Jesus Planella MoratóElena Roget Armengol

and Xavier Sanchez Martinjesus.planella@udg.edu; elena.roget@udg.edu;

xavier.sanchez@udg.edu

“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. Outline1. 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. Conclusions5. Future work

1. Introduction: Mixing in enclosed basins

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 descriptionRate of vertical exchange

Parameterizations: Buoyancy Flux: Jb0

Surface/Bottom stress: u*

Exchange coef.: Km and K

How should be parameterized from measurable

quantities?

1st. What is the mechanism that leads to turbulence?

Convection

Inflows/Outflows

Internal wave field Wind-stress forcing

Turbulent scales and numbers…Qualitativel

y

Quantitatively

Small reservoir located 100 m asl in NE Catalonia (Eastern pre-Pyrenees)Narrowed system exposed to north winds/ breeze regime

2. Study site: Boadella reservoir2.1. General description

Water inputs: Two main tributaries: Muga/Arnera river

Max. surface

area (km2)3.6 km2

Maximum depth[m] 60 m

Max. capacity[m] 62 hm2

Max. dimensions

8.7 km long1 km wide

Max. length of shoreline

[km]21 km

2. Study site: Boadella reservoir2.2. Field Campaign

On 27th and 28th in March 2010

Station point:200 m from coast

MSS profilerADCP 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. Results3.1.Water column structure: MSS

StratifiedSystem

Internal seiche

field

log10 (N2 (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

log10 (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 interflowSeiche field

External sources

of mixing: ConvectionWind-stress

3. Results3.2. Velocity field reservoir: ADCP

Along-reservoir velocity u (mm/s) vs. depth

River interflow: z~3.5 - 5.5 m and u [20,70] cm/s Internal seiche field: up to ~7 cm/s

log10 (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):  27-42.

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)

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

log10((W·kg-1)) vs. depth

3. Results3.4. Parameterizations and scaling the TKE

dissipation rateParameterization K : Dependence on Ri

440

515110542

1325

.r

RiRi·.K /

710

10545110110342

61325

.r

·.RiRi·.K /

rp

c RiRi

RiRi~K

11

p= 2/3 or 1 r =1

Lozovatsky et al. [2006]:Asymptotes linked to turbulent scales

Ric= 0.1 - 0.05Ri = 0.01

Weakly stratifiedupper 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):  387-407.

Spectral analysisK(Ri)

(W/kg)

TSSL

mppp

T

Tmp

T

pp

p

T Re,Rif~hL

KRe

KhN

Ri

hL

2

42

TKE dissipation rate from Thorpe and patch scales:

log10( (W/kg)) vs. depth

C=0.3 [LF-02]C=0.45 small stratified lake [Planella et al. 2011]

32 NL~ T Proportionality c ~ 0.64 [Dillon,1982]

NL~KN

~K T 22

Proportionality k~ 0.1

Lozovatsky and

Fernando (2002)

141

1103051

mp

mpc

cp

p

p

T

ReRe

RiRi

Cx.x.hL

mppp Re,RixNxgh~ 32 and

Planella Morató J.; Roget, E.; Lozovatsky, I.  "Statistics of microstructure patchiness in a stratified lake"  Journal of Geophysical Research-Oceans  (2011), 116, C10035.

SDP:

hp

3. Results3.4. Parameterizations and scaling the TKE

dissipation rate

TKE dissipation rate in SL??

660

10152

11818

3

.r

z~z.zuk

.

*

Wind velocity vs. time

v (m/s)

t(h)

Estimates drag coefficient, Cd: from Wüest and Lorke [2003]:

1511000440 .

d U.C

sm.~u

sm~u wind

**33 1083105

22:00

02:00

06:00

10:00

14:00

18:00

18:00

3. Results3.4. 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.

Thanks for your attention