HORIZONTAL AND TEMPORAL VARIABILITY OF TRANSPORT PROCESSES IN LAKES
by
Alexander LeBaron Forrest
B.Eng. & Soc., McMaster University, 2002 B.Sc., McMaster University, 2002 M.A.Sc., University of British Columbia, 2004
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
in
The Faculty of Graduate Studies
(Civil Engineering)
THE UNIVERSITY OF BRITISH COLUMBIA
(Vancouver)
August 2011
© Alexander LeBaron Forrest, 2011
Abstract
This work examines the three dimensional nature of three important physical transport processes in lakes: (1) convection generated from a negative surface buoyancy flux; (2) transport resulting from rotational adjustment; and, (3) underflow fate during episodic wind stirring. Vertical and horizontal temperature gradients were characterized using a combination of traditional (moorings and vertical profilers) and novel techniques (an Autonomous Underwater Vehicle) at two sites; Pavilion Lake, British Columbia, Canada and Lake Thingvallavatn, Iceland. The former site is a relatively small (5 km2), temperate lake, with comparatively low snow cover that allows solar radiation to be the dominant energy flux to the system during late winter months. Analysis of water temperature distribution in surface waters during summer and winter enabled convective patterns resulting from a negative surface buoyancy flux to be inferred. In addition to previously studied physical transport phenomena, this work has revealed the existence of a cyclonic eddy under winter ice cover in Pavilion Lake, consistent with the internal Rossby radius of deformation, extending down to ~ 14 m below the ice surface and rotating with an azimuthal speed of ~ 3 cm s-1 (as predicted by equations of cyclogeostrophic flow). Horizontal temperature transects beneath the eddy revealed temperature fluctuations associated with 1 – 2 m vertical displacements in the region 5 m directly below the eddy and are thought to be an undocumented source of mass transport. The latter field site was an embayment of a larger (88 km2) subarctic lake with a groundwater inflow that propagates through the embayment as a negatively buoyant underflow. Surface wind shear events entrain the underflow into the overlying lake water. This entrainment alters the characteristics and the ultimate fate of the underflow in the lake. Calculated entrainment of the underflow and entrainment calculated from the bulk Richardson number are in close agreement. Measurements made during these studies not only elucidated details of the three dimensional nature of known transport mechanisms but also revealed previously undiscovered modes of mass transport associated with wintertime lake hydrodynamics.
ii
Preface
While all of the research presented herein represents original work on behalf of the author, several collaborators have contributed during the editorial process, which have helped guide the creation of this final product. A long list of people also participated in each of the field deployments that helped to ensure successful operations.
Chapter 2 is based on fieldwork conducted in Pavilion Lake, British Columbia, Canada in both the summer of 2006 and the winter of 2007 by Alexander Forrest and Dr. Bernard Laval. During both deployments, I was responsible for all logistics, experimental field design, data collection, and post-processing. Dr. Bernard Laval and Dr. Roger Pieters provided guidance during the drafting of the original manuscript with Dr. Darlene S.S. Lim, the other collaborator on this work, contributing to the final editorial process. A version of this chapter has been published; Forrest, A.L., Laval, B.E., Pieters, R., and Lim, D.S.S. 2008. Convectively driven transport in temperate lakes. Limnol. Oceanogr. 53(5, part 2), 2321–2332.
Chapter 3 is a return to Pavilion Lake, BC, in the winter of 2008, by Alexander Forrest and Dr. Bernard Laval. Once again, I was responsible for all aspects of the project with Dr. Bernard Laval and Dr. Roger Pieters providing guidance during the drafting of the original manuscript and Dr. Darlene Lim, the other collaborator on this work, contributing to the final editorial process. Initial results have been published as Forrest, A.L., Laval, B.E. and Pieters, R. 2009. Under-ice convection in a temperate lake. International Association of Hydraulic Engineering and Research (IAHR). Vancouver, BC, Canada. 8 pages. The more detailed analysis presented in Chapter 3 of this thesis is in the process of being submitted.
Chapter 4 represents collaboration between Alexander Forrest, Dr. Hrund Andradóttir, and Dr. Bernard Laval during the winter of 2009 at Lake Thingvallavatn, Iceland. For this project, I was responsible for all aspects of the project, except for the measurements taken with an Aquadopp ADV, which were provided by Dr. Hrund Andradóttir. Initial results have been published as Andradóttir H.Ó., Forrest A.L., and Laval B.E. 2009. Fate of groundwater inflow in Lake Thingvallavatn during early spring ice-breakup, Proceedings of the 13th International Workshop
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on Physical Processes in Natural Waters, Sept 1 – 4, Palermo, Italy. This previous work was written equally by myself and Dr. Hrund Andradóttir. I was responsible for drafting of this chapter that has been accepted for publication to the Journal of Aquatic Sciences.
In addition to the scientific advances that have been made in this work, I have been responsible for, or coauthored, several manuscripts in an effort to document the engineering lessons learnt during the course of my PhD studies:
Forrest, A.L. and B.E. Laval. (2007). Charting lacustrine environments with UBC-GAVIA. AUV Science in Extreme Environments, Scott Polar Research Institute, Cambridge, UK. 7 pages.
Forrest, A.L. and B.E. Laval. (2007). Seasonal thermal structure of Pavilion Lake. AUV Science in Extreme Environments, Scott Polar Research Institute, Cambridge, UK. 7 pages.
Forrest, A.L., H. Bohm, B.E. Laval., E. Magnusson, E., R. Yeo, and M.J. Doble. (2007). Investigation of under-ice thermal structure: Small AUV deployment in Pavilion Lake, BC, Canada. Oceans 2007 IEEE/MTS. Vancouver BC, Canada. 9 pages.
Doble, M.J., P. Wadhams, A.L. Forrest, and B.E. Laval. (2008). AUV deployment through ice: two years of Arctic experience. Cold Regions Science and Technology. 56: 90 – 97.
Forrest, A.L., B.E. Laval, M.J. Doble, E.J. Magnusson, and R. Yeo. (2008). AUV measurements of under-ice thermal structure. Oceans 2008 IEEE/MTS. Quebec City, PQ, Canada. 10 pages.
Forrest, A.L. and B.E. Laval. (2009). From oceans to lakes - applying new tools in limnology. Journal of Ocean Technology. 4(1): 36 – 45.
Crees, T., C. Kaminski, J. Ferguson, J.M. Laframboise, A.L. Forrest, J. Williams, E. MacNeil, D. Hopkins, and R. Pederson. 2010. UNCLOS under ice survey - An historic AUV deployment in the Canadian high arctic. Oceans 2010 IEEE/MTS. Seattle, WA, USA. 8 pages.
As part of my involvement with different research groups over the course of my PhD studies, I have also been involved with authoring, or coauthoring, other works:
Lim, D.S.S., B.E. Laval, G.F. Slater, D. Antoniades, A.L. Forrest, W. Pike, R. Pieters, M. Saffari, D. Reid, D. Schulze-Makuch, D. Andersen, and C.P. McKay. (2009). Limnology of Pavilion Lake, B. C., Canada - Characterization of a microbialite forming environment. Fundamental and Applied Limnology. 173(4): 329 – 351.
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Forrest, A.L., B.E. Laval, D.S.S. Lim, D.R. Williams, A.C. Trembanis, M.M. Marinova, R. Shepard, A.L. Brady, G.F. Slater, M.L. Gernhardt, and C.P. McKay. (2009). Performance evaluation of underwater platforms in the context of space exploration. Planetary and Space Science. 58(4): 706 – 716.
Lim, D.S.S., G.L. Warman, M.L. Gernhardt, C.P. McKay, T. Fong, M.M. Marinova, A.F. Davila, D. Andersen, A.L. Brady, Z. Cardman, B. Cowie, M.D. Delaney, A.G. Fairén, A.L. Forrest, J. Heaton, B.E. Laval, R. Arnold, P. Nuytten, G. Osinski, M. Reay, D. Reid, D. Schulze- Makuch, R. Shepard, G.F. Slater, and D. Williams. (2010). Scientific field training for human planetary exploration. Planetary and Space Science. 58(6): 920 – 930.
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Table of Contents
Abstract ...... ii Preface...... iii Table of Contents ...... vi List of Tables...... viii List of Figures ...... ix Acknowledgements...... x Dedication ...... xi 1 Introduction...... 1 1.1 Review of Relevant Limnology...... 3 1.1.1 Convection Driven by a Negative Buoyancy Flux...... 3 1.1.2 Transport Resulting from Rotational Adjustment...... 6 1.1.3 Negatively Buoyant Underflows...... 8 1.2 Autonomous Underwater Vehicles...... 9 1.3 Previous Work at the Study Sites...... 12 1.4 Work Objectives...... 13 2 Convectively Driven Transport in a Temperate Lake...... 14 2.1 Introduction ...... 14 2.2 Methods ...... 15 2.2.1 Site Description...... 15 2.2.2 Data Collection ...... 16 2.3 Summertime Campaign (Cooling Heat Flux)...... 18 2.3.1 Summertime Observations...... 18 2.3.2 Surface Layer Heat Budget...... 23 2.3.3 Convective Motion...... 24 2.3.4 Intrusion Propagation...... 25 2.4 Wintertime Campaign (Radiative Heat Flux)...... 27 2.4.1 Wintertime Observations...... 28 2.4.2 Surface Heat Flux ...... 33 2.4.3 Convective Motion...... 35 2.5 Conclusions...... 36 3 A Cyclonic Eddy in an Ice-Covered Lake ...... 39 3.1 Introduction ...... 39 3.2 Methods ...... 40 3.2.1 Site Description...... 40 3.2.2 Data Collection ...... 40 3.3 Observations ...... 43 3.3.1 Observed Eddy Characteristics...... 44 3.3.2 Observed Eddy Evolution...... 48
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3.4 Discussion ...... 51 3.4.1 Eddy Behavior ...... 51 3.4.2 Beneath the Eddy...... 55 3.4.3 Eddy Erosion...... 58 3.5 Summary and Conclusions ...... 59 4 Preconditioning of an Underflow During Ice-Breakup in a Subarctic Lake ...... 61 4.1 Introduction ...... 61 4.2 Methodology...... 63 4.2.1 Site Description...... 63 4.2.2 Field Measurements...... 64 4.2.3 Net Thermodynamic Flux from Bulk Aerodynamic Formulae...... 66 4.3 Results...... 67 4.3.1 Ice Cover Erosion and Break-up...... 67 4.3.2 Inflow Characterization...... 68 4.3.3 Observed Water Column Response to Dominant Wind Regimes ...... 70 4.3.4 Vertical Characterization of the Underflow...... 72 4.3.5 Horizontal Characterization of the Underflow...... 74 4.4 Discussion ...... 78 4.4.1 Weak Wind-Forcing ...... 79 4.4.2 Strong Wind-Forcing ...... 82 4.5 Conclusions...... 85 5 Conclusions ...... 87 5.1 Research Summary ...... 87 5.2 Contributions and Recommendations...... 89 5.2.1 Autonomous Underwater Vehicles ...... 89 5.2.2 Convection Associated with a Negative Buoyancy Flux...... 90 5.2.3 Motion Resulting from Rotational Adjustment...... 91 5.2.4 Negatively Buoyant Underflow Modified Through Wind-Stirring ...... 93 Bibliography ...... 95 Appendices...... 105 Appendix A: Differential Solar Heating ...... 105 Appendix B: Entrainment Prediction...... 109 Appendix C: UBC-Gavia AUV Description ...... 111 Appendix D: Vehicle Deployments...... 120
vii
List of Tables
Table 2.1: Lake and ice characteristics at three stations in the Central Basin...... 30 Table 3.1: Summary of transects and profiles collected during the field deployment...... 43
viii
List of Figures
Figure 1.1: Typical four-layer stratification associated with radiatively driven convection ...... 5 Figure 1.2: Conceptual illustration of under-ice submerged rotating eddies...... 7 Figure 1.3: Conceptual illustration of an unsteady, negatively buoyant underflow...... 9 Figure 1.4: Schematic of UBC-Gavia...... 11
Figure 2.1: Bathymetry of Pavilion Lake with location in British Columbia, Canada...... 16 Figure 2.2: Meteorological measurements during the summertime campaign...... 19 Figure 2.3: Bin-averaged vertical temperature profiles ...... 20 Figure 2.4: Horizontal temperature transects in the surface mixed layer ...... 21 Figure 2.5: Horizontal temperature transects at the depth of the thermocline...... 22 Figure 2.6: Meteorological measurements during the wintertime campaign ...... 29 Figure 2.7: Vertical temperature profiles on three separate winter days ...... 31 Figure 2.8: Horizontal temperature measurements at the depth of the surface layer ...... 32 Figure 2.9: Horizontal temperature measurements at 6.20 m depth in the convective layer...... 33 Figure 2.10: Horizontal temperature measurements at 17.00 m depth in the convective layer.... 34
Figure 3.1: Bathymetry of Pavilion Lake with location in British Columbia, Canada...... 41 Figure 3.2: Temperature collected by the AUV using a horizontal lawnmower survey ...... 45 Figure 3.3: Horizontal transects at four depths along and across the Central Basin ...... 46 Figure 3.4: Observed temperature composite from five repetitions of 0.75 m transect ...... 47 Figure 3.5: Vertical CTD profile survey collect on 22 Feb 2008 ...... 48 Figure 3.6: Temporal evolution of horizontal temperature transects in the CL...... 49 Figure 3.7: Daily solar irradiance and observed water temperature time series...... 50 Figure 3.8: Solutions to the cyclogeostrophic flow equation for a specified density field...... 53 Figure 3.9: Along basin horizontal temperature measurements with the CL and the QL ...... 56 Figure 3.10: Multiple horizontal temperature measurements collected at 14.28 m...... 57 Figure 3.11: Summary illustration of the observed and proposed processes...... 59
Figure 4.1: Location of Silfra Bay shown relative Lake Thingvallavatn, Iceland...... 65 Figure 4.2: Position of ice cover on Silfra Bay approximated from field observations ...... 68 Figure 4.3: Observed meteorological forcing and water column response ...... 69 Figure 4.4: Bin-averaged temperature profiles taken at the deep and shallow mooring lines...... 72 Figure 4.5: Temperature contours from a CTD transect at 10:00 hrs, 24 Feb 2009...... 73 Figure 4.6: Vertical temperature profiles in Silfra Bay and the main body of the lake...... 75 Figure 4.7: Horizontal temperature profiles collected at a 2 m constant depth ...... 76 Figure 4.8: Horizontal temperature profiles collected at a 2 m constant altitude...... 78 Figure 4.9: Illustration of a 2D model of an idealized underflow system ...... 80 Figure 4.10: Predicted mixing periods based on water column mixing energy...... 84
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Acknowledgements
Over the course of this work, I have encountered many wonderful people around the world who have encouraged me to achieve more than I had ever planned. First and foremost I need to thank Bernard Laval, my supervisor, for never hesitating to support me in all my endeavors. Secondly, over the course of my research, the entire crew of talented scientists and engineers who work at Pavilion Lake have become an integral part of my research no matter how far afield I go. A special recognition must be made to Darlene Lim, Chris McKay, Dale Andersen, Donnie Reid, David Williams, and Geoff Mullins who challenged me to explore in different directions. I also need to thank Val Schmidt, Nicole Raineault, and Adam Skarke, and especially their director, Art Trembanis, from the Coastal Sediments, Hydrodynamics, & Engineering Lab at the University of Delaware, who dreams as large as I do. The welcome that I received in Iceland, especially by Hrund Andradóttir and Pasquale Amendola (University of Iceland) and Einar Sæmundsen (Thingvellir National Park Service), is the only reason that the research there was possible at all. Similarly, without the efforts of Geoff Schladow, marion wittmann, and Brant Allen (Tahoe Environmental Research Center), it would not have been possible to achieve all that we did in Lake Tahoe. A special thanks also goes to Steve McPhail, Ken Collins, and Maaten Furlong, amongst many others from the Autosub group at the National Oceanographic Centre whose efforts to put AUVs under-ice never ceases to inspire. Those people who share my polar obsession including Martin Doble, (Laboratoire d’Oceanographie de Villefranche), Chris Roper (Roper Resources), and Jeremy Wilkinson (Scottish Association for Marine Science), deserve special note, as, without them, I would have never made it to such remote corners of the world. I would also like to thank the teams from International Submarine Engineering, Defence Research and Development Canada, Memorial University and Natural Resources Canada, with special notice to James Ferguson, Chris Kaminski, Tristan Crees, Gina Miller, Ron Verrall, Jeff Williams, Richard Pederson, Peter King, and Ron Lewis who helped redefine the role AUVs can play in Arctic exploration. Finally I would like to say how proud I am to be one of the ‘Gaviators’; Bernard Laval, Harry Bohm, Richard Yeo, Eggert Magnusson, Brian McFadden, Weston Pike, Val Schmidt, Nicole Rainault, Larry Kost, Claudine Fortier, Art Trembanis, and Andrew Hamilton. I can only hope to share adventures with them again in the future.
x
Dedication
for my wife and our families
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1 Introduction
Mixing and transport in lakes is primarily driven by fluxes of mass, heat and mechanical energy through the free surface, inflows, or outflows. This work characterizes the three dimensional, time evolving nature of three important physical transport processes as they occur in two different lakes: (1) convection generated from a negative surface buoyancy flux; (2) transport resulting from rotational adjustment; and, (3) mixing induced by underflow propagation.
In open-water, away from inflows, wind shear and thermodynamic fluxes at the free surface are the primary drivers of mixing (Imberger, 1998). The energy imparted by wind shear will result in top down stirring of the water column (Findikakis and Law, 1999). When wind shear is absent, surface cooling drives convection in the surface layer (Lei and Patterson, 2006). In ice-covered conditions, the water column is effectively isolated from wind shear and warming of the ice- water interface through solar volumetric heating is one of the main sources of near-surface convection (Farmer, 1975). Vertical temperature gradients in the water column result from this radiatively driven convection (Mironov et al., 2002). Horizontal temperature gradients will result from spatial variations in volumetric heating (i.e., different levels of light attenuation in a heterogeneous ice cover; Bengtsson, 1986b).
Unlike open-water conditions where wind stirring mixes horizontal temperature gradients on timescales of hours to days, under ice-cover density anomalies potentially endure for timescales of days to months (Chao and Shaw, 1998). These longer timescales allow sufficient time for density anomalies to respond to rotational adjustment without becoming mixed into the background stratification. Although eddy formation from rotational adjustment has been studied under sea-ice (Manley and Hunkins, 1985), this constitutes a previously unobserved mass transport process under lake-ice.
In both summer and winter, stratification in the vicinity of inflows will be controlled by the buoyancy flux associated with the inflow. Depending on the sign of the buoyancy flux, lake inflows initially form overflows or underflows (Carmack et al., 1979). Underflows will propagate downslope, while entraining ambient water, to the point of neutral buoyancy. At this
1 1: Introduction point, the underflow will either form an intrusion into the ambient stratification or, if denser than the ambient stratification, continue flowing downslope as controlled by the bathymetry (Wells and Wettlaufer, 2007). Surface driven wind mixing will modify underflow behavior in those regions where depths are sufficiently small that wind stirring can entrain the underflow into the overlying water.
These physical transport processes vary on spatial scales from small (1 mm) to basin-scale (> 1 km) and timescales from seconds to years. Studies of these processes generally characterize vertical variability well; however, the associated horizontal variability has been less comprehensively studied. The ability to characterize horizontal and vertical gradients to the same resolution is logistically challenging using surface-based techniques. This work focuses on integrating observations of both vertical and horizontal temperature variability to characterize the three-dimensional, time evolving nature of basin-scale lake processes. Temporal and vertical variability were measured using traditional instrumentation (moorings and profilers) and horizontal variability was measured using an Autonomous Underwater Vehicle (AUV) as a data collection platform. This work examines waters that are both open and ice-covered. An AUV allows measurements to be made in open-water conditions that would otherwise be challenging, and measurements to be made under-ice that would otherwise be impossible.
Section 1.1 of this chapter provides an overview of the physical processes that are relevant to this work. As the AUV is a relatively novel data collection platform and influences the sampling methodology, it is described in detail in Section 1.2. A review of previous studies conducted at the study sites is given in Section 1.3. The objectives of this study are presented in Section 1.4. Chapter 2 describes the investigation of convectively driven transport in Pavilion Lake, British Columbia, Canada during evening cooling in the summer of 2006 and afternoon radiative heating in the winter of 2007. Chapter 3 describes a follow-up study in the winter of 2008, which highlights the role rotational adjustment plays under ice as a cyclonic eddy is described. To our knowledge, this is the first eddy under lake-ice to be observed and characterized. Chapter 4 explores inflow propagation in Lake Thingvallavatn, Iceland in the winter of 2009; a location where incoming groundwater forms a significant underflow into the lake. Underflow response to
2 1: Introduction the two dominant wind regimes was described through a series of measurements made during the spring ice-cover break-up.
1.1 Review of Relevant Limnology
Each of the subsequent chapters of this work explores new insights that arise from combining measurements of horizontal and vertical water column variability. This combination allows the three-dimensional, time evolving nature of physical transport processes to be better characterized. This section reviews the processes studied in each of the subsequent chapters: (1) convection associated with a negative surface buoyancy flux, (2) transport resulting from rotational adjustment, and (3) mixing induced by underflow propagation. The first and last of these processes are well-studied phenomena in lakes and observations presented in this work contribute to the knowledge of the associated three-dimensional structure. In contrast, motion associated with rotational adjustment has, to our knowledge, never been documented under lake ice.
1.1.1 Convection Driven by a Negative Buoyancy Flux
Above the temperature of maximum density, convection in freshwater lakes occurs during periods of surface heat loss (Wells and Sherman, 2001). The surface layer of lakes is defined as the portion of the water column, directly below the free surface, influenced by surface conditions. This layer is generally separated from underlying, hypolimnetic waters by a region of increasing density referred to as the metalimnion. Many have described this thermal structure in detail (Imberger, 1985; Monismith et al., 1990; Saggio and Imberger, 2001). During periods of surface wind shear away from inflows, surface wind stress is the dominant mechanism by which momentum and turbulent kinetic energy is provided to the system (Imberger, 1985). During periods where wind shear is absent, surface buoyancy flux controls mixing of the surface layer (Lei and Patterson, 2002).
The dynamics of the surface layer have been extensively studied in both the field (Imberger and Parker, 1985; Imberger and Hamblin, 1982; Monismith, 1985) and modeled using numerical techniques (Horsch and Stefan, 1988; Horsch et al., 1994; Lei and Patterson, 2006). Convection
3 1: Introduction resulting from both a uniform and differential surface heat flux has been studied extensively (e.g. Monismith et al., 1990) and has shown motion to be associated with gravitational instabilities in the form of convective plumes and density currents (Horsch and Stefan, 1988). Work in this field has modeled the various characteristics of these instabilities (e.g. entrainment, velocity, etc.; Wells and Wettlaufer, 2005). With the development of techniques that better resolve horizontal variability, plumes and density currents can now be characterized in an unprecedented way (Fer et al., 2002c).
Below the temperature of maximum density, the absorption of shortwave radiation directly below the ice surface creates a negative buoyancy flux. This flux results in gravitational instabilities, which drive convective motion in the water column (Mironov et al., 2002). At the onset of these instabilities, the typical inverse stratification associated with winter ice cover will initially be sharpened (Pieters and Lawrence, 2009) before forming the following four-layer structure (Figure 1.1): a stably stratified diffusive surface layer (SL) in direct contact with the underside of the ice; a well-mixed convective layer (CL) that deepens and warms with added heat input; an entrainment layer (EL) from which underlying fluid is mixed into the CL; and, a weakly stratified quiescent layer (QL) that continues to the bottom (Jonas et al., 2003).
As solar radiation penetrates the SL, gravitational instabilities will form at the top of the CL. These instabilities will descend as convective plumes with velocities of 1 – 10 mm s-1 (Mironov et al., 2002). These plumes will descend to the bottom of the CL, on a timescale of several minutes, where water from the QL will be entrained through penetrative convection. Through this process, the CL will deepen over time while concurrently warming at a rate of 0.025 – 0.25 ºC d-1 (Mironov et al, 2002; Jonas et al., 2003). When solar radiative forcing ceases at sunset, these motions will slowly come to a halt before starting up again during the next day.
Radiatively driven convection has been observed in the field and modeled in laboratory and numerical studies. Barnes and Hobbie (1960) were the first to report on radiatively driven convection as a physical transport process in ice-covered lakes. Farmer (1975) then made the first systematic field observations of this process in Babine Lake, Canada. Using a variety of field techniques, the temporal evolution of the thermal structure was described and compared
4 1: Introduction
Figure 1.1: Typical four-layer stratification (SL, CL, EL and QL) associated with radiatively driven convection (Pavilion Lake, BC, Canada on 18 Feb 2007). Bottom of profile represents lake depth. with a time-dependent mixed-layer model (Farmer, 1975). This thermal structure was further examined in the late spring of 1994 and 1995 in a series of studies in Lake Vendyurskoe and Lake Rindozero (Bengtsson and Svensson, 1996; Malm et al., 1997). Convective overturning was not observed in these studies as the water column was almost at the temperature of maximum density, and a slight conductivity stratification was sufficient to stabilize the system (Malm et al., 1997).
All of these studies used a combination of vertical profiling and moorings to measure temperature, conductivity, or velocity within the water column. The next evolution in instrumentation was the direct measurement of temperature microstructure, which led to estimates of the turbulent kinetic energy (TKE) dissipation rate (Jonas et al., 2003). The majority of under-ice studies to date have been based on vertical profiling techniques, either at a single
5 1: Introduction mooring location (Jonas et al., 2003) or a small number of locations along horizontal transects (Bengtsson, 1996).
Many laboratory studies have examined the problem of boundary-driven convection from distributed heat sources (Park and Whitehead, 1998) as would be associated with radiatively driven convection; few studies have described systems at or below the temperature of maximum density (Myrup et al., 1970; Ivey and Hamblin, 1989). A number of numerical studies have modeled radiatively driven convection using the mixed layer model of Farmer (1975) or some variation thereof (Patterson and Hamblin, 1988; Bengtsson, 1996). Most recently, a large-eddy simulation model was applied to simulate radiatively driven convection in the CL. This model was then coupled to a simple mixed layer model to predict layer growth (Mironov et al., 2002).
1.1.2 Transport Resulting from Rotational Adjustment
In oceans, density anomalies associated with salinity gradients may give rise to eddy formation through rotational adjustment. Under sea-ice, Chao and Shaw (1996) suggested eddies form from density anomalies resulting from shallow brine or freshening sources beneath the ice surface. These sources generally result from differential heat flux at the ice / water interface. Differential surface heating or cooling will create localized areas of ice melt (freshwater generation) or ice formation (brine generation; Smith et al., 2002). Under certain conditions, a shallow axisymmetric brine source produces a shallow cyclone and underlying anticyclone pairing. With an ice cover, the shear between the water and the ice damps the top eddy while leaving the submerged one intact. Conversely, a freshening source will generate an opposite rotation pairing (Chao and Shaw, 1998). Figure 1.2 provides a conceptual schematic of under-ice submerged rotating eddies.
Deviations of isohalines from the horizontal provide the driving force for eddy formation and maintenance. Since fresher waters rise and spread, rather than more saline waters sinking, Chao and Shaw (1998) found that the associated eddy pair was shallower and weaker for freshwater formation than for brine rejection sources of comparable strength. The observed horizontal length scale of these eddies have been found from Arctic field data to be equivalent to the Rossby radius (Timmermans et al., 2008).
6 1: Introduction
Figure 1.2: Conceptual illustration of submerged oceanic eddies under ice in the absence of currents. The dashed contours represent the region underneath the ice cover where sources of fresher or more saline waters are being applied and dashed-dot lines represent isohalines.
Since, even in open water, field studies of rotational phenomena tend to be quite complex (Kirillin et al., 2008), work to date has generally been in a laboratory. For example, Fernando et al. (1991) studied the effects of rotation and convective turbulence using a uniformly distributed heat source along the bottom of a rotating table in homogenous fluid. It was several years later that the compounding effects of stratification (Ivey et al., 1995; Levy and Fernando, 2002) and differential heating (Condie and Griffiths, 1989; Park and Whitehead, 1998) were investigated.
Using an evenly distributed heat source, and both homogenous and stratified fluids, Coates and Ivey (1997) examined the effects of varying rotation rates on eddy formation. The multiple eddies that formed spun with an equal distribution of cyclonic and anti-cyclonic rotation. Narimousa (1998) generated eddies using a localized circular region of destabilizing flux in a rotating tank with a localized saline water source. Similar to earlier work, the formation of multiple eddies was observed near the source of the negative buoyancy flux; however, over time, a much larger single eddy, approximately the same size as the source itself, formed and continued to spread both radially and vertically. This was proposed as one of to be one of the formation mechanisms of the deep-sea chimneys that are seen in the Arctic Ocean (Wadhams et al., 2004; Spall et al., 2008).
7 1: Introduction
1.1.3 Negatively Buoyant Underflows
Inflow propagation depends primarily on the density of the inflow relative to the density of the ambient water column. The density contrast between the two waters will result in the inflow being either positively or negatively buoyant. Negatively buoyant water will propagate until such depth, known as the intrusion depth, where the ambient water is of greater or equal density (Wells and Wettlaufer, 2007; Fer et al., 2002a). In cases where such a depth does not exist, the generated underflow will continue to a depth controlled by the bathymetry (Dallimore et al., 2001).
Figure 1.3 provides a schematic of a typical unsteady, density underflow (i.e., a leading front is illustrated) with an underflow of thickness, hu, moving at a constant velocity, u, as indicated.
Underflow thickness increases along the bed through entrainment (E = dhu/dx; as indicated by the dotted line) of overlying water into the underflow. Ellison and Turner (1959), and a number of studies since then, have shown that hu increases linearly as a function of distance downslope (Dallimore et al., 2001; Gu et al., 1996; Cenedese et al., 2003). This entrainment of overlying fluid into the underflow has a velocity, known as the entrainment velocity, w, associated with it.
Several authors have developed empirical models (Fischer et al., 1979), analytical solutions (Jirka, 2007) and numerical approximations (Jirka, 2003) to describe these flows in both summer and winter months (Fischer et al., 1979; Carmack et al., 1979). Several laboratory (Ellison and Turner, 1959; Hallworth et al., 1996) and numerical studies (Hetland, 2005; Rueda and MacIntyre, 2009) have examined the associated flow dynamics. A great deal of this research has focused on examining mixing induced by the underflow and the related entrainment.
Density underflows also arise from negatively buoyant convective plumes in the surface layer. These plumes will initially descend to the lakebed where they will form density currents that will begin propagating downslope as a density underflow (Horsch and Stefan, 1988). These underflows will undergo the same evolution as those generated from riverine inputs. If the underflow does not penetrate the thermocline and flow to depth, it will form an intrusion (Wells and Wettlaufer, 2007). This process will continue until the density contrast between the two
8 1: Introduction
Figure 1.3: Conceptual illustration of an unsteady, negatively buoyant underflow for a point source into a water body. layers is reduced to the point where intrusion does occur and is a function of the width of the basin, the density contrast between layers, and the buoyancy flux associated with the underflow.
1.2 Autonomous Underwater Vehicles
Autonomous Underwater Vehicles (AUVs) are tetherless unmanned submersibles, preprogrammed to execute a series of commands with minimal to no surface communications (i.e., limited operator input). As operations are untethered, AUVs are increasingly being used in environments that would be challenging to reach with traditional research vessels (e.g. under-ice; Hayes and Morrison, 2002). Another advantage of operating untethered is that, once below the surface, AUVs are decoupled from both surface and ship motion. Decoupling the data collection platform from surface influence can significantly improve the collected data quality.
The use of AUVs, as both scientific and survey platforms, is becoming more widespread as the technology becomes increasingly robust. Applications can be roughly divided between commercial, military, and academic sectors. The first of these, primarily the offshore oil
9 1: Introduction industry, is concerned with seabed surveys and pipeline tracking (Evans et al., 2003) whereas military involvement with AUVs has been almost entirely concentrated on port protection and mine countermeasures (Bovio et al., 2006). Academic applications tackle a diversity of issues from tracking harmful algal blooms (Robbins et al., 2006), to mapping deep sea vents (Yoerger et al., 2007), to space analogue research (Forrest et al., 2009).
From the perspective of limnology and oceanography (either biological, chemical or physical), one of the strengths of AUVs as platforms is the ability to sample horizontal variability in the water column to a vertical position tolerance not easily obtained by any other means. A combination of horizontal (AUV), vertical (traditional profilers), and temporal (traditional moorings) sampling techniques allows the three-dimensional, time-evolving nature of complex scalar fields in the water column to be characterized in a fashion that has previously not been possible. This is particularly relevant in ice-covered systems, as these are generally under-studied as a result of logistical and sampling challenges.
While several AUV-based studies have focused on ocean settings (e.g. Ramos et al., 2007; Statham et al., 2005), little application, with some notable exceptions (e.g. Laval et al., 2000a, Fong and Jones, 2006), has been seen in lakes. Although there have been a few scientific under- ice deployments (Ferguson et al., 1999; Wadhams et al., 2002; Hayes and Morrison, 2002; Brierly et al., 2003; McEwan et al., 2005; Nicholls et al., 2006) no studies were made under lake ice prior to the work presented here.
A focus of this work was to use UBC-Gavia (Figure 1.4), a Gavia-class AUV, as a data collection platform for high-resolution, horizontal temperature measurements using a SBE49 Fastcat Conductivity-Temperature-Depth (CTD) profiler. As illustrated, the SBE49 Fastcat CTD is positioned on top of the vehicle with the sampling intake aft of the nose of the AUV by 3 cm. Given the intake position, the sampled water is likely to be undisturbed during normal vehicle operation. Typical cruising speeds of the vehicle during the various campaigns varied from 1.2 – 1.6 m s-1. At the 16 Hz sampling frequency of the CTD, this works out to an 8 – 10 cm along
10 1: Introduction
SBE49 Fastcat CTD
Figure 1.4: Schematic of UBC-Gavia with on-board modules and SBE49 Fastcat CTD labeled. track resolution. In addition to the SBE49 Fastcat CTD, UBC-Gavia was also equipped with an additional suite of instrumentation described in Appendix C. Although these additional instruments were not used in this work, they were used for the additional vehicle deployments described in Appendix D.
Mission planning involves a series of depths and waypoints being programmed into the control software that the vehicle then follows. While there are several vehicle navigation modes for horizontal positioning (Appendix C), all rely on some form of dead-reckoning as GPS reception is not possible underwater. Dead-reckoning on this vehicle has a position error of 1 – 2 % by distance traveled. Surfacing approximately every 1000 m was designed into the AUV mission planning in order to reset this position error through reacquiring new GPS positions.
In the vertical, two vehicle operation modes are used in this work: constant depth and constant altitude (height above bottom). Vertical positioning in the water column is determined using real- time pressure data in constant depth mode and real-time altitude data in constant altitude mode. The first of these uses an onboard pressure sensor independent of the CTD whereas the second uses altitude estimates from the Acoustic Doppler Current Profiler (ADCP) module. CTD pressure data collected during constant depth mission is typically offset from the programmed depth set point. In post-processing, the mean and standard deviation are calculated from CTD pressure data to estimate a bound for the 95 % confidence interval. In a similar fashion, the 95% confidence interval bounding the altitude data was determined. The vehicle’s ability to maintain its vertical set point, as quantified by these 95% confidence interval bounds, was significantly better for constant depth (± 5 cm) than for constant altitude missions (± 20 cm). Temperature
11 1: Introduction measurements associated with the pressure data outside of these bounds were discarded. Scales for temperature variations associated with vertical vehicle motion within the background thermal stratification were calculated for constant depth missions and are presented in each of the following chapters.
1.3 Previous Work at the Study Sites
Two study sites are presented in this work; Pavilion Lake, British Columbia, Canada and Lake Thingvallavatn, Iceland. Pavilion Lake is located in central British Columbia in a limestone valley known as Marble Canyon. This is a 5 km2, predominantly groundwater fed, ultra- oligotrophic, dimictic lake that is part of an ongoing series of studies on growth processes of organosedimentary structures known as microbialites located at multiple depths along the lake bottom (Laval et al., 2000b; Lim et al., 2009; Brady et al., 2009). As part of these studies, significant effort has been spent characterizing seasonal variations of the physical environment (Lim et al., 2009). In recent years, this lake has also been designated a space analogue research site by the Canadian Space Agency, which has resulted in additional research questions around space exploration (Forrest et al., 2009; Lim et al., 2010). The focus of many of these questions has been the examination of physical transport problems during the summer months using multiple data collection platforms (e.g. AUV and manned submersible).
Lake Thingvallavatn, located in southwest Iceland, is one of the country's largest (83 km2) and deepest lakes, with a mean depth of 34 m and maximum depth of 114 m. Adalsteinsson et al. (1992) estimated that ~ 90% of its 100 m3 s-1 average discharge enters the lake as underwater springs, which are predominantly fed by melt water runoff from the Langjokull and Thorisjokull glaciers. This water percolates through basaltic glacial deposits and lavas before entering the northern shore of the lake through a series of underwater cracks (Adalsteinsson et al., 1992; Saemundsson, 1992). Although a major, multi-discipline study was undertaken in the late 1980s examining the chemistry, biology and physics of the lake (Adalsteinsson et al., 1992) the majority of studies since then have investigated the native arctic charr and stickleback fish stock found in the lake (Malmquist, 1992; Olafsdottir et al., 2006). Researchers have also examined the
12 1: Introduction role of phytoplankton (Jonasson, 1992) and zooplankton (Lindegaard, 1992) in biomass distribution and energy flow within the lake and how this is related to fish stock distribution.
1.4 Work Objectives
This work uses a combination of horizontal and vertical sampling techniques to explore the three dimensional nature of physical transport processes. The processes explored include: (1) convection resulting from a negative surface buoyancy flux in both summer and winter (Chapter 2); (2) transport under-ice resulting from rotational adjustment (Chapter 3); and, (3) underflow fate through periodic wind stirring (Chapter 4). While Chapters 2 and 4 examine processes that have been described in other studies, new insights are provided through the use of horizontal sampling. The presence of a submerged, cyclonic eddy discussed in Chapter 3, is, to our knowledge, the first such field observation ever made in a lake.
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2 Convectively Driven Transport in a Temperate Lake1
2.1 Introduction
Above the temperature of maximum density, convection in freshwater lakes occurs during periods of surface heat loss, which result in vertical mixing that can erode the diurnal thermocline (Imberger, 1985; Lei and Patterson, 2006). Seasonally, autumnal cooling erodes the seasonal thermocline (Fer et al., 2002b; Wells and Wettlaufer, 2007). Below the temperature of maximum density and under ice-cover, convection occurs during periods of shortwave radiative heating (Farmer, 1975; Mironov et al., 2002; Ellis et al., 1991). Volumetric heating of the water will result in the warming and deepening of the mixed layer. The rate at which this warming takes place is related to the ice cover characteristics, which modulate the amount of through-ice solar penetration (Jonas et al., 2003).
Basin-scale convection resulting from uniform heat flux has been well studied, as it is important in controlling lake stratification (Imberger, 1985; Fer et al., 2002a; Jonas et al., 2003). Studies have also examined transport resulting from differential heat flux (Fer et al., 2002b; Monismith et al., 1990). These studies have focused on the convection associated with differential heat flux with particular attention to shallow, near-shore regions of lakes. These regions have been demonstrated to have higher heating and cooling rates than adjacent deeper waters (Fer et al., 2002b). These higher rates result in the formation of both turbulent convective plumes and density currents in the near-shore region. A limitation to these studies is that conventional instrumentation is largely designed for vertical, rather than horizontal, sampling and is unable to highly resolve horizontal variability on short timescales.
1A version of Chapter 2 has been published. Forrest, A.L., B.E. Laval, R. Pieters, and D.S.S. Lim. (2008) Convectively driven transport in temperate lakes. Limnology and Oceanography, 53(5, part 2), 2321 – 2332.
14 2: Convectively Driven Transport
Horizontal variability in the water column is characterized in this study using an Autonomous Underwater Vehicle (AUV) as a data collection platform. Within the past decade, AUVs have seen increased application in physical oceanography with examples including: shallow hydrographic surveys of Narragansett Bay, Rhode Island (Levine et al., 1997); a survey of coastal fronts in Haro Strait, British Columbia (Nadis, 1997); deep water hydrographic and current measurements in the Strait of Sicily (Stansfield et al., 2001); turbulence gradient measurements (Thorpe et al., 2002); water renewal in hypoxic sea lochs (Overnell et al., 2002); and, AUV-based acoustic Doppler current profiler (ADCP) flow field measurement (Fong and Jones, 2006). To date, there are few examples of this technology being applied to limnology and none to under-ice limnology. Some examples of freshwater work include: flow field prediction in tidally forced lakes (Fong and Jones, 2006); internal waves in a small lake (Laval et al., 2000a); and, gravity currents associated with littoral cooling (Fer et al., 2002b).
This work details investigations of the horizontal temperature variability within the upper waters of a temperate lake in both summer and winter. Horizontal temperature measurements were made using UBC-Gavia, a small Gavia-class AUV. The next section describes the study site, the AUV, and additional instrumentation. Sections 2.3 and 2.4 describe mixing during the field campaigns in summer 2006 and winter 2007, respectively. Studying convection in the surface waters in both summer and winter is important as mass transport resulting from a destabilizing heat flux will control vertical stratification of the water column in temperate lakes during periods of little to no surface wind shear. This condition is sometimes present during periods of low wind in the summer and is typical with winter ice cover.
2.2 Methods
2.2.1 Site Description
Pavilion Lake is a 5 km2 temperate lake located in central British Columbia in a limestone valley known as Marble Canyon (Figure 2.1a). Pavilion Lake has three basins (North, Central, and South Basins) orientated along the longitudinal axis of the lake joined by sills 6 – 10 m deep.
15 2: Convectively Driven Transport
Figure 2.1: (a) Bathymetry of Pavilion Lake with British Columbia location inset (filled circle – location of meteorological station). Contours represent 20 m intervals with maximum depth of 61 m in the Central Basin. The area outlined with a dashed line is enlarged in (panels b and c): (b) summertime AUV transects (solid lines): transect in mixed layer (2.12 m depth) on left and transect along the thermocline (7.02 m depth) on right (filled inverted triangle – location of vertical CTD profiling); and, (c) Wintertime AUV transect (solid line) for all depths (open square – point of launch and recovery; open inverted triangle – site of vertical CTD profiling; X – locations of ice profiling Stations H1 – H3 indicated from north to south).
This lake is primarily groundwater fed with no year round surface inflows and a single regulated outflow at the top end of the North Basin. AUV-based work presented in this study was conducted in the Central Basin at the widest point of the lake. This work is part of a series of studies examining this relatively deep (maximum recorded depth of 61 m) freshwater lake, which has evidence of macro-scale growth of organosedimentary structures known as microbialites (Laval et al., 2000b).
2.2.2 Data Collection
The investigation was divided into two campaigns: summer (08 – 09 Aug 2006) and winter (21 – 22 Feb 2007). The primary focus of these campaigns was to characterize the horizontal temperature variability using UBC-Gavia. In the summer campaign, UBC-Gavia cycled twelve times along 500 m, constant-depth, transects in the surface mixed layer (2.12 m depth) and above
16 2: Convectively Driven Transport the seasonal thermocline (7.02 m depth; Figure 2.1b). In the winter campaign, 300 m transects were conducted in the stratified surface layer (0.50 m depth), and the convectively mixed layer (6.20 and 17.00 m depth; Figure 2.1c). During both campaigns, a tolerance of 10 cm (i.e., ± 5 cm away from the mean AUV depth determined in post-processing) was used; data points associated with depths exceeding this tolerance are not reported.
Horizontal temperature transects were collected with a Seabird Electronics SBE-49 Conductivity-Temperature-Depth (CTD) profiler mounted on UBC-Gavia. As configured for this deployment, the vehicle was approximately 2.4 m in length, 0.2 in diameter, and 55 kg dry weight in air. Although the maximum velocity of the vehicle is approximately 3 m s-1, a cruising speed of 1.6 m s-1 was selected for an along track resolution of ~ 10 cm. The vehicle navigated using a combined RDI Workhorse Navigator Doppler velocity log (DVL) and a Kearfott inertial navigation system (INS). As the water column depths exceeded the range of the DVL (> 30 m), periodic surface intervals (i.e., vehicle surfacing with zero thrust) were required to reset accumulated position error with new GPS fixes. As these surface intervals are not possible under ice, mission lengths were reduced from 12 km (summer) to 3 km (winter). Vertical temperature profiles were collected with a Seabird SBE19plus CTD profiler (vertical resolution ~ 20 cm). These profiles were collected continuously during the vehicle deployment at a fixed location along the mission transects (Figure 2.1b – filled inverted triangle; Figure 2.1c – open inverted triangle).
During both campaigns, meteorological data (wind speed and direction, air temperature, relative humidity, and incoming shortwave solar radiation) were measured with a Campbell Scientific CR1000 weather station. This station was positioned on the shoreline at a point near the sill separating the North and Central Basins (Figure 2.1a). Wind parameters were sampled every 10 seconds and then averaged every 15 minutes. All other parameters were averaged over 30 minute intervals. During the summer campaign, the surface water temperature (estimated using the average temperature values from CTD casts during the testing period) was used with the meteorological data to estimate the surface heat flux using bulk aerodynamic formula. In this estimation, near-neutral stability of the atmosphere above the lake surface was assumed (i.e.,
17 2: Convectively Driven Transport
za/Lo < 0.5; where za is the measurement height above the water surface and Lo is the Monin- Obukhov length in the atmospheric boundary layer). Calculation of the bulk transfer coefficients associated with the bulk aerodynamic formulae was solved iteratively to estimate the latent and sensible heat flux (Launiainen 1995; Launiainen and Cheng, 1998; Heikinheimo et al. 1999).
During the winter campaign, three ice profiles were collected on 25 Feb 2007, 3 days after the AUV sampling, at the approximate beginning (Station H1), middle (Station H2), and end (Station H3) of the AUV transect in the Central Basin (Figure 2.1c). Depths of white and black ice were recorded for each profile. No precipitation was recorded in the three-day interval between AUV and ice sampling. Samples of the white and black ice were collected in polyethylene bags, melted at room temperature, and then transferred to sample bottles. These samples were then measured for conductivity using a Guildline Portasal. Vertical temperature profiles were collected from each of these three holes.
2.3 Summertime Campaign (Cooling Heat Flux)
Over the course of the summertime campaign, a series of horizontal temperature transects were collected in the epilimnion and above the seasonal thermocline. In the epilimnion, a horizontal temperature gradient was observed to gradually weaken during evening cooling. Measurements along the thermocline suggest the propagation of an intruding density current. Detailed observations are summarized in Section 2.3.1. Section 2.3.2 presents a heat budget for the observed cooling of the surface mixed layer. The negative buoyancy flux associated with this cooling is responsible for thermal instabilities that take the form of convective plumes and density currents discussed in Section 2.3.3.
2.3.1 Summertime Observations
Figure 2.2 summarizes the observed weather conditions from 08 – 11 Aug 2006, including the AUV sampling period on 09 Aug 2006 (grayed-out). A peak wind speed of ~ 5 m s-1 was observed prior to the sampling period, while wind speed ranged from 1 – 2 m s-1 during the
18 2: Convectively Driven Transport
Figure 2.2: Meteorological measurements during the summertime campaign: (a) wind speed; (b) wind direction (0 and 360º represent north); (c) air temperature; (d) relative humidity; (e) incident shortwave radiation. Grayed period indicates time of AUV deployment on 09 Aug 2006. sampling period (Figure 2.2a). Wind direction was generally from the southeast until the sampling period when the wind was from the west (Figure 2.2b). Temperature, humidity, and shortwave radiation (Figure 2.2c – e) all show warm, dry days leading up to the sampling period and then cloudy, more humid conditions the following day.
Between 20:00 hrs and 23:30 hrs on 09 Aug 2006, 138 consecutive vertical CTD profiles were collected at a location in the approximate center of the AUV track (Figure 2.1b – filled inverted triangle). During this time, the AUV was collecting horizontal temperature transects at two constant depths (Figure 2.3 – dashed-dot lines). Figure 2.3 shows the average temperature of all casts with the two dotted lines representing the standard deviation of temperature along the profile. The seasonal thermocline depth (where ∂T ∂z is a maximum) was 8.40 m. The