Three Tree Point Cruise 1; Jul. 31 – Aug 8
The objective of the study is to observe the formation and dissipation of a headland eddy. Included in this is examination of the baroclinic response of the flow to the sloping headland and of the vertical structure of vorticity within an eddy. The selected site for the observations is Three Tree Point in Puget Sound, WA. Cruise 1 will take place from July 31 through Aug. 7, making use of the UW/APL R/V Miller. Personnel will include G. Pawlak, P. MacCready, K. Edwards, R. McCabe, W. Martin and E. Boget.
Our primary goals for the first cruise are to verify that flow structure (i.e., a headland eddy) exists and to characterize the velocity and density field within that structure. Secondary goals include diagnosing the variability of the flow field and determining the viability of the use of phase-averaged spatial sampling (i.e. Geyer and Signell, 1990) which is a goal for a future cruise (2002). In addition, the cruise is intended to shed light on the longevity of eddies as well as on their baroclinic structure.
The cruise plan focuses primarily on eddies formed to the south of Three Tree Point during the flood tide although some effort will be devoted to examining the north, ebb tide eddy. After leaving from UW on day 1, the Miller will be based out of Des Moines marina, located about 30 minutes south of 3Tree.
Drogued Drifter Deployments
Five Brightwaters 104A drifters will be used in cruise 1. The preliminary deployment strategy for these involves release of the drifters at the start of the flood tide, just to the north or at the tip of the headland. The first attempt will be to deploy these in a straight cross-shore line as near to simultaneously as possible. Other strategies are: incline the line into the flow, release all the drifters at one location simultaneously, release drifters at one location at a set time interval or release drifters during an established flood tide at the expected center of an eddy.
The first drifter deployment is for one half tidal cycle. Subsequent deployments will last 24 hours or more. Drifter positions will be monitored continuously (via GONIO and ARGOS) while the Miller is at the study site and these may be recovered and redeployed if need be. Locations will also be obtained daily before departing from Des Moines as well as via cell phone link while aboard the Miller.
The Miller will have a 12 ft. inflatable boat with a 15 hp outboard for use retrieving drifters that may have beached.
Drifters will be marked by marker flags, reflective tape and photoactivated strobes and will be labeled with warnings identifying them as UW property including contact information.
CTD Survey
In order to establish the background baroclinic structure, a set of CTD observations will be carried out at set stations over a single flood tide. This is scheduled for the first full cruise day to allow for debugging drifter issues since the CTD survey does not depend on drifter locations or success in identifying an eddy.
The survey will cover the entire study area, but will focus on the temporal evolution in the lee of the headland, within one tidal excursion. The survey pattern consists of three nearly (14 deg CCW) north-south lines (a,b and c, even numbers north, odd south) spaced east-west by 500 m. The spacing within each line varies so that stations are more densely spaced near the ridge. The inner stations (0, 1, 2, 3, 4) are spaced at 400 m and the outer stations are spaced at 600 m.
Brief surveys will be conducted later in the cruise at key locations to provide a baseline for the background flow and to diagnose variability of the stratification.
The CTD profiles will be conducted using a self powered, internally logging SeaCat CTD from UW Pooled Equipment. The Miller will have a hydraulic winch for raising and lowering the package. We will also borrow a downrigger set-up from Eric D’Asaro to serve as a backup.
Moored ADCP
A 300 KHz ADCP will be deployed in a gimbaled bottom mount at the start of the cruise. This will be placed at the tip of the headland at a depth of 50m. The mooring will be recovered early on the final cruise day, to allow time for any difficulties in recovery. The mooring will provide a reference tidal signal as well high resolution velocity data near the eddy generation point. The mooring location will be as near to the location of the March cruise mooring as possible, possibly at slightly greater depth: 47.451 deg Lat 122.387 deg Lon.
Eddy-following CTD surveys
Once an eddy has been identified from real-time drifter tracks obtained via GONIO, a set of CTD profiles will be conducted in the vicinity of the eddy. A set of at least three profiles will provide data on the three-dimensional structure of the eddy. Five profiles will be optimal.
Eddy-following ship-mounted ADCP surveys
Surveys with a ship-mounted 150 KHz ADCP will be carried out around eddies that can be identified from the real-time drifter tracks. These will be rapid, single bowtie pattern surveys within boxes of about 1 km. Axes should be orthogonal to the flow to simplify analysis.
Cross-channel ship-mounted ADCP surveys
Cross channel surveys with the ship-mounted 150 KHz ADCP will allow a ‘shuttered’ view of the passing current using the assumption of frozen flow. A passing eddy structure can be reconstructed then from the time series of rapid cross-channel surveys. The channel just north of 3Tree is about 4.5 km. A 4 km transect can be completed in just over 15 minutes, cruising at 8 kts.
The transects will be used with passing drifters to identify released eddy structures. The velocity data from multiple cuts then will be useful to characterize the eddy spindown process.
The cross-channel surveys will be conducted at various streamwise locations. One strategy will be to conduct transects during the ebb following a strong flood where the flood tide eddy has been seeded with drifters. Transects on the north side of the headland will aim to capture the passing flood tide eddy as it is released from the south side. In addition, the transects can be conducted while drifters are tracked to ID eddies.
Ship-mounted ADCP spatial surveys
ADCP surveys will be conducted at set locations in the double bowtie pattern planned for a Barnes cruise in 2002. This will be continued through an entire 13 hour tidal cycle to create a three-dimensional picture of the flow evolution. If we average 6 kts, a 1 km double bowtie pattern can be completed in less than one hour. In addition to providing a preliminary view of the 3D flow structure, this will allow us to assess the viability of the more complete surveys planned for 2002.
The trapezoidal survey patterns (see survey scheme) are designed to cover the complete eddy structure.
Spatial survey pattern:
Pattern will be a double bowtie starting at corner #1: a-b-c-d-a-e-c-f-a
CTD and ADCP station coordinates
|
Survey
1 lengths: leg
A: 0.985 leg
B: 0.800 leg
C: 1.612 leg
D: 0.870 leg
E: 1.400 leg
F: 0.800 total
for dbl bowtie #1: 9.065 Survey
track corners, Box 1: Corner # 1 -122.3840, 47.4505 ; 122 23.040, 47 27.030 Corner
# 2 -122.3921, 47.4435 ; 122
23.523, 47 26.612 Corner
# 3 -122.3946, 47.4505 ; 122
23.678, 47 27.030 Corner
# 4 -122.3740, 47.4466 ; 122
22.441, 47 26.794 |
Survey
2 lengths: leg
A: 1.612 leg
B: 0.800 leg
C: 2.154 leg
D: 1.000 leg
E: 2.000 leg
F: 1.400 total
for dbl bowtie # 2: 12.733 Survey
track corners, Box 2: Corner
# 1 -122.3740 47.4466 ; 122
22.441, 47 26.794 Corner
# 2 -122.3895 47.4366 ; 122
23.369, 47 26.193 Corner
# 3 -122.3921 47.4435 ; 122
23.523, 47 26.612 Corner
# 4 -122.3637 47.4409 ; 122
21.822, 47 26.454 |
