Boundary Layer Off Research Articles

Dynamics of the Coastal Boundary Layer off Perth, Western Australia

Abstract Field measurements were used to investigate circulation, mixing processes, and variations in temperature and salinity in the coastal boundary layer off Perth, Western Australia. This region is characterized by a low amplitude diurnal tidal regime, a strong southwesterly summer sea breeze, and a topography dominated by submerged barrier reefs. Subinertial current dynamics were analyzed using a simplified depth-averaged alongshore momentum equation. Offshore of the reefs, the results showed a balance between wind stress, alongshore pressure gradient, and bottom friction and acceleration forces. The alongshore pressure gradient contributed to accelerating the water body, but wind was the dominant driving force for a majority of the time. Onshore of the reefs (lagoonal waters), the alongshore dynamic balance was mainly between wind stress and bottom friction. The temperature variability in Perth coastal waters was in agreement with the seasonal variations in the net air–sea heat flux. The exchange ra...

Factors influencing the mesoscale variations in marine stratocumulus albedo

Measurements of both horizontal gradients and vertical profiles of aerosols, cloud droplets and thermodynamic parameters in the cloud topped marine boundary layer off of central California are presented. They suggest that, while aerosols can indeed modulate cloud albedo, other parameters such as sea surface temperature may similarly affect cloud albedo. Additionally, the impact of aerosols, through sedimentation and precipitation, on cloud optical depths and thus albedo is not always in accord with conventional expectations and can either enhance or decrease the albedo, depending on ambient conditions. Taken together, these results suggest that current estimates of indirect forcing by aerosols could be significantly in error.

Factors influencing the mesoscale variations in marine stratocumulus albedo

Measurements of both horizontal gradients and vertical profiles of aerosols, cloud droplets and thermodynamic parameters in the cloud topped marine boundary layer off of central California are presented. They suggest that, while aerosols can indeed modulate cloud albedo, other parameters such as sea surface temperature may similarly affect cloud albedo. Additionally, the impact of aerosols, through sedimentation and precipitation, on cloud optical depths and thus albedo is not always in accord with conventional expectations and can either enhance or decrease the albedo, depending on ambient conditions. Taken together, these results suggest that current estimates of indirect forcing by aerosols could be significantly in error.

Assimilation of moored velocity data in a model of coastal wind‐driven circulation off Oregon: Multivariate capabilities

Horizontal current measurements from an array of moored acoustic Doppler profilers are assimilated sequentially into a model of coastal wind‐driven circulation off Oregon during the upwelling season of May–August 2001. Model results are compared against independent moored and ship survey data to document a positive effect of velocity data assimilation (DA) on other oceanic variables of interest such as the sea surface height (SSH), temperature, potential density, surface salinity, and near‐bottom turbulence parameters. Significant improvement is achieved for the nearshore SSH even when data are assimilated from only two moorings at an alongshore distance of 50 km from the SSH verification site. At 45°N, in an area of simple shelf bathymetry with relatively small alongshore variations, the model (even without DA) provides a good description of the isopycnal structure on a cross‐shore section. At 44.2°N, over complicated bathymetry, velocity DA may improve the slope of isopycnals but at the same time not necessarily the density values themselves. Data assimilation based on a time‐invariant representation of the forecast error covariance may inhibit spatial variability on horizontal scales smaller than the assumed forecast error decorrelation scale. An experiment involving assimilation of both velocity and moored salinity measurements demonstrates that moored velocity DA improves transport of buoyant surface water. The level of improvement in the near‐bottom turbulent dissipation and bottom stress found with the DA model indicates that it is suitable for future studies of spatial and temporal variability in the bottom boundary layer off Oregon.

Light-induced growth of phytoplankton collected during the winter from the benthic boundary layer off Oregon, USA

Despite the importance of the spring phytoplankton bloom off Oregon as a food source for zooplankton, little is known about the sources of phytoplankton seed stock for the bloom or its tim- ing. Experiments were conducted in the late winter to determine if the benthic boundary layer (BBL) could be a source of viable phytoplankton and to determine to what extent their growth was limited by light. Water collected on 3 dates from the BBL over the inner-shelf and mid-shelf in January 2003 was used for growth experiments run in shipboard incubators held at 10°C. The water was exposed to in situ nutrient concentrations and to 4 light levels and 2 daylengths (9 and 12 h), representative of winter and spring daylengths. Significant increases in chl a and particulate organic matter (POM) were observed in high light and medium light treatments at both daylengths. Chl a increased mini- mally (≤0.3 µg l -1 ) in the low light treatments, while no significant increase in POM occurred. Chl a decreased in the control (no light) treatment throughout the experiments. Blooms that developed dur- ing these incubations were dominated by Thalassiosira spp., Actinopytchus sp., Asterionellopsis glacialis and other diatoms, consistent with in situ phytoplankton community composition during the spring. Growth occurred at light levels that were generally 40 to 50% of that measured in surface waters in January, suggesting that light levels at the surface are conducive to growth, but that mixing prevents significant growth during the winter.

Large-Scale Structure of the June–July 1996 Marine Boundary Layer along California and Oregon

Abstract Data from surface stations, profilers, long-range aircraft surveys, and satellites were used to characterize the large-scale structure of the marine boundary layer off of California and Oregon during June and July 1996. To supplement these observations, June–July 1996 averages of meteorological fields from the U.S. Navy’s operational Coupled Ocean–Atmospheric Mesoscale Prediction System (COAMPS) model were generated for the region. Model calculations show a broad band of fast northerly surface winds exceeding 7 m s−1 extending along the California–Oregon coast. Buoy-measured peaks of 7.1 m s−1 off Bodega Bay, 7.2 m s−1 off Point Piedras Blancas, and 8.8 m s−1 near Point Conception were reported. Mean winds at the buoys located 15–25 km offshore are generally faster than those at coastal stations, and all station winds are faster in the afternoon. The aircraft and station observations confirm that an air temperature inversion typically marks the top of the marine boundary layer, which deepens offs...

Properties of Flow within the Coastal Boundary Layer off Long Island, New York

Properties of flow within the nearshore waters off the southern coast of Long Island are established by statistical analysis of data from a moored current meter array and by concurrent hydrographic information. Upwelling-downwelling episodes dominate subdiurnal frequency fluctuations of the flow within a coastal boundary layer of about 10 km width. The structure of the boundary layer appears to be baroclinic during the presence of a pycnocline, and is also influenced by frictional effects because of the increase of bottom depth with distance from shore. Significant mean offshore flow is evidenced by measurements taken in autumn and late summer, despite negligible mean wind stress during the same periods. This flow is most likely related to the large longshore density gradients which are consistently inferred from hydrographic data taken in the vicinity. During events of roughly constant wind heading, the cross-shore flow of a particular direction is often confined within a distinct density band. This has made it possible to infer cross-shore flow which was otherwise unresolved by the current meter array.