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GLERL 2002 Milestone Reports
GLERL 2002 Milestone Home
GOAL: SUSTAIN HEALTHY COASTS
OBJECTIVE 2: Promote Clean Coastal Waters to Sustain Living Marine Resources and to Ensure Safe Recreation, Healthy seafood, and Economic vitality
PM: Number of Coastal and Great Lake States Provided with Improved Predictive Capabilities and Understanding of Coastal Processes
Milestone: Report on estimates of the Lagrangian time and space scales associated with coastal circulation in southern Lake Michigan.
Scientist: Michael McCormick
Purpose: In the coastal regions of large lakes and oceans the horizontal gradients of dissolved chemicals and suspended materials are often far greater in the offshore than in the alongshore direction. Therefore, the mechanisms driving cross-isobath (depth) circulation play a critical role in maintaining the water quality in coastal regions. In the Laurentian Great Lakes the absence of any tidal currents and their smaller basin geometry, relative to oceanic conditions, leaves a velocity field that is dominated by wind forcing. Time variability in the surface wind stress in both magnitude and direction results in a relatively weak background circulation pattern. Under conditions like these there is a greater potential impact for storms to be a major mechanism for the offshore flux of coastal materials.
As part of a National Science Foundation and NOAA sponsored study an extensive array of fixed current meter moorings and satellite-reporting drifting buoys were used in the coastal region of southeastern Lake Michigan as part of an effort to determine the statistics associated with offshore and longshore transport. The observational program began in the fall of 1997 and ended in early summer 2000.
With recent improvements in Lagrangian positioning technology, with GPS and sophisticated microprocessor equipped drifters, they have become even more useful tools for studying coastal circulation. In this report, we describe our findings from an April 1999 Lagrangian experiment on the coastal waters of Lake Michigan.
Efforts: To estimate the Lagrangian flow statistics and their relevant Lagrangian time and spatial scales a total of 25 CODE-type drifters were deployed at several sites in Lake Michigan. The CODE-type drifters have excellent water-tracking ability with little wind induced slippage (DAVIS 1985). Clearwater Instrumentation's ClearSat GPS/ARGOS drifters were used exclusively for the Lagrangian measurements. The buoys are configured with a 1 m by 1 m cruciform drogue which renders a center-of-effort of approximately 0.8 m below the water surface. Attached to the drifter bottom was an 8 m long cable, 3 mm in diameter, terminated with a 1 kg Danforth type anchor. Midway down the cable a small float was attached to make the assembly neutrally buoyant and decouple its motion from the drifter while letting the anchor hang no more than 5 m below the drifter. The low drag profile of this assemblage was estimated to have little impact on the drifter's trajectory yet would help to save the buoy from possible destruction by anchoring it out of the surf zone. The GPS positions were fixed at 30 minute time intervals and internally stored and encoded to allow data compression for up to 17 positions to be transmitted over ARGOS and thereby enable high resolution coverage through remote access.
The drifters were deployed over the first two weeks in April 1999 at five locations. Each deployment episode consisted of one drifter at each of the five sites. This would be repeated in two days, weather permitting, until all of the drifters were deployed. This strategy would enable both large spatial coverage as well as some information on the temporal variability. The stations were located along the 20 m depth contour and the buoys were deployed by personnel from the United States Coast Guard operating from stations near Chicago, Illinois; Michigan City, Indiana; and St. Joseph, Michigan. Two of the sites were near Chicago, two were near Michigan City (Michigan City East, MCE; and Michigan City West, MCW), and one just west of St. Joseph (SJ). Figure 1. shows the study location and a composite of all available trajectories reported from 22 of the drifters.
The Lagrangian time and length scales estimate the time and distance over which the drifter's motion remains correlated. If the drifter's trajectory were able to absolutely track a given parcel of water then the drifter derived scale estimates would more accurately describe the surface flow than when water parcels are poorly tagged due to buoy slippage. Wind and wave induced slippage, in general, increases dispersion and under these conditions the calculated Lagrangian time and length scales may underestimate the true time and spatial distances over which the flow field is correlated. Nonetheless, Lagrangian statistics are consistent with similarly designed and configured drifters such that they do provide useful insights into system dynamics. All of the calculations are based upon various applications of the residual velocity fluctuations and in particular, the velocity autocorrelation function. The Lagrangian time and length scales were calculated from the seven longest in duration drifter tracks.
The data suggest that the net basin circulation appears to be as a single cyclonic gyre. Each of the drifters showed a net positive counter-clockwise longshore velocity with an overall mean longshore velocity of 3.2 cm/s. Frequent reversals in the flow field are evident from both the drifter tracks and by the large difference between the drifter's scalar speed and velocity averages. Correspondingly, each of the drifters showed a net offshore transport with an overall mean velocity of 1.3 cm/s. Over a three-week time interval the drifters traveled a total excursion distance of approximately 145 km with a net cyclonic transport of 58 km accompanied with a net offshore displacement of 24 km.
The Lagrangian integral time and length scales showed far greater variability than seen in the velocity data. Not unexpectedly the longshore flow showed much higher correlation times and longer correlation distances than observed in the offshore data. Overall mean integral longshore scales of 32.4 h and 8.1 km were calculated compared to offshore scale values of 8.1 h and 3.1 km. The statistics based upon drifters released at SJ suggest a more nearly isotropic (no preferred orientation) flow in that region while the MCW results showed a preference for higher offshore correlations in direct contrast with MCE, which showed a longshore dominance. Both MCW and MCE showed closer agreement in drifter trajectories deployed five days apart from the same site, than between the two sites for drifters deployed at the same time. Furthermore, the relatively small dispersion shown by the drifters from MCW and MCE compared to SJ suggests complex circulation resulting from the interaction between coastal wind features with basin scale dynamics and local bottom topography.
Customers: General Great Lakes science community Significance: The excellent tracking capability of these drifters resulted in Lagrangian space and time scale estimates that were up to three times larger than any other estimate ever reported in the Great Lakes. This has important implications for modeling dispersion rates in surface waters.
Success: This phase of the research was completed in 2001 and will provide better estimates of surface dispersion, which could lead to better predictive water quality models.
Next Steps: Future efforts will employ detailed descriptions of the over water wind field, as well as model simulations of the entire southern basin, in an attempt to quantify the temporal and spatial evolution of the Lagrangian velocity field.
Figure 1. Twenty-two drifter tracks from April 1999 on Southern Lake Michigan. The initial starting locations are highlighted in green and markers are displayed every 24 h along each individual trajectory.
Milestone Home Thermal Structure Monitoring Project Page
Last updated: July 19, 2002 mbl