Hydrodynamic characteristics of Lake Victoria based on idealized 3D Lake Model Simulations
Richard O. Anyah
Center for Environmental Prediction, Department of Environmental Sciences, Rutgers University, New Brunswick, NJ 08901
Fredrick SemazziNorth Carolina State University, Department of Marine, Earth and Atmospheric
Sciences&
Department of Mathematics, North Carolina State University
Lian XieNorth Carolina State University, Department of Marine, Earth & Atmospheric
Sciences
Submitted
Journal of Geophysical ResearchOceansOctober 2006
Corresponding Author email: [email protected]
1
Abstract
Thermodynamic and hydrodynamic characteristics of Lake Victoria are investigated
based on idealized simulations using a 3Dlake model. A suite of simulations with an
elliptic(oval) geometry and prescribed wind speed (surface wind stress), lakeatmosphere
temperature difference and vertical temperature profile were performed. The time
evolutions of lake temperature as well as the currents (circulation characteristics) at
different depths and/or points are analyzed in order to understand the lake’s response to
certain aspects of surface forcing conditions. Similarities and differences between the
features simulated in a typical tropical lake (Lake Victoria) and typical midlatitude
lake(s) based on the effects of the coriolis force are also examined.
Our simulations revealed a number of unique features in the temperature.
Considered at different points on the lake surface, the temperature of both runs with or
without effect of coriolis force equilibrates after almost the same time(between 3040
days). However, there is a conspicuous difference in the vertical temperature profiles of
the two runs(cases). For example, the MIDLAT run is characterized by a ‘domeshaped’
profile in the bottom layers(40m and deeper) after 30 days of model integration, in
contrats to the VICTORIA case which is nearly isothermal over the full water column.
Perhaps one of the most significant outcome of the present study is that the twogyre
circulation pattern shown in the VICTORIA case after 30 days of model integration is
also present in the simulations with observed lake bathymetry.
2
1 Introduction Lakes are valuable natural resources that play crucial roles in shaping the
characteristics of subcontinental to continental scale water systems through modulation
of regional climates. Precisely, the importance of lakes include; (i) source of fresh water
for the neighboring human communities and industrial development, (ii) hydroelectric
power supply, (iii) irrigation, (iv) transportation, and (v) sanctuaries for a complex variety
of ecological systems. Besides, large lakes are natural museums where indicators of the
rhythms in paleoclimatic/paleoenvironmental changes/variability (Kutzbach, 1990) can
be reconstructed.
Lake Victoria in East Africa, with a total surface area of 69,000km2(IDEAL, 2003)
is the largest lake in the tropics, and second in size only to Lake Superior whose surface
area is about 82,100km2 Song et al., 2004). The Lake(Victoria) exerts significant
influence on the ambient atmosphere and surrounding regions (Sun et al., 1999;; Anyah
and Semazzi, 2004; Anyah et al., 2006) on a scale comparable to the North America
Laurentian Great Lakes(Kelly et al., 1997). However, while a number of observational
and modeling studies have investigated and demonstrated the influence of the Great
Lakes in the modification, development and intensification of atmospheric systems such
as snow storm(Schwab and O’Connor, 1994; Schwab and Bedford, 1994; Beletsky et al.,
1999, among others), not many studies of similar scope have been conducted for the
largest tropical lake, Lake Victoria.
3
Beletsky and Schwab, 2001 have shown that the influence of lakes on the
atmospheric systems on a variety of scales not only affects the regional weather and
climate variability, but also the water levels, thermal structure and lake circulations. Large
lakes are important agents that influence the overlying atmospheric circulations, while the
atmospheric forcing also affects the lake’s thermal structure and consequently the lake
circulation. The heat transfer at the lakeatmosphere interface is influenced by a number
of meteorological variables, among them air temperature, humidity, wind stress and solar
radiation. The lake, on the other hand, responds through both radiative and turbulent heat
transfers and heating/cooling. Because inland lakes have relatively large heat capacity,
they can have pronounced subseasonal/seasonal influence on regional climates,
especially due to diurnal or seasonal lags in their intrabasin heat transport compared to
the surrounding land. Large lakes also display both nearshore and offshore (midlake)
dynamical regimes typical in coastal oceans (Csanady, 1982). However, in contrast to the
relatively stable main oceanic gyres, lake currents lack persistence and depend more on
shortterm atmospheric forcing because of the comparatively small size (Beletsky et al.,
1999). Nevertheless, despite the weak currents in the lakes combined with lack of
persistence, lake circulation is quite important for ecological and water resources
management issues. Besides, large lakes are important regulators of regional climates
(Anyah and Semazzi, 2004; Schwab and Beletsky, 2001, Anyah et al., 2006).
Hydrodynamics of most large inland lakes is highly variable due to the differences in
geometry, surrounding topographies, hydrological and geochemical loadings as well as
meteorological exposures (Schwab and Bedford, 1994; Beletsky and Schwab, 2001). The
4
interplay between wind stress and heat flux in combination with Lake Bathymetry makes
circulation patterns in large lakes very complex (Beletsky et al., 1999). While current
flows in midlatitude lakes are strongly dictated by geostrophic balance, circulation
regimes in low latitude (tropical) lakes (e.g Lake Victoria) may be completely different
due to the reduced influence of the coriolis force, despite the fact that they experience
large effect (meridional variation of coriolis force). Laird et al.(2003) and Cooper etβ
al.(2000) have shown that although the strength of lake circulations may be reduced by
excluding latent heating and solar radiation processes while maintaining only the surface
wind stress forcing, the overall circulation structures are sustained. In the present study ,
we mainly investigate the effect of wind stress forcing on Lake Victoria circulation in an
idealized modeling setup. We do not include the effects of latent heating and solar
radiation processes. Success in understanding of some of the theories developed from
simple (idealized) simulations of the lake hydrodynamics can be extrapolated to
understand the results obtained from more realistic and often complex cases
(simulations).
In the present study a 3DLake model, based on the Princeton Ocean Model (POM)
is applied to simulate the hydrodynamic properties of Lake Victoria based on idealized
lake geometry(bathymetry) and wind stress forcing. Detailed discussion of the
changes/modifications made to POM to simulate freshwater Lake Victoria can be found
in Song et al.,2004 and Anyah et al.,2006). We also compare, using simplified idealized
simulations, the similarities and differences between simulated circulation patterns of a
typical tropical lake (Lake Victoria) and a typical midlatitude lake. POM model has been
5
applied to successfully study the hydrodynamics of several closed (inland) lakes (eg.,
Schwab and Bedford, 1994: Great lakes; Kuan et al., 1994: Lake Erie; O’Connor and
Schwab, 1994: Great Lakes; Beletsky et al., 1997: Lake Michigan; Zavatarelli and Mellor,
1995: Mediterranean Sea). Beletsky et al. (1996) demonstrated that in order to evaluate
the performance of POM or any three dimensional lake models in coastal environment or
large inland lakes, it is important to study model responses for the basic case of upwelling
and Kelvin wave propagation with idealized wind forcing and simple topographies. Thus,
unlike real world simulation where several factors can influence coastal/lake
hydrodynamics simultaneously, in an idealized geometry and surface forcing, it is
relatively easier to isolate the influence of topographic effects and/or wind forcing.
However, it is also important to note that verification of some of the lake circulation
theories/characteristics simulated by theoretical (numerical) models has often been
riddled with difficulties in obtaining adequate observations (Bennett, 1997). This is
particularly true for studies over Lake Victoria where no comprehensive observational
data acquisition or monitoring has been taking place
A brief description of the POM(3Dlake)model as well as the design of the idealized
experiments is presented in the next section, section 2. Results and discussions are given
in section 3, while summary and conclusions are presented in section 4.
2 Model description and experimental design
2.1 POM model
We have used POM version pom2k (Blumberg and Mellor, 1987), which is a three
dimensional, nonlinear primitive equation, finite difference ocean model. The model uses
6
a mode splitting technique to solve for the 2D barotropic mode of the free surface currents
and the 3D baroclinic mode associated with the full three dimensional temperature,
turbulence and current structure. The barotropic mode uses a shorter time step, while the
baroclinic mode uses relatively longer time step. Both modes are constrained by the CFL
computational stability criteria. The model is based on a splitexplicit Eulerian scheme in
which the internal and external modes are integrated separately to optimize computational
efficiency. The model includes a 2.5 turbulence closure submodel (Mellor and Yamada,
1974) with an implicit time scheme for vertical mixing. Detailed description of
modifications made to POM model to simulate the hydrodynamic characteristics of Lake
Victoria can be found in Song et al.,2004 and Anyah et al.,2006, while the standard POM
model description can be found in Mellor and Yamada (1987) and is also available
online at;
http://www.aos.princeton.edu/WWWPUBLIC/htdocs.pom/PubOnLine/POL.html#USER
S_GUIDE
2.2 Formulation of elliptic bathymetry/geometry for Lake Victoria
An elliptic (oval) geometry is adopted for Lake Victoria since it closely approximates
real geometry of the lake(Figure 1).The oval is rotated about 60o along the zaxis to
further mimic the true orientation of Lake Victoria as shown in figure 1. The bathymetry
is flat at the bottom with minimum and maximum depths of 10m and 80m, respectively.
Though the actual surface area of Lake Victoria is estimated at about 69,000km2(IDEAL,
2003),the elliptic lake geometry used in the present study is having an approximate
7
surface of 62800km2(i.e the long radius is about 200km and the short radius is about
100km).
3 Results and Discussions3.1 Vertical temperature profile
First, we examined the changes of lake surface temperature (LST) and the vertical
stratification by performing two parallel simulations forced with uniform wind stress.
Both runs involve a continuous integration period of 60 days with a constant (easterly)
wind stress of 104 m2s2 (~3ms1). The observed wind speed qround Lake Victoria are
shown to be generally within the range of 35ms1( Ochumba, 1996). Easterly trades, with
occasional southeast/northeast components, dominate the prevailing flow over Lake
Victoria basin. circulations.
The only difference in the two runs is the coriolis parameter, which is kept constant
at 104s1 in the first run representing a hypothetical midlatitude lake(hereafter MIDLAT),
but is set at about zero(i.e108) in the second run(hereafter VICTORIA). The primary
objective in these experiments was to examine some of the fundamental similarities and
differences in the evolution of the thermodynamic and hydrodynamic properties between
lake Victoria and a ‘typical’(hypothetical) midlatitude lake, which is strongly influenced
by coriolis force.
The initial temperature profile for our idealized simulations is shown in figure 2. This
profile was based on limited point observations over Lake Victoria archived by the
Fisheries department (e.g Ochumba, 1996) indicating that the upper 40m layer of the lake
depth is usually characterized by isothermal conditions most of the year. Hence, at the
8
beginning of our model integrations isothermal conditions are prescribed over the upper
20m layer of the lake, at the climatological LST value of 24oC. The temperature then
decreases gradually(linearly) with depth following a near logarithmic profile over the next
20m layer until it reaches 21oC at 40m depth. Thereafter the temperature remains
isothermal again (at 21oC) until the bottom of the lake.
In figure 3, the changes in the vertical profile of temperature over the central point in
the idealized oval (elliptic) lake indicate that after 5 days (figure 3a) of integration the
mixed layer at the top in the VICTORIA run has stretched down to the 30m depth, while
in the MIDLAT case, the initial temperature stratification/profile remained almost
unchanged. However, in both cases it can be seen that the initial temperature has reduced
by about 0.5oC at the top. Ten days later (figure 3b), the MIDLAT case has become
remarkably cooler within the 1050m layer(depth) by about 2oC compared to the
VICTORIA case. It is also interesting to note that after 2 months of integration, the full
80m depth of the lake in VICTORIA case is fully mixed, while in the MIDLAT case,
where the effect of coriolis force is supposed to be stronger the full water column has not
completely mixed and thus the temperature is still relatively well stratified. The possible
mechanism that inhibits complete mixing in the MIDLAT case compared to VICTORIA
case can be explained as follows.
Since both simulations are initialized with a temperaturedependent stratification, the
vertical stratification during the model integration also depends on the total energy
balance and mixing processes of the lake. The vertical temperature profile in the
MIDLAT case is also influenced by the fact the ‘ideal’ lake is not initially in geostrophic
9
balance given that we apply constant (unidirectional) surface wind (stress) forcing. The
MIDLAT simulation is subjected to considerable ‘geostrophic adjustment’ during model
integration, a process that also reorganizes the sources and sinks of the (potential and
kinetic) energy within the lake. This is not the case with VICTORIA run where the effect
of the coriolis force is negligible and thus the circulation is less(not) influenced by
geostrophy.
The second mechanism, which may also be linked to the differences in the
temperature profiles between MIDLAT and VICTORIA as shown in figure 3 relates to
the theory that when a fluid undergoes geostrophic adjustment only a fraction of the
potential energy (PE) is converted into kinetic energy (KE) of the finally geostrophically
adjusted state (see detailed explanation in Grimshaw et al., 1998). Since wind(stress) is
the major external input responsible for mixing in our experiments; wind adds KE to the
lake and thus reorganizes the existing PE, based on energy conservation principles, by
converting part of that energy to KE. Consistent with earlier postulations by e.g
Csanady(1974), wind affects the lake through the shear (traction) it imparts on the water
surface. This shear drags the water in the downwind direction, adding kinetic energy and
causing surface currents and the ‘set up’ whereby the mean lake surface downwind is
tilted upward compared to the upwind side of the lake, causing a pressure gradient on the
lake surface. This(‘set up’)results in basin wide circulation, with the bottom water return
currents compensating/replacing the surface water motion via upwelling/downwelling on
the downwind/upwind sides of the lake. As a consequence of geostrophic adjustment in
the MIDLAT case part of the PE is gradually converted into KE, thus it takes relatively
10
longer time for the water column to mix. The opposite is the cane in VICTORIA run
where the conversion of PE to KE is not constrained by geostrophic adjustment, and thus
takes place relatively faster, leading to rapid mixing.
3.2 Temperature Evolution at different points of the lake
Our idealized simulations reveal the following characteristic evolution of the lake
temperature at different points and depths. Over the northeast quadrant the surface
temperatures in both MIDLAT and VICTORIA simulations appear to reach steady state
about the same time (after 40 days), despite the differences in the value of the coriolis
parameter(large in MIDLAT). A conspicuous feature over the southeast(figure 4a) and
northeast(figure 4b) quadrants, which are on the upwind side of the elliptic lake is the
apparent upwelling of relatively colder water from the lower lake layers(i.e induced by the
impulse of the wind acting at the surface) making the initial surface temperature to cool
faster within the first two days, then begins to oscillate back and forth before reaching
equilibrium at about 22oC after about 40 days of model integration. While the sudden
temperature drop may be associated with the rapid upwelling of colder water from the
bottom layers of the lake, it could as well be the instability in the model during its spin up
phase. Over the southwest(figure 4c) and northwest(figure 4d) quadrants located
downwind of our uniform wind stress forcing, the surface temperatures remain relatively
warmer as a result of downwelling instigated by the ‘set up’ effect of the wind stress,
before equilibrating after about 30 days. However, the MIDLAT simulated temperature is
consistently warmer than the surface temperature simulated in the VICTORIA case by
about 2oC.
11
At 40m depth, the temporal evolution of temperature suggest that the lake
temperature evolution and mixing to appear to be largely influenced by the wind, with
and the effect of coriolis force being negligible. In both MIDLAT and VICTORIA runs
the temperature reaches equilibrium state at about the same time(4045 days), and at
about the same value(22oC). This is likely a manifestation of the fact that since both cases
are initialized with similar temperature stratification their total heat content is the same.
Since no heat is added into the system (lake), both surface and deeplayer temperatures
will tend to equilibrate at approximately the same value (i.e 22oC) as a consequence of
heat redistribution which in our experiments is mainly driven by the
upwelling/downwelling currents, triggered largely by the surface ‘set up’ by wind stress
forcing. However, it is interesting to note that at 40m depth, the lake temperature in the
MIDLAT, as opposed to VICTORIA run, the lake begins to warm up very fast within the
first ten days over the northeast quadrant (figure 4e). This can be attributed to geostrophic
adjustment process in MIDLAT case, which leads to entrainment of warm water from the
upper mixed layers of the lake into the layers below the thermocline, thus extending
(deepening) the mixed layer.
However, over the northwest(figure 4g) and southwest(figure 4h) quadrants, due to
upwelling, there seem to be uniform warming in both runs during the first 5 days, both
reaches equilibrium after about 40 days. Again, since the heat content of the two idealized
cases is the same the two simulations tend to equilibrate at the same temperature.
3.3 Cross section of simulated vertical temperature profile
12
Figure 5 presents a comparison of the horizontal cross sections of temperature
profiles between MIDLAT and VICTORIA simulations after 2, 15, and 30 days of
integration. After 2 days of model integration the temperature profile in the upper layers
of the lake in the MIDLAT run (figure 5a) is significantly different from the VICTORIA
case (figure 5b). Rather surprising is that after 2 days the temperature of the upper 20m
layer in the MIDLAT remained isothermal across entire lake, except over the eastern
boundary, which is upwind relative to the wind stress forcing. Conversely, temperature
within the same layer appear well mixed in the VICTORIA case, especially over the
western boundary of the lake (downwind of the surface forcing). These distinct
differences can be attributed to the following mechanism(s).
First, despite the stirring effect of wind at the surface and upwelling on the upwind
side in the MIDLAT case, it takes a longer time to mix the upper 20m layer compared to
the VICTORIA case due to geostropic adjustment as explained earlier. In the contrary(i.e
VICTORIA case), the stirring effect by the surface wind forcing and the subsequent
surface pressure gradient (‘set up’) build up accelerates the conversion of PE into KE,
thus facilitating rapid and efficient mixing of the upper layers of the water column. In
general, with constant wind stress forcing at the surface, it takes about two weeks(15
days:figure 5c) for the entire water column in the VICTORIA case to become well mixed
(isothermal). However, in the MIDLAT case, the water column is still significantly
stratified. This is still manifested even after 30 days, where the central part of the lake is
still well stratified, although the maximum water column temperature has cooled by about
2oC compared to the initial value, just like in the VICTORIA case. The other distinct
13
difference between the two runs is the presence/absence of the ‘dome’ shaped
thermocline in the MIDLAT/VICTORIA run shown in figures 5e and 5f, respectively
.The ‘dome’ shaped thermocline manifested in the MIDLAT simulations is consistent
with earlier studies(e.g Schwab et al.,1995) that have postulated that geostrophic
circulation around a ‘dome’ shaped thermocline leads to enhanced cyclonic circulations
in large and medium sized lakes during periods of stratification.
3.4 Lake currents
Figures 6ai shows surface currents (circulation patterns) in the two simulations with
idealized bathymetry as well as simulation with ‘real’ lake bathymetry after 5, 15, and 30
days of model integration. It is apparent that despite similar wind stress forcing, the
differences in the surface currents are quite significant between the MIDLAT and
VICTORIA runs after just 5 days of integration (figures 6a,b). As mentioned in the
previous sections, the circulation differences are most likely associated with
absence/presence of geostrophy in the VICTORIA/MIDLAT run. After two weeks of
model integration, there seem to be no distinct features observable in the circulation
pattern in both runs. However, a month (30days) later, very distinct features in the surface
currents are exhibited in both MIDLAT and VICTORIA simulations. For, example, a
single gyre(anticyclonic) circulation stretching across the entire lake basin is simulated
in MIDLAT(figure 6e), while in the VICTORIA twogyre(counterrotating) circulation
currents are distinctively simulated(figure 6g). Interestingly, the twogyre( counter
rotating) circulation patterns are also manifested in the simulation with real lake
bathymetry(figure 6i). It should be noted that the latter simulation was done with coarser
14
resolution(20km) as opposed to 1km resolution in the idealized simulations These unique
features characterizing the lake’s circulation patterns, especially in the VICTORIA run
suggest that the gyre circulations are not necessarily dependent on the lake size and
rotation due to coriolis force, but perhaps mainly driven by the impact of uniform surface
wind forcing. This is in fact consistent with many previous studies that have found similar
patterns in both small lakes such as Lake Biwa in Japan (Endoh et al.(1995) and large
lakes such as Lake Michigan( Schwab , 1983, 2003).
Perhaps one of the most significant outcome of the present study is that the twogyre
circulation pattern shown in the VICTORIA case after 30 days of model integration is
also present in our simulated lake currents in the reallake run(hereafter REALBATH).
This demonstrates that, to some extent, realistic lake circulation characteristics are
captured in our idealized simulations. This is probably the first study to demonstrate that
such gyre circulation patterns are present in the lowlatitude (tropical) lakes where the
effect of coriolis force is negligible; implying the circulation is likely being driven by
surface wind rather than rotational effects imposed by coriolis force. Furthermore, our
results are consistent with those of earlier studies that have shown that a horizontally
uniform wind tend to generate a twogyre(counterrotating) circulation pattern in
stratified lakes based on simple(idealized) bathymetry(e.g. Bennett,1974;Schwab and
Beletsky,1997;Ufuk,2004). The mechanism responsible for such circulation pattern(gyre)
is based on the fact that in a stratified lake uniform surface wind forcing tends to generate
stronger current in the downwelling sections(downwind) of the lake compared to the
15
upwelling sections(upwind) due to decrease (or the asymmetry) in vertical mixing and
bottom friction.
4 Summary and conclusionsThe primary objective of the present study was to investigate some of the fundamental 3D
hydrodynamic characteristics of Lake Victoria. The 3D lake model applied in the present
is based on the ‘freshwater’ version of the POM model. Two sets of simulations, one with
idealized elliptic lake geometry(bathymetry) and another based on realistic(observed)
lake bathymetry derived from digitized lake data are performed. In both cases, the lake
surface forcing is based on uniform(constant) easterly wind (stress), which is consistent
with the fact that the prevailing wind over the lake basin is dominantly easterly trades
throughout the year. The effects of short and longwave radiation are ignored.. The lake
temperature was initialized with the following profile; isothermal(24oC) within upper
20m layer, decreaseing gradually(linearly) with depth following a near logarithmic profile
within the next 20m layer until it reaches 21oC at 40m depth and thereafter the
temperature remains isothermal again (21oC) until the bottom(70m).
The second part of our study focused on the simulated differences/similarities
between ‘typical’ midlatitude lake and ‘typical’ tropical lake(Victoria), with both cases
based on the same idealized elliptic bathymetry/geometry and same depth. The
differences in the two lakes and their hydrodynamics are primarily controlled by setting
coriolis parameters. The third set of simulations were performed with the actual(real) lake
bathymetry.
16
Our simulations revealed a number of unique features in the temperature evolution
(profiles) at the surface and lower depths during the 2month integration period.
Considered at different points on the lake surface, the temperature of both MIDLAT and
VICTORIA runs equilibrates after almost the same time(between 3040 days). However,
there is a conspicuous difference in the vertical temperature profiles of the two
runs(cases). For example, the MIDLAT run is characterized by a ‘domeshaped’ profile
in the deeper lower layers(40m and deeper) after 30 days of model integration.
Conversely, the temperature profile in the VICTORIA case reaches near isothermal over
the full water column after similar period of integration.
Another peculiar feature shown in the simulated circulation patterns is the presence
of twocounter rotating gyres in the VICTORIA run, but only a single anticyclonic gyre
in the MIDLAT run after one month of model integration. The circulation gyres shown in
the VICTORIA run, with an idealized bathymetry, is also apparent in the simulation with
actual(observed) bathymetry, just after 20 days of model integration.
Finally, it is important to note that whereas our simulated results show physical
features in the lake circulation and temperature patterns that are consistent with previous
studies(mostly focused on midlatitude lakes), the present study ignored effects of short
and longwave radiation, as well as the variations in the heat fluxes. The focus of the
present study was to examine the impact of uniform wind forcing of the fundamental
features of the 3D hydrodynamics and thermodynamics of Lake Victoria. However, the
combined effects of radiation and heat fluxes and wind stress are worth considering in
future investigations.
17
Acknowledgements
This research was supported by NSF grant # ATM0438116. The model experiments were
performed at the North Carolina State University High Performance Center and at
National Center for Atmospheric Research (NCAR). NCAR is sponsored by NSF. We
would like also to thank Ufuk from Istanbul Technical University for helping with the
lake grid generating program.
18
Figure Captions
Figure 1: Idealized geometry/bathymetry for Lake Victoria
Figure 2: Initial temperature profile
Figure 3: Vertical temperature profiles after (a) 5 days (b) 10 days (c) 30 days month (d) 60 days
Figure 4: Surface and 40m depth temperature evolution at points located over the four qadrants of the lake
Figure 5: Cross section of temperature profiles after 2, 15 and 30 days of model integration.
Figure 6: Comparison of the simulated surface currents in the MIDLAT, VICTORIA and REALBATH runs
19
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(a) (b)
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Figure 1
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Figure 2
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(a) (b)
(c) (d)
Figure 3
(a) (b)
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(c) (d)
(e) (f)
(g) (h)
Figure 4
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(a) (b)
(c) (d)
(e) (f)
Figure 5
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(a) (b) (c)
(d) (e) (f)
(g) (h) (i)
Figure 6
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