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” Environmental Physics Group, Department of Physics University of Girona (UdG), Catalonia, Spain

1. Outline 1.Introduction: Mixing in enclosed basins. 2.Study site: Boadella Reservoir  General description  Field Campaign 3.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 4.Conclusions 5.Future work

Aim of this work: 1. Obtain small-scale microstructure data from a uprising measurement system in a small stratified reservoir. 2. Describe turbulence characteristics in depth and time during the field campaign. 3. Validate and provide applicable parameterizations of mixing for modelling small enclosed basins. Enclosed aquatic systems Complex systems: Wide variety of mixing mechanisms involved in Quantitative description Rate of vertical exchange Parameterizations: Buoyancy Flux: J b 0 Surface/Bottom stress: u * Exchange coef.: K m and K  How should be parameterized from measurable quantities? 1 st. What is the mechanism that leads to turbulence? Convection Inflows/Outflows Internal wave field Wind-stress forcing Turbulent scales and numbers… QualitativelyQuantitatively

Small reservoir located  100 m asl in NE Catalonia (Eastern pre-Pyrenees) Narrowed system exposed to north winds/ breeze regime 2. Study site: Boadella reservoir 2.1. General description Water inputs: Two main tributaries: Muga/Arnera river Max. surface area (km 2 ) 3.6 km 2 Maximum depth[m]  60 m Max. capacity[m] 62 hm 2 Max. dimensions 8.7 km long 1 km wide Max. length of shoreline [km] 21 km

2. Study site: Boadella reservoir 2.2. Field Campaign On 27th and 28th in March 2010 Station point: 200 m from coast MSS profiler ADCP profiler Uprising system from shoreline: 75 casts: Every 15 min (22 h) Low speed: ~0.4 m/s Depth: ~22 m MSS profiler: Water column: 0.5 m bins Sampling rate: 0.03 Hz (5.5 h) ADCP profiler:

3. Results 3.1.Water column structure: MSS Stratified System Internal seiche field log 10 (N 2 (s -2 )) vs. depth z (m) 18:00 20:00 22:00 0:00 2:00 4:00 6:00 8:00 10:00 12:00 14:00 Temperature (ºC) vs. depth 18:00 20:00 22:00 0:00 2:00 4:00 6:00 8:00 10:00 12:00 14:00 log 10 (turb. (ppm )) vs. depth z (m) Epilimnion (SL): Mixed Layer Convection Restratification Metalimnion(TH): Strongly stratified Hipolimnion(HL): Weakly stratified ºC ppm ppb Internal source of mixing: River interflow Seiche field External sources of mixing: Convection Wind-stress

3. Results 3.2. Velocity field reservoir: ADCP Along-reservoir velocity u (mm/s) vs. depth River interflow: z~ m and u  [20,70] cm/s Internal seiche field: up to ~7 cm/s log 10 (u (mm/s)) 10:00 11:00 12:00 13:00 14:00 15:00 Internal sources of mixing: 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):

3. Results 3.3. Turbulent Stratified Sub-Layer River interflow:  High velocities ( v>20 cm/s; Sh 2 >0.01 s -2 )  High stratification ( 5·10 -4 s -2 <N 2 <5·10 -3 s -2 )  Low Richardson numbers ( Ri<0.07 )  High TKE dis. rates (  >2.75·10 -6 W·kg -1 ) 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 Good sub-layer to validate parameterizations: Very low Ri (small errors) Based on patch length log 10 (  (W·kg -1 )) vs. depth

3. Results 3.4. Parameterizations and scaling the TKE dissipation rate Parameterization K  : Dependence on Ri p= 2/3 or 1r =1 Lozovatsky et al. [2006]: Asymptotes linked to turbulent scales Ri c = Ri  = 0.01 Weakly stratified upper ocean layer Lozovatsky et al. [2006] Lozovatsky, I.; Roget, E.; Fernando, H.J.S.; Figueroa, M.; Shapovalov, S. "Sheared turbulence in a weakly stratified upper ocean." Deep-Sea Research Part I-Oceanographic Research Papers (2006):  Spectral analysis K(Ri)  (W/kg) TSSL

TKE dissipation rate from Thorpe and patch scales: log 10 (  (W/kg)) vs. depth C=0.3 [LF-02] C=0.45 small stratified lake [Planella et al. 2011] Proportionality c ~ 0.64 [Dillon,1982] Proportionality k~ 0.1 Lozovatsky and Fernando (2002) and Planella Morató J.; Roget, E.; Lozovatsky, I. "Statistics of microstructure patchiness in a stratified lake" Journal of Geophysical Research-Oceans (2011), 116, C SDP:  h p 3. Results 3.4. Parameterizations and scaling the TKE dissipation rate

 TKE dissipation rate in SL?? Wind velocity vs. time v (m/s) t(h) Estimates drag coefficient, C d : from Wüest and Lorke [2003]: 22:00 02:00 06:00 10:00 14:00 18:00 18:00 3. Results 3.4. Parameterizations and scaling the TKE dissipation rate

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 thermocline at ~3.5 m depth from the surface: Parameterizations for vertical transport (diffusivities) are in good agreement with parameterizations based on Richardson number and Thorpe scales  Estimated dissipation rates from diffusivities obtained from parameterizations are in accordance to dissipation rates estimated from spectral analysis.  Internal seiche field is also observed during the whole field campaign: Obtained diffusivities fit also reasonable well to parameterizations proposed in literature.

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.

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