced mixing during the cold season determines whether deep waters remain anoxic during longer periods of time or if the oxygen reservoir is replenished during winter. Different transition metals such as V, Cr, Mn, Fe, Co, Mo show a broad range of behaviour with respect to adsorption, complexation and solubility depending on the redox conditions. Their concentration and distribution in lake sediments offers some potential as proxy indicators for deep water mixing and the frequency and magnitude of strong wind events. Varved lake sediments covering the last 100 years offer an opportunity to test and calibrate focusing patterns of transition metals with limnological and meteorological time series.

Effect of localised nutrient enrichment on the shallow epilithic periphyton of oligotrophic Lake Taupo, New Zealand

Abstract A localised increase in nutrient concentrations in the shallow littoral zone of Lake Taupo, New Zealand, resulted from injection of secondary‐treated, nitrified sewage effluent into the groundwater close to the lake. Coincident with this enrichment, there was an increase in periphyton abundance. This increase was restricted to the zone of enrichment and a biomass of up to 1000 mg chlorophyll a m−2 was attained. There was also a change in species composition from a diatom/heterocystous cyano‐bacteria‐dominated assemblage to one dominated by two diatoms, Synedra ulna and Gomphoneis herculeana. Interpretation of the effect of nutrient enrichment on periphyton was complicated by occasional dramatic biomass declines in response to physical variables, notably strong wind events and lake level changes.