“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
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:
Complex systems:
Quantitative description
Enclosed
Wide variety of mixing
Rate data
of vertical
exchange
aquatic
systems small-scale microstructure
1. Obtain
from
a
mechanisms involved in
uprising measurement system in a small stratified
1st. What
is the mechanism
How should be parameterized
reservoir.
leads toturbulence
turbulence?characteristics
from measurable
2. that
Describe
in depth quantities?
and time
during the field campaign.
3. Validate and provide applicable parameterizations of
Parameterizations:
Convection
mixing for modelling small enclosed
basins.
Buoyancy Flux: Jb0
Internal wave field
Surface/Bottom stress: u*
Wind-stress forcing
Exchange coef.: Km and K
Inflows/Outflows
Qualitatively
Quantitatively Turbulent scales and numbers…
2. Study site: Boadella reservoir
2.1. 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
Max. surface
area (km2)
3.6 km2
Maximum
depth[m]
60 m
Max.
capacity[m]
62 hm2
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
MSS profiler:
Uprising system from shoreline:
75 casts: Every 15 min (22 h)
Low speed: ~0.4 m/s
Depth: ~22 m
ADCP profiler:
Water column: 0.5 m bins
Sampling rate: 0.03 Hz (5.5 h)
3. Results
3.1.Water column structure: MSS
Stratified
System
Convection
Restratification
Epilimnion
(SL): of
Internal source
Mixed
Layer
mixing:
River interflow
Metalimnion(TH):
Strongly
Seiche
field
External
sources of
stratified
mixing:
Hipolimnion(HL):
Convection
Weakly
stratified
ppb
ºC
Internal
seiche
Wind-stress
field
2 (s-2(ppm
(turb.
))
log10 (N
))
vs.vs.
depth
z (m)
depth
z (m)
Temperature (ºC) vs. depth
4:00
18:00 20:00 22:00 0:00 2:00
6:00 8:00 10:00 12:00 14:00
4:00
18:00 20:00 22:00 0:00 2:00
6:00 8:00 10:00 12:00 14:00
ppm
3. Results
3.2. Velocity field reservoir: ADCP
Along-reservoir velocity u (mm/s) vs. depth
log10 (u (mm/s))
10:00
11:00
12:00
13:00
14:00
15:00
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.
3. Results
3.3. 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
3. Results
3.4. Parameterizations and scaling the TKE
dissipation rate
Parameterization K :
Dependence on Ri
p= 2/3 or 1
Lozovatsky et al. [2006]:
Asymptotes linked to turbulent scales

Ri 


K  ~ 1 
 Ri c 
p
r =1

1  Ri
 Ri






r
Ric= 0.1 - 0.05
Ri  = 0.01
K   4.3·105  1  10  Ri 
2 / 3
 1  5  Ri   4.5·106
1
r 2  0.71
Spectral analysis
K(Ri)
Weakly stratified
upper ocean layer
TSSL
K   4.5·105  1  5  Ri 
2 / 3
r 2  0.44
 1  5  Ri 
1
Lozovatsky et al. [2006]
 (W/kg)
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.
3. Results
3.4. Parameterizations and scaling the TKE
dissipation rate
TKE dissipation rate from Thorpe and patch scales:
 ~ L2T  N 3
K ~

N
2
Proportionality c ~ 0.64
[Dillon,1982]
 K  ~ L2T  N
Proportionality k~ 0.1
log10( (W/kg)) vs. depth
SDP:

hp
Lozovatsky
and
Fernando
(2002)
4

N 2 hp
Ri p 
2
LT
L

K
T

 T ~ f Ri p , Re mp 
hp
hp
KT

Re mp  
1

1
 Ri p  4  Rempc 
LT
  1 

 1.5  x  0.03 x  C  1 



hp
Remp 
 Ricp  
C=0.3 [LF-02]
C=0.45 small stratified lake
[Planella et al. 2011]
 ~ hp2  g x N 3and xRi p , Remp 
Planella Morató J.; Roget, E.; Lozovatsky, I. "Statistics of microstructure patchiness in a stratified lake" Journal of Geophysical
Research-Oceans (2011), 116, C10035.
3. Results
3.4. Parameterizations and scaling the TKE
dissipation rate
 TKE dissipation
rate in SL??
k u*3

z
  5.110 8 z 1.18 ~ z 1
r 2  0.66
u* ~ 5 10 3
Wind velocity vs. time
m
m
 u*wind ~ 3.8 10 3
s
s
Estimates drag coefficient, Cd:
from Wüest and Lorke [2003]:
v
(m/s)
Cd  0.0044 U101.15
18:00
22:00
02:00
06:00
10:00
14:00
18:00
t(h)
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
 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.
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