Idealized simulation of hydrodynamic characteristics of Lake Victoria that potentially modulate regional climate

Transcription

1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 29: (2009) Published online 11 December 2008 in Wiley InterScience ( Idealized simulation of hydrodynamic characteristics of Lake Victoria that potentially modulate regional climate Richard O. Anyah a * and Fredrick Semazzi b a Department of Natural Resources Management and Engineering, University of Connecticut, 1376 Storrs Road, Storrs CT 06269, USA b Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, NC , USA ABSTRACT: This study explores, through three-dimensional (3D)-lake model simulations, the unique thermodynamic and hydrodynamic characteristics of Lake Victoria that can potentially modulate the lake catchment climate. The simulations are mostly based on idealized forcing due to lack of sufficient observed data. A suite of simulations with an elliptic (oval) geometry and prescribed wind speed (surface wind stress), lake atmosphere temperature difference and vertical temperature profile are performed. The time evolutions of lake temperature as well as the currents (circulation characteristics) at different depths and/or points are analysed 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 mid-latitude lake based on the effects of the Coriolis force are also examined. Our simulations revealed a number of unique features in the time and space evolutions and profiles of the lake 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 30 and 40 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 dome-shaped profile in the bottom layers (40 m and deeper) after 30 days of model integration, in contrast to the VICTORIA case which is nearly isothermal over the full water column. Perhaps one of the most significant outcomes of the present study is that the two-gyre circulation pattern shown in the VICTORIA case after 30 days of model integration is also present in the simulations with observed lake bathymetry. Even more significant is that our results with a fully coupled regional climate-3d lake hydrodynamics simulate more realistic evaporation/evapotranspiration and precipitation over the lake surface and immediate environs. Copyright 2008 Royal Meteorological Society KEY WORDS Lake Victoria influence on regional climate; Lake Victoria hydrodynamics; lake currents; time and space evolutions of lake temperatures Received 18 September 2007; Revised 24 September 2008; Accepted 30 September Introduction Lake Victoria in East Africa (Figure 1), with a total surface area of km 2 (IDEAL, 2003) is the largest lake in the tropics, and second in size only to Lake Superior whose surface area is about km 2 (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., 1998). However, while a number of observational and modelling 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 Bedford, 1994; O Connor and Schwab, 1994; Beletsky et al., 1999, among others), not many * Correspondence to: Richard O. Anyah, Department of Natural Resources Management and Engineering, University of Connectictut, 1376 Storrs Road, Storrs CT 06269, USA. Richard.anyah@uconn.edu studies of similar scope have been conducted for the largest tropical lake, Lake Victoria. 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 lake atmosphere 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 sub-seasonal/seasonal influence on regional climates, especially due to diurnal or seasonal lags in their intra-basin heat transport compared to the surrounding land. Large lakes also display both nearshore and offshore (mid-lake) dynamical regimes Copyright 2008 Royal Meteorological Society

2 972 R. O. ANYAH AND F. SEMAZZI Figure 1. Map of East Africa, with Lake Victoria located almost in the middle. typical in coastal oceans (Csanady, 1982). However, in contrast to the relatively stable main oceanic gyres, lake currents lack persistence and depend more on short-term 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 (Beletsky and Schwab, 2001; Anyah and Semazzi, 2004; 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 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 mid-latitude lakes are strongly dictated by geostrophic balance, circulation regimes in low-latitude lakes may be completely different due to the reduced influence of the Coriolis force, despite the fact that they experience large β-effect (meridional variation of Coriolis force). Laird et al. (2003) 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 modelling 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. In the present study a three-dimensional (3D)-lake 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. The primary focus of this study is to investigate whether by representing the 3D-circulation characteristics of Lake Victoria play any role in the redistribution of lake surface temperatures (LSTs) that consequently lead to different characteristics in lake atmosphere interactions, reflected in rainfall distribution with the lake surface and surrounding regions. 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 mid-latitude lake. POM model has been applied to successfully study the hydrodynamics of several closed (inland) lakes (e.g. 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. (1997) demonstrated that to evaluate the performance of POM or any 3D-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 numerical models has often been riddled with difficulties in obtaining adequate observations (Bennett, 1977). 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 (3D-lake) model as well as the design of the idealized experiments is presented in Section 2. Results and discussions are given in Section 3, while summary and conclusions are presented in Section Model Description and Experimental Design 2.1. POM model We have used POM version pom2k (Blumberg and Mellor, 1987), which is a 3D, non-linear primitive equation, finite difference ocean model. The model uses a

3 IDEALIZED SIMULATION OF HYDRODYNAMIC CHARACTERISTICS OF LAKE VICTORIA 973 (a) (b) Figure 2. Idealized geometry/bathymetry for Lake Victoria. mode-splitting technique to solve for the two-dimensional barotropic mode of the free surface currents and the 3D baroclinic mode associated with the full 3D 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 split-explicit Eulerian scheme in which the internal and the external modes are integrated separately to optimize computational efficiency. The model includes a 2.5 turbulence closure sub-model (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 Blumberg and Mellor (1987) and is also available online at; pom/pubonline/pol.html#users 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 2). The oval is rotated about 60 along the z-axis to further mimic the true orientation of Lake Victoria as shown in Figure 2. The bathymetry is flat at the bottom with minimum and maximum depths of 10 and 80 m, respectively. Although the actual surface area of Lake Victoria is estimated at about km 2 (IDEAL, 2003), the elliptic lake geometry used in the present study is having an approximate surface of km 2 (i.e. the long radius is about 200 km and the short radius is about 100 km). 3. Results and Discussions 3.1. Vertical temperature profile First, we examined the changes of 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 10 4 m 2 s 2 ( 3 ms 1 ). The observed wind speed over Lake Victoria region is shown to be generally within the range of 3 5 ms 1 (Ochumba, 1996). Easterly trades, with occasional southeast/northeast components, dominate the prevailing flow over Lake Victoria basin. The only difference in the two runs is the Coriolis parameter, which is kept constant at 10 4 s 1 in the first run representing a hypothetical mid-latitude lake (hereafter MIDLAT), but is set at about zero (i.e )inthe 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 3(a). This profile was based on limited point observations over Lake Victoria archived by the Fisheries department (e.g. Ochumba, 1996) indicating that the upper 40 m layer of the lake depth is usually characterized by isothermal conditions most of the year. In Figure 3, the changes in the vertical profile of temperature over the central point in the idealized ellipseshaped lake indicate that after 5 days (Figure 3(b)) of integration the mixed layer at the top in the VICTORIA run has stretched down to the 30 m depth, while in the

4 974 R. O. ANYAH AND F. SEMAZZI Figure 3. Vertical temperature profiles after (a) initial (b) 5 days (c) 10 days (d) 30 days month and (e) 60 days. 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 0.5 C at the top. After 10 days (Figure 3(c)), the MIDLAT case has become remarkably cooler within the m layer by about 2 C compared to the VICTORIA case. It is also interesting to note that after 2 months of integration, the full 80 m 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.

5 IDEALIZED SIMULATION OF HYDRODYNAMIC CHARACTERISTICS OF LAKE VICTORIA 975 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 temperature-dependent 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 balance given that we apply constant uniform surface wind stress forcing. The MIDLAT simulation is subjected to considerable Coriolis force 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 influenced by Coriolis very little. The second mechanism, which may also be linked to the differences in the temperature profiles between MID- LAT and VICTORIA as shown in Figure 3 relates to the theory that since wind (stress) is the major external input responsible for mixing in our experiments; wind adds KE to the lake and thus converts part of the existing PE to KE. Consistent with earlier postulations by, e.g. Csanady (1974), wind affects the lake through the shear force it imparts on the water surface. This shear drags the water in the downwind direction, adding kinetic energy causing significant pressure gradient on the lake surface. This results into 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 significant influence of Coriolis force in the MIDLAT case, part of the PE is gradually converted into KE, thus it takes relatively longer time for the water column to mix. The opposite is the case in VICTORIA run where the conversion of PE to KE is not influenced by Coriolis force, but primarily by wind stress and thus takes place relatively faster, leading to rapid mixing 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 4(a)) and northeast (Figure 4(b)) 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 2 days, then begins to oscillate back and forth before reaching equilibrium at about 22 C 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 4(c)) and northwest (Figure 4(d)) quadrants located downwind of our uniform wind stress forcing, the surface temperatures remain relatively warmer as a result of downwelling instigated by the uniform 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 2 C. At 40 m depth, the temporal evolution of temperature suggests 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 (40 45 days), and at about the same value (22 C). 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 deep-layer temperatures will tend to equilibrate at approximately the same value (i.e. 22 C) 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 40 m depth, the lake temperature in the MIDLAT, as opposed to VICTORIA run, the lake begins to warm up very fast within the first 10 days over the northeast quadrant (Figure 4(e)). 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 4(g)) and southwest (Figure 4(h)) 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 Cross-section of simulated vertical temperature profile Figure 5 presents a comparison of the horizontal crosssections 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 5(a)) is significantly different from the VIC- TORIA case (Figure 5(b)). Rather surprising is that after 2 days the temperature of the upper 20 m 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,

6 976 R. O. ANYAH AND F. SEMAZZI Figure 4. Surface and 40-m depth temperature evolution at points located over the four quadrants of the lake. 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 20 m layer compared to the VICTORIA case due to geostropic adjustment as explained earlier. For Victoria, the stirring effect by the surface wind forcing and the subsequent surface pressure gradient build up accelerates the conversion of

7 IDEALIZED SIMULATION OF HYDRODYNAMIC CHARACTERISTICS OF LAKE VICTORIA 977 Figure 5. Cross-section of temperature profiles after 2, 15 and 30 days of model integration. 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 2 weeks (15 days: Figure 5(c)) 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 2 C compared to the initial value, just like in the VICTORIA case. The other distinct difference between the two runs is the presence/absence of the dome-shaped thermocline in the MIDLAT/VICTORIA run shown in Figure 5(e) and (f), respectively. The dome-shaped thermocline manifested in the MIDLAT simulations is consistent with earlier

8 978 R. O. ANYAH AND F. SEMAZZI Figure 6. Comparison of the simulated surface currents in the MIDLAT, VICTORIA and REALBATH runs. studies (e.g. Schwab et al., 1995) that have postulated that geostrophic circulation around a dome-shaped thermocline leads to enhanced cyclonic circulations in largeand medium-sized lakes during periods of stratification Lake currents Figure 6(a) (i) 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 (Figure 6(a) and (b)). As mentioned in the previous sections, the circulation differences are most likely associated with absence/presence of significant influence of Coriolis force in the VICTORIA/MIDLAT run. After 2 weeks of model integration, there seem to be no distinct features observable in the circulation pattern in both runs. However, 30 days later, very distinct features in the surface currents are exhibited in both MIDLAT and VICTORIA simulations. For, example, a single gyre

9 IDEALIZED SIMULATION OF HYDRODYNAMIC CHARACTERISTICS OF LAKE VICTORIA 979 (anti-cyclonic) circulation stretching across the entire lake basin is simulated in MIDLAT (Figure 6(e)), while in the VICTORIA two-gyre (counter-rotating) circulation currents are distinctively simulated (Figure 6(g)). Interestingly, the two-gyre, counter-rotating, circulation patterns are also manifested in the simulation with real-lake bathymetry (Figure 6(i)). It should be noted that the latter simulation was done with coarser resolution (20 km) as opposed to 1 km 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; Schwab and Beletsky, 2003). Perhaps one of the most significant outcomes of the present study is that the two-gyre circulation pattern shown in the VICTORIA case after 30 days of model integration is also present in our simulated lake currents in the real-lake run (hereafter REALBATH). Although there is need for more detailed examination, beyond the scope of the present study, our results suggest that somehow 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 low-latitude lakes where the effect of coriolis force is negligible; implying that 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 two-gyre (counter-rotating) circulation pattern in stratified lakes based on simple (idealized) bathymetry (e.g. Bennett, 1974; 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 upwelling sections (upwind) due to decrease (or the asymmetry) in vertical mixing and bottom friction Evaluation of simulated evaporation and precipitation with or without 3D-lake hydrodynamics In this section, we present a comparison of the simulated surface evapotranspiration/evaporation and precipitation for the month of November 2002 for simulations with prescribed LST in RegCM3 and one where RegCM3 is fully coupled to a 3D-lake model. RegCM3 is a 3D, sigma-coordinate, primitive equation regional climate model. Detailed description of the atmospheric model, RegCM3, can be found in Giorgi et al. (1993), with latest updates in the model physics parameterizations and dynamics described in Anyah et al. (2006). The 3D-lake model is based on the modified version of POM as is described earlier in Section 2.1, but further details can be found also in Song et al. (2004) and Anyah et al. (2006). The coupled model simulations are forced with initial and boundary conditions derived from NCEP reanalysis. TRMM satellite precipitation estimates have been used to evaluate precipitation. However, it was not possible to evaluate surface evaporation because of lack of any reliable observations. What is clear in Figure 7 is the difference between the simulated surface evaporation (E) amounts in the simulations with and without 3D hydrodynamics of the lake. The difference in the amount is more pronounced over the lake surface. In the simulation with 3D-lake model, the spatial distribution of E is also quite different compared to the simulations without the lake. Since E over the lake is influenced by the LST, the spatial distribution in Figure 8(b) shows that by accounting for 3D-hydrodynamics the lake circulation (currents) helps to redistribute LST in a way that is not possible in the case where the lake is represented by simplified one-dimensional (1D)-thermal diffusion equation (Figure 7(a)) that is not capable of redistribution lake Figure 7. RegCM3 Simulated November 2002 surface evaporation in mm (a) with 1D-lake model and (b) with 3D-lake model.

10 980 R. O. ANYAH AND F. SEMAZZI (a) (b) lake surface presented in Figure 8(a) does not compare quite well with the TRMM estimates (Figure 8(c)). On the other hand, the RegCM3-3D lake coupled modelsimulated rainfall amount and distribution over the lake surface and its environs ( Figure 8(b)) is more consistent with the TRMM estimates. In particular, the highest amount of rainfall is simulated on the western sections of the lake as is also represented by TRMM estimates. This is possible because by incorporating 3D-lake hydrodynamics the vertical and horizontal distribution of LST is possible and thus the lake atmosphere interactions are reasonably improved. It is very important to simulate the lake surface rainfall amounts and distribution as accurately as possible to resolve the hydrological cycle of the Lake Victoria Basin correctly given the Lake s water balance is dominated by precipitation and evapotranspiration terms as shown in some previous studies. 4. Summary and Conclusions (c) November TRMM Satellite estimates Figure 8. RegCM3 Simulated November 2002 precipitation total (a) with 1D-lake model, (b) with 3D-lake model and (c) TRMM satellite estimate. temperature in both vertical and horizontal directions during the model integration. In Figure 8, the simulated rainfall total (mm) during November 1998 with and without 3D-lake hydrodynamics is compared with the TRMM satellite estimates. The simulated rainfall amount and distribution over the The 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 long-wave radiation are ignored. The lake temperature was initialized with the following profile; isothermal (24 C) within upper 20 m layer, decreasing gradually (linearly) with depth following a near logarithmic profile within the next 20 m layer until it reaches 21 C at 40 m depth and thereafter the temperature remains isothermal again (21 C) until the bottom (70 m). The second part of our study focused on the simulated differences/similarities between typical mid-latitude 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 was performed with the actual (real) lake bathymetry. Our simulations revealed a number of unique features in the temperature evolution (profiles) at the surface and lower depths during the 2-month 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 30 and 40 days). However, there is a conspicuous difference in the vertical temperature profiles of the two runs (cases). For example, the MIDLAT run is characterized

11 IDEALIZED SIMULATION OF HYDRODYNAMIC CHARACTERISTICS OF LAKE VICTORIA 981 by a dome-shaped profile in the deeper lower layers (40 m 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 two-counter-rotating gyres in the VICTORIA run, but only a single anticyclonic gyre in the MIDLAT run after 1 month of model integration. The circulation gyres shown in the VICTO- RIA 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 mid-latitude lakes), the present study ignored effects of short- and long-wave 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. Acknowledgements This research was supported by NSF Grant # ATM 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 also thank Ufuk from Istanbul Technical University for helping with the lake grid-generating program. 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