DEVELOPMENT OF A COASTAL OCEAN LAGRANGIAN (COOL) FLOAT Dave Hebert Graduate School of Oceanography University of Rhode Island Narragansett, RI 02882-1197 Tel: (401) 874-6610 Fax: (401) 874-6728 email:
[email protected] Tom Rossby Graduate School of Oceanography, University of Rhode Island Tel: (401) 874-6521 Fax: (401) 874-6728 quad email:
[email protected] Mark Prater Graduate School of Oceanography, University of Rhode Island Tel: (401) 874-6512 Fax: (401) 874-6728 email:
[email protected] Award #: N0000149610356
LONG-TERM GOALS To understand the structure and dynamics of the cross-shelf/vertical circulation under di erent forcing conditions such as Ekman upwelling-favourable winds. To determine how small-scale mixing processes a ect this circulation.
OBJECTIVES
A new oat, the COastal Ocean Lagrangian (COOL) oat, based on established RAFOS oat technology at the University of Rhode Island, is being designed and constructed in this project. This COOL oat will be designed to follow a water parcel in all three directions. The nal oat will actively change its density in order to follow the water parcel if it changes its density either through mixing or solar heating.
APPROACH
The design of a standard f/h oat (Rossby et al. 1994), which is an isopycnal RAFOS oat (Rossby et al. 1986) capable of changing its volume (and density) to several pre-determined values, was modi ed to produce the COOL oat (Figure 1). The rst modi cation to the f/h oat was the addition of a low-power compass and vanes (Figure 1). As water ows vertically past the oat, the oat will rotate and be measured by the compass. The second modi cation to the oat will be the control circuitry of the volume changer (vocha) part of the oat. The oat will change its volume until as much water ows past the oat in the opposite direction. After this volume adjustment has been made, the oat should be located in the same water parcel as it was initially placed even though the density of the water may have changed. The present design consists of sixteen predetermined volumes which are used at preprogrammed intervals. After the initial testing of the oat, the vocha will modi ed to supply an almost in nite number of volumes with a small modi cation to the vocha system.
Form Approved OMB No. 0704-0188
Report Documentation Page
Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.
1. REPORT DATE
3. DATES COVERED 2. REPORT TYPE
30 SEP 1997
00-00-1997 to 00-00-1997
4. TITLE AND SUBTITLE
5a. CONTRACT NUMBER
Development of a Coastal Ocean Lagrangian (COOL) Float
5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER
6. AUTHOR(S)
5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
8. PERFORMING ORGANIZATION REPORT NUMBER
University of Rhode Island,Graduate School of Oceanography,Narragansett,RI,02882 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)
10. SPONSOR/MONITOR’S ACRONYM(S) 11. SPONSOR/MONITOR’S REPORT NUMBER(S)
12. DISTRIBUTION/AVAILABILITY STATEMENT
Approved for public release; distribution unlimited 13. SUPPLEMENTARY NOTES 14. ABSTRACT
15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: a. REPORT
b. ABSTRACT
c. THIS PAGE
unclassified
unclassified
unclassified
17. LIMITATION OF ABSTRACT
18. NUMBER OF PAGES
Same as Report (SAR)
5
19a. NAME OF RESPONSIBLE PERSON
Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18
Figure 1. Upper left panel: Two possible orientations of the vanes on the COOL oat. Upper right panel: The compressee (cylinder with piston) and 12 kHz pinger at bottom of the oat. Lower left panel: Deployment of a quasi-isobaric (no compressee) COOL oat. Lower right panel: Deployment of the isopycnal COOL oat. The compressee is attached below the pinger and used as the drop weight.
WORK COMPLETED
Several engineering tests have been completed. Last year di erent vane designs were tested with a prototype oat (Rajamony et al. 1996). This year the COOL oat was deployed in the open ocean in a region where there should be a small diapycnal velocity but vertical velocities due to internal waves. Although a standard isopycnal RAFOS
oat appears to follow the vertical displacement of an isopycnal by internal waves, it is not clear whether the vanes will a ect the oat's motion. Also, if there is some slippage and the oat rotates, the oat will believe that there is a vertical (diapycnal) velocity and thus change its density to follow the water parcel. It is necessary to determine how the oat responds in the real ocean in order to determine the true diapycnal velocity and
how often the oat should change its density. We want to make sure that this densityadjustment process is a not a positive feedback one. Eight deployments of the COOL oat, using a variety of vane angles and with or without a compressee, have been made with total deployment periods ranging from 6 to 13 hours (Hebert et al. 1997). Six of these deployments were o the continental shelf south of Rhode Island and the other two deployments were made o of Oregon during an engineering cruise of Dr. Moum of Oregon State University. The oat records temperature and pressure every 64 s and compass heading every 8 s during a 12 hr mission (every 4 s during a 6 hr mission). This data is recorded internally and downloaded after recovery of the oat.
RESULTS In each deployment, the oat descends to the predetermined density surface. As it sinks, the oat rotates rapidly as water ows past it (Figure 2). Then, the oat remains on a density surface while being advected by the horizontal currents. This portion of the deployment will be referred to as the mission. The oat is tracked acoustically from the ship. If there is a vertical ow past the oat, the oat will rotate. To determine this vertical velocity, it is necessary to relate the speed of the oat's rotation to a vertical velocity. Thus, at the end of the oat mission, the oat makes 5 VOCHA moves which changes the oat's density and makes it move vertically. The pressure and compass data allows us to determine the rotation rate { velocity calibration for the oat. 15
50
14
100
13
150
12
200
11
250 Compass Angle (degrees)
360
DESCENT
MISSION
DOWN
UP
UP
DOWN
ASCENT
Temperature (°C)
Pressure (dbars)
COOL Float 2, Deployment 2 (Isopycnal) 0
10
Water Motion Downward Past Float
300 240
Water Motion Upward Past Float
180 120 60 0 0
2
4
6 8 Time (hours)
10
12
14
Figure 2. Pressure, temperature (top panel) and compass heading (bottom panel) recorded by the COOL oat for a deployment.
Near the end of the deployment, the oat burns a release wire, drops a weight and returns to the surface for recovery. A asher, located at the top of the oat, is activated for easy spotting at night. The oat is recovered simply using a boat hook and attaching lines to the bail (Figure 1). For this example of a COOL oat deployment (Figure 2), it is evident that the
oat was on a density surface whose properties and dynamics were changing during its mission (Figure 3). CTD casts taken near the oat show signi cant variability of temperature and salinity in this region. Post-cruise analysis of satellite images indicate that the oat was deployed near the edge of a warm-core ring. 11.20
150
11.15
160
11.10
170
11.05
180
11.00
Vertical Velocity (cm/s)
1.5
Temperature (°C)
Pressure (dbars)
COOL Float 2, Deployment 2 (Isopycnal) 140
− ∂P / ∂t ω / 15° regression ( ∂T / ∂t ) / ( ∂T / ∂P )
1 0.5 0 −0.5 −1 150
200
250
300 350 400 Time (minutes)
450
500
550
Figure 3 Top panel: Temperature and pressure recorded by the COOL oat while on an isopycnal surface. Bottom panel: Vertical velocity of the oat (,@P=@t) and vertical velocity past the oat based on temperature change [(@T=@t)=(@T=@P )] and rotation (!) of the oat. Data has been smoothed with a Butterworth lter having a half-power point at 10 min.
The COOL oat must be moving from one water mass to another during its mission (Figures 3). The oat mission can be divided into two parts. For the rst half of the mission, the oat is at a constant pressure with internal waves present but seeing a decrease in water temperature and rotating due to a vertical velocity past it (the positive w during this period). In the second half of the mission, the oat is not seeing a temperature change or rotating; however, its pressure is increasing. As expected, the vertical velocity estimated from pressure (,@P=@t) is dominated by internal waves. The high frequency oscillations in the vertical velocity based on temperature [[(@T=@t)=(@T=@P )] is due to bit noise of the temperature data, even though the data has been smoothed over 10 minutes (Figure 3). The analysis of all eight missions are presently underway. Comparisons of vertical velocities with XBT and CTD pro les ( rst six deployments) will be made
IMPACT/APPLICATIONS The production of a COOL oat will allow many investigators to study coastal ocean processes. The Graduate School of Oceanography has made their oat technology available to the oceanographic community as evident through the commercially available RAFOS oat.
REFERENCE Hebert, D., M. Prater, J. Fontaine and T. Rossby. 1997: Results from the test deployments of the COastal Ocean Lagrangian (COOL) oat, Graduate School of Oceanography Technical Report 97-2, University of Rhode Island (in prep.) Rajamony, J., S. Peterson, J. Fontaine, D. Hebert, T. Rossby and M. Prater. 1996: Vane Design for the COastal Ocean Lagrangian (COOL) Float, Graduate School of Oceanography Technical Report 96-8, University of Rhode Island, 24pp. Rossby, T., D. Dorson and J. Fontaine. 1986: The RAFOS system, J. Atmos. Ocean. Techn., 10, 609-617. Rossby, T., J. Fontaine and E.C. Carter, Jr. 1994: The f/h oat | measuring stretching vorticity directly, Deep-Sea Res., 41, 975-992. Details of all eight deployments can be found at: http://micmac.gso.uri.edu/hebert/cool oat
