Time-mean Surface Research Articles

Observation-based estimates of heat and freshwater exchanges from the subtropical North Atlantic to the Arctic

Continuous measurements from the OSNAP (Overturning in the Subpolar North Atlantic Program) array yield the first estimates of trans-basin heat and salinity transports in the subpolar latitudes. For the period from August 2014 to May 2018, there is a poleward heat transport of 0.50 ± 0.05 PW and a poleward salinity transport of 12.5 ± 1.0 Sv across the OSNAP section. Based on the mass and salt budget analyses, we estimate that a surface freshwater input of 0.36 ± 0.05 Sv over the broad subpolar-Arctic region is needed to balance the ocean salinity change created by the OSNAP transports. The overturning circulation is largely responsible for setting these heat and salinity transports (and the derived surface freshwater input) derived from the OSNAP array, while the gyre (isopycnal) circulation contributes to a lesser, but still significant, extent. Despite its relatively weak overturning and heat transport, the Labrador Sea is a strong contributor to salinity and freshwater changes in the subpolar region. Combined with trans-basin transport estimates at other locations, we provide new estimates for the time-mean surface heat and freshwater divergences over a wide domain of the Arctic-North Atlantic region to the north and south of the OSNAP line. Furthermore, we estimate the total heat and freshwater exchanges across the surface area of the extratropical North Atlantic between the OSNAP and the RAPID-MOCHA (RAPID Meridional Overturning Circulation and Heat-flux Array) arrays, by combining the cross-sectional transports with vertically-integrated ocean heat and salinity content. Comparisons with the air-sea heat and freshwater fluxes from atmospheric reanalysis products show an overall consistency, yet with notable differences in the magnitudes during the observation time period.

Using Drifter Velocity Measurements to Assess and Constrain Coarse-Resolution Ocean Models

AbstractProperly fitting ocean models to observations is crucial for improving model performance and understanding ocean dynamics. Near-surface velocity measurements from the Global Drifter Program (GDP) contain valuable information about upper-ocean circulation and air–sea fluxes on various space and time scales. This study explores whether GDP measurements can be used for usefully constraining the surface circulation from coarse-resolution ocean models, using global solutions produced by the consortium for Estimating the Circulation and Climate of the Ocean (ECCO) as an example. To address this problem, a careful examination of velocity data errors is required. Comparisons between an ECCO model simulation, performed without any data constraints, and GDP and Ocean Surface Current Analyses Real-Time (OSCAR) velocity data, over the period 1992–2017, reveal considerable differences in magnitude and pattern. These comparisons are used to estimate GDP data errors in the context of the time-mean and time-variable surface circulations. Both instrumental errors and errors associated with limitations in model physics and resolution (representation errors) are considered. Given the estimated model–data differences, errors, and signal-to-noise ratios, our results indicate that constraining ocean-state estimates to GDP can have a substantial impact on the ECCO large-scale time-mean surface circulation over extensive areas. Impact of GDP data constraints on the ECCO time-variable circulation would be weaker and mainly limited to low latitudes. Representation errors contribute substantially to degrading the data impacts.

Processes Shaping the Frontal-Scale Time-Mean Surface Wind Convergence Patterns around the Gulf Stream and Agulhas Return Current in Winter

ABSTRACTHigh-resolution satellite observations and numerical experiments have revealed local enhancement of time-mean surface wind convergence along the axes of warm western boundary currents and divergence slightly poleward. A recent study has suggested that frequent occurrence of persistent atmospheric fronts and sea level pressure (SLP) troughs along a sea surface temperature (SST) front are responsible for shaping the frontal-scale wind convergence and divergence contrast as seen in the wintertime climatology near the Kuroshio Extension (KE). These events tend to induce surface wind convergence with moderate magnitude. Through atmospheric reanalysis with high-resolution SST, the present study reveals that, as in the vicinity of the KE, surface wind convergence with moderate magnitude and divergence with moderate-to-extreme magnitude are found to play a primary role in shaping the climatological-mean wind convergence–divergence contrasts across the SST fronts near the Gulf Stream (GS) and Agulhas Return Current (ARC) in winter. In contrast, strong-to-extreme convergence events associated with synoptic-scale atmospheric disturbances are found to yield horizontally uniform time-mean wind convergence. Furthermore, cluster analysis and case studies suggest that persistent atmospheric fronts and SLP troughs are responsible for inducing moderate wind convergence also near the GS and ARC. Thus, these features are consistent with their counterpart near the KE, but the impacts of the ARC tend to be substantially weaker, probably due to its cooler SST among other potential factors.

The Time-Scale-Dependent Response of the Wintertime North Atlantic to Increased Ocean Model Resolution in a Coupled Forecast Model

AbstractThis study uses initialized forecasts and climate integrations to evaluate the wintertime North Atlantic response to an increase of ocean model resolution from ~100 km [low-resolution ocean (LRO)] to ~25 km [high-resolution ocean (HRO)] in the European Centre for Medium-Range Weather Forecasts Integrated Forecasting System (ECMWF-IFS). Importantly, the simulated impacts are time-scale dependent such that impacts in subseasonal and seasonal forecasts cannot be extrapolated to climate time scales. In general, mean biases are reduced in HRO relative to LRO configurations and the impact is increased at longer lead times. At subseasonal to seasonal lead times, surface heating anomalies over the Gulf Stream are associated with local increases to the poleward heat flux associated with transient atmospheric eddies. In contrast, surface heating anomalies in climate experiments are balanced by changes to the time-mean surface winds that resemble the steady response under linear dynamics. Some aspects of air–sea interaction exhibit a clear improvement with increased resolution at all lead times. However, it is difficult to identify the impact of increased ocean eddy activity in the variability of the overlying atmosphere. In particular, atmospheric blocking and the intensity of the storm track respond more strongly to mean biases and thus have a larger response at longer lead times. Finally, increased ocean resolution drives improvements to subseasonal predictability over Europe. This increase in skill seems to be a result of improvements to the Madden–Julian oscillation and its associated teleconnections rather than changes to air–sea interaction in the North Atlantic region.

Processes Shaping the Frontal-Scale Time-Mean Surface Wind Convergence Patterns around the Kuroshio Extension in Winter

AbstractHigh-resolution satellite observations and numerical simulations have revealed that climatological-mean surface wind convergence and precipitation are enhanced locally around the midlatitude warm western boundary currents (WBCs) with divergence slightly to their poleward side. While steep sea surface temperature (SST) fronts along the WBCs have been believed to play an important role in shaping those frontal-scale atmospheric structures, the mechanisms and processes involved are still under debate. The present study explores specific daily scale atmospheric processes that are essential for shaping the frontal-scale atmospheric structure around the Kuroshio Extension (KE) in winter, taking advantage of a new product of global atmospheric reanalysis. Cluster analysis and case studies reveal that a zonally extending narrow band of surface wind convergence frequently emerges along the KE, which is typically observed under the surface northerlies after the passage of a developed synoptic-scale cyclone. Unlike its counterpart around the cyclone center and associated cold front, the surface convergence tends to be in moderate strength and more persistent, contributing dominantly to the distinct time-mean convergence/divergence contrast across the SST front. Accompanying ascent and convective precipitation, the band of convergence is a manifestation of a weak stationary atmospheric front anchored along the SST front or generation of a weak meso-α-scale cyclone. By reinforcing the ascent and convergence, latent heating through convective processes induced by surface convergence plays an important role in shaping the frontal-scale atmospheric structure around the KE.

Vorticity Balance off the South Shore of Oahu, Hawaii, Derived by High-Frequency Radio Doppler Current Observations

AbstractHigh-frequency radio Doppler surface current observations off the south shore of Oahu, Hawaii, are used to calculate the vorticity equation at a ~2-km spatial resolution in terms of a time-dependent and time-mean surface balance. First-order terms are mean advection of mean vorticity, vortex stretching, and a residual, which is treated as unquantified terms such as wind stress curl, bottom pressure torque, and noise. The most striking feature in the 2-yr time-mean vorticity balance is the anticorrelation between advection of vorticity and vortex stretching implying that potential vorticity (PV) advection is the most dominant mechanism in the area. Several terms in the depth-integrated vorticity balance were also estimated. The bottom pressure torque acts as a first-order term only in areas of shallow topography. A PV analysis resulted in the 50-m Penguin Bank steering the westward Hawaiian Lee Current.

The Gulf Stream Convergence Zone in the Time-Mean Winds

Abstract The distribution of surface divergence in the northwest Atlantic is investigated using 10 years of satellite wind observations from QuikSCAT and a 1-yr simulation from the COAMPS atmospheric model. A band of time-mean surface convergence overlies the Gulf Stream [called here the Gulf Stream convergence zone (GSCZ)] and has been attributed previously to a local boundary layer response to Gulf Stream SST gradients. However, this analysis shows that the GSCZ results mainly from the aggregate impacts of strong convergence anomalies associated with storms propagating along the storm track, which approximately overlies the Gulf Stream. Storm surface convergence anomalies are one to two orders of magnitude greater than the time-mean convergence and produce a highly asymmetric divergence distribution skewed toward convergent winds. The sensitivity of the sign and magnitude of the time-mean divergence to extreme weather events is demonstrated through analysis using an extreme-value filter, conditional sampling based on rain occurrence, and comparison to its median and mode. Vertical velocity and surface pressure are likewise affected by strong storms, which are characterized by upward velocity and low surface pressure. Storms are thus an important process in shaping the mean state of the atmosphere in the northwest Atlantic. These results are difficult to reconcile with the prevailing view that SST “anchors” surface convergence, upward vertical velocity, and increased rain over the Gulf Stream through a local boundary layer adjustment mechanism.

Impact of Rossby Wave Breaking on U.S. West Coast Winter Precipitation during ENSO Events

Abstract This study demonstrates that water vapor transport and precipitation are largely modulated by the intensity of the subtropical jet, transient eddies, and the location of wave breaking events during the different phases of ENSO. Clear differences are found in the potential vorticity (PV), meteorological fields, and trajectory pathways between the two different phases. Rossby wave breaking events have cyclonic and anticyclonic regimes, with associated differences in the frequency of occurrence and the dynamic response. During La Niña, there is a relatively weak subtropical jet allowing PV to intrude into lower latitudes over the western United States. This induces a large amount of moisture transport inland ahead of the PV intrusions, as well as northward transport to the west of a surface anticyclone. During El Niño, the subtropical jet is relatively strong and is associated with an enhanced cyclonic wave breaking. This is accompanied by a time-mean surface cyclone, which brings zonal moisture transport to the western United States. In both (El Niño and La Niña) phases, there is a high correlation (>0.3–0.7) between upper-level PV at 250 hPa and precipitation over the west coast of the United States with a time lag of 0–1 days. Vertically integrated water vapor fluxes during El Niño are up to 70 kg m−1 s−1 larger than those during La Niña along the west coast of the United States. The zonal and meridional moist static energy flux resembles wave vapor transport patterns, suggesting that they are closely controlled by the large-scale flows and location of wave breaking events during the different phase of ENSO.

A modeling study of surface rainfall processes associated with a torrential rainfall event over Hubei, China, during July 2007

The surface rainfall processes associated with the torrential rainfall event over Hubei, China, during July 2007 were investigated using a two-dimensional cloud-resolving model. The model integrated the large-scale vertical velocity and zonal wind data from National Centers for Environmental Prediction (NCEP)/Global Data Assimilation System (GDAS) for 5 days. The time and model domain mean surface rain rate was used to identify the onset, mature, and decay periods of rainfall. During the onset period, the descending motion data imposed in the lower troposphere led to a large contribution of stratiform rainfall to the model domain mean surface rainfall. The local atmospheric drying and transport of rain from convective regions mainly contributes to the stratiform rainfall. During the mature periods, the ascending motion data integrated into the model was so strong that water vapor convergence was the dominant process for both convective and stratiform rainfall. Both convective and stratiform rainfalls made important contributions to the model domain mean surface rainfall. During the decay period, descending motion data input into the model prevailed, making stratiform rainfall dominant. Stratiform rainfall was mainly caused by the water vapor convergence over raining stratiform regions.

Unsteady compressible flow simulation around low-aspect-ratio rectangular flat wings

Unsteady compressible flow solutions around a sharp-edged rectangular flat wing of an aspect ratio of 1.0 are obtained using a parallel Reynolds-averaged Navier–Stokes algorithm. The simulated cases are obtained for Mach numbers of 0.42–0.87 and Reynolds numbers of the order of 200 000–300 000 in the transitional flow regime up to an angle of attack of 15°. A Degani–Schiff modification for the Baldwin–Lomax turbulence model is used to distinguish between the boundary layers and the primary and secondary vortices over the flat plate. Unsteady vortex shedding frequencies and Strouhal number variation with the angle of attack are obtained by using the fast Fourier transform. The flow around the low-aspect-ratio rectangular flat wing is strongly dominated by the leading- and side-edge vortices and flow separations. The numerical results are compared with the prior experimental data acquired at the von Karman Institute that shows a mix of laminar/transitional flow. In the simulations, top surface pressures of the rectangular flat wing are well predicted except in the leading-edge region. Time-mean averaged surface flow topologies are also found to be in good agreement with the experiment. Complex flow structures, including the primary and secondary separation bubbles and the footprints of the primary and secondary vortex cores, are well captured through the use of the modified algebraic turbulence model.

Sea State Bias in Radar Altimetry Revisited

Sea state bias (SSB) is calculated anew for each radar altimeter. The input data are sea surface height (h) differences, separated in time by at most one repeat cycle, or the difference between measured h and a time-mean sea surface, an approach called here “differences from the mean” (DFM). In order to release data to users soon, early estimates of SSB are computed from short time series. This work has three objectives: 1) show that the DFM approach introduces spurious values in the estimated SSB, 2) quantify the SSB error caused by short time series, and 3) estimate SSB for OSTM.

Sea surface freshwater flux estimates from GECCO, HOAPS and NCEP

Surface net freshwater flux fields, estimated from the GECCO ocean state estimation effort over the 50 yr period 1951–2001, are compared to purely satellite-based HOAPS freshwater flux estimates and to the NCEP atmospheric re-analysis net surface freshwater flux fields to assess the quality of all flux products and to improve our understanding of the time-mean surface freshwater flux distribution as well as its temporal variability. Surface flux fields are adjusted by the GECCO state estimation procedure together with initial temperature and salinity conditions so that the model simulation becomes consistent with ocean observations. The entirely independent HOAPS net surface freshwater flux fields result from the difference between SSM/I based precipitation estimates and fields of evaporation resulting from a bulk aerodynamic approach using SSM/I data and the Pathfinder SST. All three products agree well on a global scale. However, overall GECCO seems to have moved away from the NCEP/NCAR first guess surface fluxes and is often closer to the HOAPS data set. This holds for the time mean as well as for the seasonal cycle.

Sea surface freshwater flux estimates from GECCO, HOAPS and NCEP

Surface net freshwater flux fields, estimated from the GECCO ocean state estimation effort over the 50 yr period 1951–2001, are compared to purely satellite-based HOAPS freshwater flux estimates and to the NCEP atmospheric re-analysis net surface freshwater flux fields to assess the quality of all flux products and to improve our understanding of the time-mean surface freshwater flux distribution as well as its temporal variability. Surface flux fields are adjusted by the GECCO state estimation procedure together with initial temperature and salinity conditions so that the model simulation becomes consistent with ocean observations. The entirely independent HOAPS net surface freshwater flux fields result from the difference between SSM/I based precipitation estimates and fields of evaporation resulting from a bulk aerodynamic approach using SSM/I data and the Pathfinder SST. All three products agree well on a global scale. However, overall GECCO seems to have moved away from the NCEP/NCAR first guess surface fluxes and is often closer to the HOAPS data set. This holds for the time mean as well as for the seasonal cycle.

Cloud microphysical and rainfall responses to zonal perturbations of sea surface temperature: A cloud-resolving modeling study

The cloud microphysical and rainfall responses to zonal perturbations of sea surface temperature (SST) are investigated by analyzing the equilibrium simulation data (from day 31–40) obtained from a series of two-dimensional cloud-resolving simulations with a zonal model domain of 768 km. Four experiments imposed by zonal SST perturbations of wavenumbers 1 (SST29Z1), 2 (SST29Z2), 4 (SST29Z4), and 8 (SST29Z8) are compared to the control experiment imposed by zonally uniform SST (SST29). The model domain mean SST is 29 °C, and the two-dimensional cloud-resolving model with a cyclic lateral boundary is also imposed by zero vertical velocity and constant zonal wind. The time and model domain mean surface rain rates in SST29Z1, SST29Z2, and SST29Z8 are about 10% larger than those in SST29, whereas the mean surface rain rates in SST29Z4 and SST29 are similar. The analysis of mean surface rainfall budgets shows that local water vapor and hydrometeor changes play important roles in determining the differences and similarities in mean surface rain rate between the perturbation experiments and the control experiment. Both convective and stratiform rain rates are larger in SST29Z1 and SST29Z2 than in SST29 due to the smaller advection of rain from convective regions into raining stratiform regions and the larger vapor condensation rates associated with the larger water vapor convergence over raining stratiform regions in SST29Z1 and SST29Z2. The convective rain rates are larger in SST29Z4 and SST29Z8 than in SST29 because of the larger condensation rates associated with the larger water vapor convergence over convective regions in SST29Z4 and SST29Z8. The stratiform rain rates in SST29Z4 and SST29Z8 are smaller than in SST29 due to the smaller vapor condensation rates and smaller collection rates of cloud water by rain over raining stratiform regions in SST29Z4 and SST29Z8.

Sensitivity of convective and stratiform rainfall to sea surface temperature

The sensitivity of surface rainfall and associated cloud microphysical processes to sea surface temperature (SST) and its diurnal variation is investigated with a series of two-dimensional cloud-resolving model experiments. Experiments with time-invariant SST of 27 °C (SST27) and 31 °C (SST31) and experiments with diurnal SST amplitudes of 1 °C (SST29D1) and 2 °C (SST29D2) and time-mean SST of 29 °C are, respectively, compared with the control experiment with time-invariant SST of 29 °C (SST29). The model with a cyclic lateral boundary is imposed by zero vertical velocity and a constant zonal wind and is integrated for 40 days to reach quasi-equilibrium states for all experiments. Although the time and model domain mean surface evaporation rate is smaller in SST27 than in SST29, the addition of mean local atmospheric drying rate in SST27 leads similar mean surface rain rates in the two experiments. The smaller surface evaporation associated with the colder SST causes the larger local atmospheric drying rate over clear sky regions and the smaller local atmospheric moistening rate in SST27 than in SST29. The larger surface evaporation over clear sky regions associated with the warmer SST causes the larger water vapor transport to convective regions that leads to the larger convective rain rate and thus the larger mean surface rain rate in SST 31 than in SST29. The mean local atmospheric drying and decrease of mean hydrometeor concentration in SST29D1 and increase of mean hydrometeor concentration in SST29 cause the larger mean surface rain rates in SST29D1 than in SST29. The slight increase of mean surface rain rate with increasing diurnal SST amplitude is simply caused by increase of mean surface evaporation rate.

Responses of tropical deep convective precipitation systems and their associated convective and stratiform regions to the large‐scale forcing

AbstractResponses of convective and stratiform rainfall to large‐scale forcing in the tropical deep convective regime are investigated by analysing data from a two‐dimensional cloud‐resolving model simulation. The model is forced by large‐scale vertical velocity, zonal wind, horizontal advection, and sea‐surface temperature observed and derived from TOGA COARE. The imposed large‐scale vertical velocity shows that the ascending motion averaged from 22–27 December 1992 is significantly stronger than that averaged from 3–8 January 1993. Thus, the periods of 22–27 December 1992 and 3–8 January 1993 are, respectively, identified as strong‐forcing (SF) and weak‐forcing (WF) phases. Although the convective rain rates are similar in the two phases, stratiform rain rate is larger in the SF phase than in the WF phase, which leads to larger time and spatial mean surface rain rate in the SF phase. Similar rates of water vapour convergence and vapour condensation and collection of cloud water by rain over water‐hydrometeor‐dominated convective regions are responsible for similar convective rain rates in the two phases. Water vapour divergence is larger in the WF phase than in the SF phase, which causes smaller vapour condensation and deposition rates in the WF phase. As a result, the collection rate of cloud water by rain and the melting rate of graupel to rain are smaller in the WF phase than in the SF phase, which accounts for the smaller stratiform rain rate in the WF phase. Copyright © 2008 Royal Meteorological Society

Flow Around a Circular Cylinder with Tangential Blowing near a Plane Boundary (2nd Report, A Study on Unsteady Characteristics)

The wake oscillation of a circular cylinder, which is placed near a rigid plane boundary, with tangential blowing from a surface slot is investigated experimentally under the condition of Re=2.7 ×104. The time-mean surface pressure measurements on the circular cylinder, flow visualizations and velocity fluctuation measurements were carried out for various momentum coefficient Cμ and clearances between the circular cylinder and the rigid plane boundary changed. Based on these experiments it is found that periodic velocity fluctuation, which is associated with vortex shedding (Karman vortices), is suppressed by the tangential blowing. However, in the case of the cylinder placed near the rigid plane boundary, the reverse flow occurs in the clearance between the cylinder and the boundary, in particular Cμ increases. The level of flow unsteadiness naturally increases. Moreover, velocity fluctuation measurements revealed that the velocity fluctuation classified into two cases, one being the velocity fluctuation occurring Periodically (H/D>0.1) and the other occurring non-periodically (H/D=0.1).

The Double-ITCZ Problem in IPCC AR4 Coupled GCMs: Ocean–Atmosphere Feedback Analysis

Abstract This study examines the double–intertropical convergence zone (ITCZ) problem in the coupled general circulation models (CGCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). The twentieth-century climate simulations of 22 IPCC AR4 CGCMs are analyzed, together with the available Atmospheric Model Intercomparison Project (AMIP) runs from 12 of them. To understand the physical mechanisms for the double-ITCZ problem, the main ocean–atmosphere feedbacks, including the zonal sea surface temperature (SST) gradient–trade wind feedback (or Bjerknes feedback), the SST–surface latent heat flux (LHF) feedback, and the SST–surface shortwave flux (SWF) feedback, are studied in detail. The results show that most of the current state-of-the-art CGCMs have some degree of the double-ITCZ problem, which is characterized by excessive precipitation over much of the Tropics (e.g., Northern Hemisphere ITCZ, South Pacific convergence zone, Maritime Continent, and equatorial Indian Ocean), and are often associated with insufficient precipitation over the equatorial Pacific. The excessive precipitation over much of the Tropics usually causes overly strong trade winds, excessive LHF, and insufficient SWF, leading to significant cold SST bias in much of the tropical oceans. Most of the models also simulate insufficient latitudinal asymmetry in precipitation and SST over the eastern Pacific and Atlantic Oceans. The AMIP runs also produce excessive precipitation over much of the Tropics, including the equatorial Pacific, which also leads to overly strong trade winds, excessive LHF, and insufficient SWF. This suggests that the excessive tropical precipitation is an intrinsic error of the atmospheric models, and that the insufficient equatorial Pacific precipitation in the coupled runs of many models comes from ocean–atmosphere feedback. Feedback analysis demonstrates that the insufficient equatorial Pacific precipitation in different models is associated with one or more of the following three biases in ocean–atmosphere feedback over the equatorial Pacific: 1) excessive Bjerknes feedback, which is caused by excessive sensitivity of precipitation to SST and overly strong time-mean surface wind speed; 2) overly positive SST–LHF feedback, which is caused by excessive sensitivity of surface air humidity to SST; and 3) insufficient SST–SWF feedback, which is caused by insufficient sensitivity of cloud amount to precipitation. Off the equator over the eastern Pacific stratus region, most of the models produce insufficient stratus–SST feedback associated with insufficient sensitivity of stratus cloud amount to SST, which may contribute to the insufficient latitudinal asymmetry of SST in their coupled runs. These results suggest that the double-ITCZ problem in CGCMs may be alleviated by reducing the excessive tropical precipitation and the above feedback-relevant errors in the atmospheric models.

Flow Around a Circular Cylinder with Tangential Blowing near a Plane Boundary (1st Report, A Study on Fluid Force)

The flow around a circular cylinder, which is placed near a rigid plane boundary, with tangential blowing is investigated experimentally under the condition of Re=2.7×104. The time-mean surface pressure measurements on the circular cylinder and flow visualizations were carried out for various momentum coefficient Cμ and clearances between the circular cylinder and the rigid plane boundary. The fluid force on the circular cylinder was calculated from these surface pressure distributions. The results of experiment show that the lift reaches the maximum value at certain Cμ and then starts decreasing, and this tendency has never been observed for the cylinder placed in the open flow field. It is considered that the phenomenon is attributed to change in the flow pattern around the circular cylinder, more specifically, due to the occurrence of reverse flow in the gap. In addition, the characteristics of the lift forces can be represented by newly defined nominal jet coefficient C'μ and nominal lift coefficient C'L.

An experimental study of the surface drift currents in a wave flume

The Lagrangian surface drift current induced by surface gravity waves in a wave flume has been investigated experimentally by the particle tracking method. It was observed that in most regions of the flume, the time-mean surface drift current was in the opposite direction to that of the wave propagation. The secondary current in the form of a pair of longitudinal vortices caused by the lateral boundaries was analyzed. It is suggested that the convection of the vorticity generated by the wave-absorber and the lateral boundaries is an important factor in the determination of the time-mean drift in a wave flume.