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Jesus Planella Morató Elena Roget Armengol and Xavier Sanchez Martin

“ Upraising measurements for the study of convective mixing at the upper mixed layer of a lake ”. Jesus Planella Morató Elena Roget Armengol and Xavier Sanchez Martin jesus.planella@udg.edu; elena.roget@udg.edu; xavier.sanchez@udg.edu. Environmental Physics Group, Department of Physics

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Jesus Planella Morató Elena Roget Armengol and Xavier Sanchez Martin

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  1. “Upraising measurements for the study of convective mixing at the upper mixed layer of a lake” Jesus PlanellaMorató Elena Roget Armengol and Xavier Sanchez Martin jesus.planella@udg.edu; elena.roget@udg.edu; xavier.sanchez@udg.edu Environmental Physics Group, Department of Physics University of Girona (UdG), Catalonia, Spain

  2. 1. Outline • Introduction: Mixing in enclosed basins. • Study site: BoadellaReservoir • General description •  Field Campaign • Results: •  Water column structure: Analysis of MSS profiler • data. •  Velocity field: Analysis of ADCP data. •  The Turbulent Stratified Sub-Layer: TSSL. • Parameterizations and scaling the TKE dissipation • rate • Conclusions • Future work

  3. 1. Introduction: Mixing in enclosed basins Aim of this work: Complex systems: Wide variety of mixing mechanisms involved in Quantitative description Rate of vertical exchange Enclosed aquatic systems Obtain small-scale microstructure data from a uprising measurement system in a small stratified reservoir. Describe turbulence characteristics in depth and time during the field campaign. Validate and provide applicable parameterizations of mixing for modelling small enclosed basins. 1st. What is the mechanism that leads to turbulence? How should be parameterized from measurable quantities? Parameterizations: Buoyancy Flux: Jb0 Surface/Bottom stress: u* Exchange coef.: Km and K Convection Internal wave field Wind-stress forcing Inflows/Outflows Qualitatively Quantitatively Turbulent scales and numbers…

  4. 2. Study site: Boadella reservoir2.1. General description Small reservoir located 100 m asl in NE Catalonia (Eastern pre-Pyrenees) Narrowed system exposed to north winds/ breeze regime Water inputs: Two main tributaries: Muga/Arnera river

  5. 2. Study site: Boadella reservoir2.2. Field Campaign MSS profiler: On 27th and 28th in March 2010 Uprising system from shoreline: 75 casts: Every 15 min (22 h) Low speed: ~0.4 m/s Depth: ~22 m Station point: 200 m from coast MSS profiler ADCP profiler ADCP profiler: Water column: 0.5 m bins Sampling rate: 0.03 Hz (5.5 h)

  6. 3. Results3.1.Water column structure: MSS Convection Restratification Stratified System Temperature (ºC) vs. depth Epilimnion (SL): Mixed Layer Internal source of mixing: River interflow Seiche field Metalimnion(TH): Strongly stratified External sources of mixing: Convection Wind-stress ppb ºC Hipolimnion(HL): Weakly stratified Internal seiche field log10 (turb. (ppm )) vs. depth z (m) log10 (N2 (s-2 )) vs. depth z (m) ppm 18:00 18:00 22:00 22:00 0:00 0:00 2:00 2:00 4:00 4:00 8:00 8:00 10:00 10:00 14:00 14:00 20:00 20:00 6:00 6:00 12:00 12:00

  7. 3. Results3.2. Velocity field reservoir: ADCP Along-reservoir velocity u (mm/s) vs. depth log10 (u (mm/s)) 15:00 11:00 10:00 12:00 13:00 14:00 Internal sources of mixing: River interflow: z~3.5 - 5.5 m and u [20,70] cm/s Internal seiche field: up to ~7 cm/s Forcat, F.; Roget, E.; Figueroa, M.; Sanchez, X.  "Earth rotation effects on the internal wave field in a stratified small lake: Numerical simulations."  Limnética  30 (2011):  27-42.

  8. 3. Results3.3. Turbulent Stratified Sub-Layer River interflow: • High velocities (v>20 cm/s; Sh2 >0.01 s-2) • High stratification (5·10-4 s-2 <N2 <5·10-3 s-2) • Low Richardson numbers (Ri<0.07) • High TKE dis. rates (>2.75·10-6 W·kg-1) log10((W·kg-1)) vs. depth Good sub-layer to validate parameterizations: Very low Ri (small errors) Based on patch length Well-defined sublayer: Turbulent Stratified Sub-layer (TSSL) Analogous to STZ (Str. Turb. Zone) (LF-02) TSBL (Wrinkel & Gregg 2002) Presence of SDP described in LF-02

  9. 3. Results3.4. Parameterizations and scaling the TKE dissipation rate Parameterization K : Dependence on Ri p= 2/3 or 1 r =1 Weakly stratified upper ocean layer Ric= 0.1 - 0.05 Ri  = 0.01 Lozovatsky et al. [2006]: Asymptotes linked to turbulent scales  Spectral analysis K(Ri) TSSL Lozovatsky et al. [2006]  (W/kg) Lozovatsky, I.; Roget, E.; Fernando, H.J.S.; Figueroa, M.; Shapovalov, S.  "Shearedturbulence in a weaklystratifiedupperocean."  Deep-Sea ResearchPart I-OceanographicResearchPapers   (2006):  387-407.

  10. 3. Results3.4. Parameterizations and scaling the TKE dissipation rate TKE dissipation rate from Thorpe and patch scales: log10( (W/kg)) vs. depth Proportionality c ~ 0.64 [Dillon,1982] SDP:  hp Lozovatsky and Fernando (2002) Proportionality k~ 0.1 C=0.3 [LF-02] C=0.45 small stratified lake [Planella et al. 2011] and PlanellaMorató J.; Roget, E.; Lozovatsky, I.  "Statistics of microstructurepatchiness in a stratifiedlake"  Journal of GeophysicalResearch-Oceans  (2011), 116, C10035.

  11. 3. Results3.4. Parameterizations and scaling the TKE dissipation rate  TKE dissipation rate in SL?? Wind velocity vs. time v (m/s) Estimates drag coefficient, Cd: from Wüest and Lorke [2003]: t(h) 18:00 10:00 14:00 02:00 18:00 22:00 06:00

  12. 4. Conclusions  Uprising measurements were done satisfactorily: Data were obtained up to surface. Our results correlates well with expected results in SL (LOW profile).  Uprising measurements allows to describe qualitatively the convective process in the mixed layer.  River interflow is identified in the upper part of the main thermoclineat ~3.5 m depth from the surface: Parameterizations for vertical transport (diffusivities) arein good agreement with parameterizations based on Richardson number and Thorpe scales  Internal seiche field is also observed during the whole field campaign: Obtained diffusivities fit also reasonable well to parameterizations proposed in literature.  Estimated dissipation rates from diffusivities obtained from parameterizations are in accordance to dissipation rates estimated from spectral analysis.

  13. 4. Future work  Parameterization during the night period: Test the parameterizations of convective turbulence and convective + wind-driven turbulence in the mixed layer.  Simulations: 1. How the internal seiche field interacts when convection and wind stress are present. 2. How the river interflow interacts with other turbulent mixing processes.

  14. Thanks for your attention

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