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Simulated Physical Mechanisms Associated with Climate Variability over
Lake Victoria Basin in East Africa
RICHARD O. ANYAH
Center for Environmental Prediction, Department of Environmental Sciences, Rutgers, The State University of New Jersey,
New Brunswick, New Jersey
FREDRICK H. M. SEMAZZI
Department of Marine, Earth and Atmospheric Sciences, and Department of Mathematics, North Carolina State University,
Raleigh, North Carolina
LIAN XIE
Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina
(Manuscript received 5 October 2005, in final form 21 March 2006)
ABSTRACT
A fully coupled regional climate, 3D lake modeling system is used to investigate the physical mechanisms
associated with the multiscale variability of the Lake Victoria basin climate. To examine the relative
influence of different processes on the lake basin climate, a suite of model experiments were performed by
smoothing topography around the lake basin, altering lake surface characteristics, and reducing or increas-
ing the amount of large-scale moisture advected into the lake region through the four lateral boundaries of
the model domain. Simulated monthly mean rainfall over the basin is comparable to the satellite (Tropical
Rainfall Measuring Mission) estimates. Peaks between midnight and early morning hours characterize the
simulated diurnal variability of rainfall over the four quadrants of the lake, consistent with satellite esti-
mates, although the simulated peaks occur a little earlier. It is evident in the simulations with smoothed
topography that the upslope/downslope flow generated by the mountains east of the lake and the land–lake
breeze circulations play important roles in influencing the intensity, the location of lake/land breeze fronts,
and the horizontal extent of the land–lake breeze circulation, as well as lake basin precipitation. When the
lake surface is replaced with marsh (water hyacinth), the late night and early morning rainfall maximum
located over the western sector of the lake is dramatically reduced. Our simulations also indicate that
large-scale moisture transported via the prevailing easterly trades enhances lake basin precipitation signifi-
cantly. This is in contrast to the notion advanced in some of the previous studies that Lake Victoria
generates its own climate (rainfall) through precipitation–evaporation–reprecipitation recycling only.
1. Introduction
The primary objective of the present study is to char-
acterize and understand the relative roles of several
physical mechanisms associated with the coupled lake–
atmosphere climate variability over Lake Victoria basin
in East Africa on diurnal through interannual time
scales. Lake Victoria basin is one of the agriculturally
productive areas in East Africa and thus a major bread-
basket for the region. Besides having an agriculturally
rich hinterland, the lake also supports and sustains im-
portant fisheries, maintains an energy supply (hydro-
electric power), and is a potent source of both domestic
and industrial water supply. The lake is also one of the
primary sources of the river Nile (the longest river in
the world), which could be viewed as the hydrological
“placenta” and lifeline of semiarid countries down-
stream, which include the Sudan, Ethiopia, and Egypt.
Lake Victoria basin, situated within a shallow conti-
nental sag between the two arms of the East Africa
Corresponding author address: Dr. Richard Anyah, Center for
Environmental Prediction, Department of Environmental Sci-
ences, Rutgers, The State University of New Jersey, New Bruns-
wick, NJ 08901.
E-mail: anyah@cep.rutgers.edu
3588 MONTHLY WEATHER REVIEW VOLUME 134
© 2006 American Meteorological Society
MWR3266
Great Rift Valley system (Fig. 1), provides an environ-
ment conducive for complex interactions and integra-
tions between regionally induced and large-scale circu-
lation systems. These include topographic and lake-
induced circulations (Song et al. 2004; Anyah and
Semazzi 2004), circulations associated with widespread
variations in land cover/land use characteristics, mon-
soonal circulations associated with the thermal contrast
between land and the nearby Indian Ocean (Okeyo
1987; Mukabana and Pielke 1996), and the influence of
the humid Congo air mass emanating from the tropical
(Congo) rain forest (Anyamba 1984; Sun et al. 1999a).
The general climate of the lake basin ranges from a
modified equatorial type with substantial rainfall occur-
ring throughout the year, particularly over the lake and
its vicinity, to a semiarid type characterized by inter-
mittent droughts over some areas located even within
short distances from the lakeshore. However, the sea-
sonal rainfall is characterized by a bimodal regime:
March–May and October–December, locally known as
long and short rainy seasons, respectively. During the
short rainy season, rainfall is more widespread over the
entire lake basin during the month of November (As-
nani 1993). The seasonal cycle of rainfall is mainly con-
trolled by the north–south migration of the intertropi-
cal convergence zone (ITCZ) across the region,
whereas the diurnal cycle is dominated by lake/land
breeze circulations. Large-scale precipitation over the
lake basin is mainly initiated from the southeastern
(eastern) Indian Ocean monsoon flow that transports
maritime moisture into the interior of East Africa. The
humid Congo air mass also significantly boosts convec-
tion and overall rainfall amounts received over the
western and northwestern parts of the lake (Anyamba
1984). However, a quasi-permanent trough that lingers
over Lake Victoria (Asnani 1993) due to locally in-
duced convection, orographic influence, and land–lake
thermal contrast tends to favor convection over the
lake basin throughout the year. In terms of interannual
variability, the Lake Victoria basin climate is charac-
terized by periodic episodes of anomalously wet or dry
conditions associated with SST anomalies over equato-
rial Indian Ocean (e.g., 1961 floods) and also Pacific
Ocean SST perturbations (e.g., 1997/98 ENSO-related
floods).
However, the complex interactions between regional
and large-scale processes and their associated modulat-
ing influence on the regional climate are not yet well
investigated quantitatively. Furthermore, many previ-
ous investigators have primarily used empirical meth-
ods that are based mainly on the scarce observations
over the region and thus do not offer adequate scope to
sufficiently unveil the cause–effect relationships be-
tween regional climate variability and an individual
process or combination of processes. Such cause–effect
relationships may only be understood through the nu-
merical modeling approach that also accounts for the
lake–atmosphere coupled variability. Hence, in the
present study, we have applied a fully coupled regional
climate–three-dimensional lake modeling system–
Princeton Ocean Model (RegCM3–POM; Song et al.
2002, 2004; Blumberg and Mellor 1987; Anyah 2005) to
investigate how the interactions among local and large-
scale factors (processes) modulate the Lake Victoria
basin climate. A coupled modeling system provides a
rich test bed for examining the response of the lake
basin (and regional climate) to an individual process or
a combination of processes through a suite of system-
atic sensitivity simulations.
We investigate the role of complex topography
(steep terrain on both sides of the lake) and whether it
helps to organize, enhance, and/or suppress the devel-
opment of convective activity over the lake basin and
surrounding areas. Also, we have examined how
changes in lake surface characteristics, as typified by
the invasion of a large swath of the lake surface by
water hyacinth weed, alters the lake–atmosphere inter-
actions and eventually the lake basin climate. In addi-
tion, experiments are performed to investigate the ex-
tent to which large-scale moisture transported via the
prevailing monsoonal flows affects the overall climate
(rainfall pattern) over the lake basin. We focus on the
short rains due to the large spatial and temporal vari-
ability of rainfall experienced during this season com-
pared to the long rainy season (Nicholson 1996). As a
result, the regional climate during the short rains is
highly sensitive to perturbations in both large- and lo-
cal-scale climate systems (Sun et al. 1999a).
Mesoscale lake-effect circulations have been shown
to develop through complex interactions of an array of
environmental and geographic variables such as lake–
air temperature differences, wind speed, lower-
tropospheric stability, lake shape, or bathymetry (Laird
et al. 2003a,b). McPherson (1970) established that the
distribution of the thermal surface gradient (i.e., caused
by shoreline configuration) and interaction with the
ambient wind may enhance or diminish the low-level
convergence and vertical circulation within the lake ba-
sins. This is consistent with the study by Hostetler and
Giorgi (1992), who noted that increased simulated pre-
cipitation in the presence of large inland lakes was due
to an increase in overlake evaporation that adds water
vapor to the prevailing flow systems, thereby enhancing
convective instability and precipitation associated with
such systems. This is more pronounced when the lake
DECEMBER 2006 ANYAH ET AL. 3589
FIG. 1. (a) Terrain height around Lake Victoria basin; areas higher than 1200 m are shaded (NKK high-
lands). (b) Horizontal cross section of the elevation around Lake Victoria basin along 1°S (KH represents
Kenya highlands).
3590 MONTHLY WEATHER REVIEW VOLUME 134
surface temperatures are much warmer than the over-
lying atmosphere.
Fraedrich (1972) made a significant contribution to
the understanding of this problem over Lake Victoria
in East Africa by investigating the dynamics of noctur-
nal circulations and the frequent development of thun-
derstorms over the northwestern/western sector(s) of
the lake using an analytical (climatological) model. The
dynamical processes linked to the nocturnal circulation
patterns were found in this study to be associated with
three-way interactions among the diurnal land–lake
breeze circulations, the upslope or downslope moun-
tain or valley winds, and prevailing (large scale) mon-
soonal flow. The resultant “nonlinear”interactions fa-
vor strong convergence over the western sector of the
lake at night but over land areas east of the lake catch-
ment during the day (Okeyo 1987). The diurnal and
monthly rainfall variability is also closely linked with
this flow pattern (Mukabana and Pielke 1996), with the
western sector receiving more rainfall than the eastern
sector in terms of the monthly mean totals. Asnani
(1993) also showed that rainfall over the lake is about
30%–35% more than over the surrounding land areas.
Ba and Nicholson (1998) also used satellite data and
showed that the frequency of cold cloud duration over
the lake is about 25%–30% greater than over the sur-
rounding land, although the estimated overlake rainfall
pattern was found to be highly correlated with basin-
wide rainfall (Mistry and Conway 2003). Thus, it is un-
likely that causes of the dramatic lake level fluctuations
experienced during the 1961 and 1997/98 flood episodes
over the region can be well understood without ac-
counting for the contribution from large-scale forcing
to regional rainfall variability. After all, the bimodal
rainfall pattern associated with the passage of the ITCZ
across eastern Africa is also well marked in the over-
lake rainfall variability (Nicholson 1998; Mistry and
Conway 2003), indicating that the rainfall pattern over
the lake basin is also partly driven by fluctuations in the
large-scale climatic conditions.
Recently, Song et al. (2002, 2004) developed a fully
coupled regional climate modeling system (RegCM2–
POM) to simulate the coupled lake–atmosphere cli-
mate variability over Lake Victoria basin. They dem-
onstrated that adopting the traditional modeling ap-
proach in which the three-dimensional lake
hydrodynamics and thermodynamics are neglected, as
in cases where the lake is represented by a simple 1D
thermal equation (Sun et al. 1999b; Anyah and Semazzi
2004), is not entirely satisfactory for Lake Victoria.
Their results further demonstrated that the fully
coupled RegCM2–3D model simulated more realistic
climate conditions over eastern Africa and Lake Vic-
toria basin compared to observations and results from
the standard RegCM2–1D model adopted in Hostetler
et al. (1993), Sun et al. (1999b) and Anyah and Semazzi
(2004).
In the present study, the response of lake basin and
regional climate to nonlinear interactions among topo-
graphic-induced circulations, land–lake breeze circula-
tions, and large-scale (prevailing) monsoonal flow is
investigated based on an enhanced and improved ver-
sion of the RegCM3–3D lake modeling system (Anyah
2005). Additionally, the impacts of changes in the
physical characteristics of the lake surface (as exempli-
fied with a recent invasion of a large swath of the lake
by water hyacinth) on the lake and basinwide climate
variability are examined. A brief description of the
coupled modeling system and the design of numerical
experiments is given in section 2. Results and discus-
sions are presented in section 3, while the summary and
conclusions are given in section 4.
2. Description of component models of the
RegCM3–POM system
a. RegCM3 model
RegCM3 is a three-dimensional primitive equation
atmospheric model (Pal et al. 2005, manuscript submit-
ted to Bull. Amer. Meteor. Soc.). It is an improved and
augmented version of the National Center for Atmo-
spheric Research (NCAR) RegCM2 (Giorgi et al.
1993a,b). The model uses a terrain-following (sigma
pressure) vertical coordinate system. The radiation
physics calculations are based on the NCAR Commu-
nity Climate Model version 3 (CCM3) GCM radiation
scheme (Kiehl et al. 1996) that includes a component
for computing the effects of greenhouse gases (NO
2
,
CH4, and CFCs), aerosols, and cloud ice. The land sur-
face physics parameterizations are based on the Bio-
sphere–Atmosphere Transfer Scheme version 1e
(BATS1e: Dickinson et al. 1993) in which a standard
surface drag coefficient based on surface-layer similar-
ity theory is applied to calculate sensible heat, water
vapor (latent heat), and momentum fluxes.
Two major enhancements in the cloud and precipi-
tation processes have been implemented in RegCM3
since its earlier version, RegCM2. First, moisture is
prognosticated based on cloud water formation, advec-
tion and mixing by turbulence, and reevaporation and
conversion of cloud water into rain via a bulk auto-
conversion term. Second, the large-scale precipitation
parameterization, subgrid explicit moisture scheme
(SUBEX; Pal et al. 2000), is used to account for non-
convective clouds and model-resolved precipitation.
SUBEX accounts for the subgrid variability in clouds
DECEMBER 2006 ANYAH ET AL. 3591
by relating the average gridcell relative humidity to
cloud fraction and cloud water based on the formula-
tion by Sundqvist et al. (1989).
Further modifications and customization of RegCM3
for the equatorial eastern Africa domain were carried
out at North Carolina State University’s Climate Simu-
lation Laboratory (Anyah 2005). These followed the
criteria applied in Sun et al. (1999a,b) during the first
application of NCAR RegCM2 for simulating the re-
gion’s climate and in Song et al. (2002) during the initial
development of a coupled RegCM2–POM system for
Lake Victoria basin.
b. Three-dimensional Lake Victoria model based
on POM
The Princeton Ocean Model (Blumberg and Mellor
1987) is a three-dimensional, nonlinear primitive equa-
tion, finite difference ocean model. The model uses 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 tem-
perature, turbulence, and current structure. The baro-
tropic mode uses a shorter time step, while the baro-
clinic mode uses a relatively longer time step. Both
modes are constrained by the Courant–Friedrichs–Lewy
(CFL) computational stability criteria. The model is
based on a split-explicit Eulerian scheme in which the
internal and external modes are integrated separately
to optimize computational efficiency. The model in-
cludes a 2.5 turbulence closure submodel (Mellor and
Yamada 1974) with an implicit time scheme for vertical
mixing. The equation of state (Mellor 1991) is used to
calculate density as a function of temperature, pressure,
and salinity. POM is currently one of the most widely
used ocean models and has also been extensively used
for studying coastal estuaries and inland lake basins. A
detailed description of POM can be found in Blumberg
and Mellor (1987). Modifications made to the POM
used in this study for freshwater Lake Victoria can be
found in Song et al. (2002, 2004) and Anyah (2005). The
coupling of the atmosphere and the lake is through the
fluxes of momentum, sensible heat, longwave radiation,
moisture, latent heat, and shortwave radiation (Fig. 2),
and details are available in Song et al. (2004).
c. Design of numerical experiments
We first performed RegCM3 model runs over a rela-
tively larger domain covering 15°N–8°S, 10°–55°E, at a
spatial resolution of 60 km (i.e., Big Brother Experi-
ments) for the short rains seasons over a 5-yr period
(1998–2002). The initial and boundary conditions were
taken from the 6-hourly National Centers for Environ-
mental Prediction (NCEP) reanalysis (Kalnay et al.
1996). In these experiments, a simple 1D lake model
represented Lake Victoria. For the lake basin experi-
ments with the RegCM3–3D lake modeling system, the
model domain covered 5°N–7°S, 25°E–41°E, which en-
compasses the whole of the Lake Victoria catchment.
The initial and boundary conditions were derived from
the 6-hourly output of the Big Brother Experiments.
The physical mechanisms associated with interac-
tions between the lake and regional topography were
investigated by performing the following experiments:
•Experiment 1 (CTRL): Control case in which the ter-
rain and land surface/land use characteristics are un-
altered.
•Experiment 2 (TPALL): Land surface/land use char-
acteristics are as in CTRL, except the terrain height
all around the lake basin is smoothed such that the
maximum height is 1300 m above mean sea level
FIG. 2. Schematic of the coupled atmosphere–lake system
(modified from Song et al. 2004; LW represents longwave radia-
tion, SW is shortwave radiation,
is wind stress, Lq is latent heat,
and SH is sensible heat).
3592 MONTHLY WEATHER REVIEW VOLUME 134
(AMSL) and is just above the approximate lake sur-
face elevation (⬃1200 m).
•Experiment 3 (TPEA): Same as in TPALL, except
only the terrain between the lake and Indian Ocean is
smoothed.
•Experiment 4 (LBOG): Similar to CTRL, except the
lake is replaced with bog/marsh (swamp) in order to
examine the basinwide climate response to changes
in lake surface characteristics. We believe that this
experiment mimics the ongoing changes in the lake
surface characteristics imposed by the invasion of a
large swath of the lake surface by water hyacinth.
Seven additional experiments were performed to in-
vestigate the role that large-scale moisture transported
via the four lateral boundaries of our model domain
(covering Lake Victoria basin) plays in the overall lake
basin rainfall variability.
•Experiment 5 (Qe-20): Large-scale moisture [mixing
ratio (q)] along the eastern lateral boundary is re-
duced by 20%. This means that the incoming large-
scale moisture through the eastern boundary is rela-
tively drier than in the control case. This is done prior
to interpolating the boundary and initial conditions
onto the model grids in order to avoid any inconsis-
tencies in the physics and dynamics of the model dur-
ing integrations.
•Experiment 6 (Qe-50): Same as in Qe-20, except
large-scale moisture is 50% drier than in the control.
•Experiment 7 (Qe-80): Same as in Qe-20, except
large-scale moisture is 80% drier than in the control.
•Experiment 8 (Qw-50): Same as in Qe-50, except
large-scale moisture forcing over the western bound-
ary is 50% drier than in the control.
•Experiment 9 (Qs-50): Same as in Qe-20, except
large-scale moisture forcing over the southern
boundary is 50% drier than in the control.
•Experiment 10 (Qn-50): Same as in Qe-20, except
large-scale moisture forcing over the northern
boundary is 50% drier than in the control.
•Experiment 11 (Qa-50): Same as in Qe-20, except
large-scale moisture forcing over all four lateral
boundaries is 50% drier than in the control.
3. Results and discussion
a. Comparison of simulated and satellite rainfall
The RegCM3–3D lake coupled model-simulated di-
urnal, seasonal, and interannual variability of rainfall
are mainly evaluated using the National Aeronautics
and Space Administration (NASA) Tropical Rainfall
Measuring Mission (TRMM) satellite estimates (avail-
able online at http://disc.gsfc.nasa.gov/data/datapool/
TRMM_DP/). TRMM data over the lake are currently
some of the most comprehensive observational surro-
gates available for evaluating overlake simulated rain-
fall because of the lack of high-resolution in situ obser-
vations. The precipitation radar aboard the TRMM Mi-
crowave Imagery (TMI) satellite is also capable of
detecting below-cloud rainfall and is thus a suitable tool
for estimating rainfall over Lake Victoria that has a
strong diurnal cycle of cloud cover (Kummerow et al.
2000; Ba and Nicholson 1998).
The 5-yr average (1998–2002) of the simulated rain-
fall in November is compared to TRMM estimates av-
eraged over the same period and is presented in Fig. 3.
On the other hand, in Figs. 4a,b, 4c,d, and 4e,f, the
simulated and TRMM rainfall are compared in Novem-
ber 1998, 2000, and 2002, respectively. These three
years represent periods with relatively different large-
scale climatic regimes. The November 1998 short rains
season coincided with the mature phase of La Niña
conditions, while November 2002 coincided with mod-
FIG. 3. Five-year rainfall average in November (1998–2002) for
the (a) RegCM3–POM simulation and (b) TRMM.
DECEMBER 2006 ANYAH ET AL. 3593
FIG. 4. Comparison between RegCM3–POM simulated monthly mean rainfall and TRMM estimates over Lake Victoria basin in
November (a) 1998 model, (b) 1998 TRMM, (c) 2000 model, (d) 2000 TRMM, (e) 2002 model, and (f) 2002 TRMM.
3594 MONTHLY WEATHER REVIEW VOLUME 134
erate El Niño conditions. We treat November 2000 as a
near-”normal”season over our study area. Further-
more, the month of November, which is the middle of
the short rains season, is generally associated with more
widespread rainfall over the lake basin (Asnani 1993),
and thus the rainfall characteristics during the month
are nearly representative of the overall rainfall pattern
and/or fluctuations during the entire season.
In November 1998 (Figs. 4a,b), the model generally
simulates more rainfall over the entire lake basin com-
pared to the TRMM. Large amounts of rainfall are
simulated over the western and northwestern sectors of
the lake, with the maximum peak located slightly to the
southwest. The TRMM rainfall maximum is located
over the southwestern section of the lake surface as
well, although the peak amount is about 180 mm com-
pared to over 280 mm simulated by the model, a dif-
ference of about 50%. In addition, the model simula-
tions show a secondary region of enhanced rainfall to
the east of the lake (approximately located over the
Nandi-Kericho-Kisii (NNK) highlands; see Fig. 1),
which is conspicuously missing in the TRMM estimates.
Since 1998 was the first year of the TRMM mission, it
has been noted that many errors in the algorithms used
in computing satellite rainfall estimates had not been
corrected (Kummerow et al. 2000). However, the in-
ability of the RegCM3–POM coupled model to ad-
equately resolve rainfall over the elevated regions east
of Lake Victoria may also be associated with the mod-
el’s relatively coarse spatial resolution (20 km) as well
as a deficiency in the model’s precipitation physics,
leading to unrealistically high amounts of simulated
rainfall.
In Figs. 4c,d, both the simulated and TRMM rainfall
in November 2000 are characterized by higher rainfall
amounts over the western and northwestern sector of
the lake compared to the surrounding areas. In addi-
tion, the overall simulated rainfall pattern is reasonably
consistent with the TRMM estimates (Fig. 4c). How-
ever, the simulated rainfall pattern and amount is not in
good agreement with TRMM estimates over the high-
lands east of the lake. The exceptionally high amounts
of rainfall simulated are likely due to the deficiency of
the model dynamics/physics in capturing the rainfall
pattern over the high terrain east of the lake at the
present spatial resolution of 20 km. But the relatively
dry conditions (less rainfall) simulated over the hinter-
lands to the west of the lake qualitatively agree with the
low values seen in the TRMM estimates.
Figures 4e,f shows that the simulated rainfall in No-
vember 2002 over the northern and northwestern quad-
rant of the lake surface is in general agreement with
TRMM estimates, in terms of both the location of rain-
fall maximum as well as the spatial distribution. How-
ever, significant differences are evident between simu-
lated and TRMM rainfall over the southwestern sector
of the lake. TRMM rainfall estimates over the south-
western part of the lake surface are relatively higher
than the simulated amount by over 50%. However, the
simulated dry conditions over the land areas northeast
of the lake catchment are consistent with the TRMM
estimates (Fig. 4f).
In Figs. 5a,b, the simulated area-averaged rainfall
over the northern half of the lake (0°–1°S) and the
southern half (1°–2°S) are compared with the TRMM
estimates. Over the northern half of the lake, the model
overestimates the rainfall total in four out of the five
years (i.e., 1998, 2000, 2001, and 2002) compared to
FIG. 5. Overlake averaged rainfall in the (a) northern and (b)
southern half, and (c) overlake rainfall anomaly based on the
1998–2002 average.
DECEMBER 2006 ANYAH ET AL. 3595
TRMM estimates. On the other hand, over the south-
ern half, the simulated rainfall totals are lower than the
TRMM rainfall for three out of the five years (i.e., 1999,
2001, and 2002). However, the model and TRMM are
in relatively better agreement over the southern half of
the lake compared to the northern half. The negative
differences between the model and TRMM over both
halves in November 1998 (La Niña) are generally con-
sistent with previous studies, which have shown that
during La Niña events, most parts of East Africa in-
cluding the lake basin tend to experience below normal
rainfall amounts (e.g., Ogallo 1988; Nicholson 1996).
This is also apparent in Fig. 5c, which shows the
anomaly of the overlake rainfall based on a 5-yr (1998–
FIG. 6. Overlay of 850-hPa mean flow on convective precipitation over the lake basin at
(a) 0300 and (b) 1500 LST.
3596 MONTHLY WEATHER REVIEW VOLUME 134
2002) mean. Both simulated and satellite (TRMM) es-
timated anomalies are negative, although the anomaly
in the satellite estimate is relatively larger.
Overall, the coupled RegCM3–POM reasonably re-
produces the spatial and temporal variability of Lake
Victoria basin rainfall (Figs. 3–5), which is consistent
with TRMM satellite estimates. It is, however, clearly
evident that during all 5 yr covered by our simulations,
the simulated overlake rainfall amounts are higher than
over the surrounding land areas by almost 30%–50%.
This is consistent with TRMM estimates, although the
simulated values are much more exaggerated.
b. Diurnal cycle of circulation and precipitation
over Lake Victoria
The mean circulation pattern and associated land or
lake breeze circulation driven convection (convective
precipitation) is given in Fig. 6. The peak of the noc-
turnal circulation generally occurs between midnight
and early morning hours, when the lake surface is much
warmer than the surrounding land areas (Fraedrich
1972). Conversely, the peak lake breeze circulation oc-
curs between late afternoon and early evening (Okeyo
1987), when the adjoining land surface is much warmer
than the lake surface. Figure 6a shows the simulated
mean circulation pattern in the morning at 0300 LST
for the month of November. The circulation pattern
over the lake basin is characterized by flow conver-
gence over the western sector of the lake consistent
with previous studies (e.g., Fraedrich 1972; Okeyo 1987;
Datta 1981; Song et al. 2002).
Figure 6b shows the mean circulation pattern over
the lake basin at 15 LST, when the lake breeze circu-
lation is expected to be fully developed. The model
clearly captures outflow from the lake surface and the
apparent location of the lake breeze front east of the
lake, around 35°E. However, the exact location of the
land breeze (nocturnal circulation) front is not clearly
resolved in the model simulations. The simulated land
breeze circulation is also relatively weaker and the ap-
FIG. 7. Three-hourly total rainfall over four different quadrants over the lake in November 1998.
DECEMBER 2006 ANYAH ET AL. 3597
proximate location of the land breeze front is confined
within the western rim of the lake surface. It is also
important to note that Lake Victoria forms a quasi-
permanent trough and is surrounded by high mountain
ranges on both sides, thus, often the statically stable air
over the lake during the day partly determines the in-
tensity of the lake breeze circulation. The simulated
convective precipitation associated with the times of
peak land–lake breeze is consistent with the simulated
circulation patterns. It can be seen that a significant
amount of rainfall is simulated over the western sector
of the lake around 3 LST, but outside the lake surface
over the land areas and east of the lake basin (over the
NNK highlands; Fig. 1) around 15 LST.
Figures 7, 8, and 9 show the total simulated and
TRMM estimated diurnal rainfall variability over four
different quadrants of the lake surface for November
1998, November 2000, and November 2002, respec-
tively. The four quadrants are 1°⫻1°square boxes
(⬃10,000 km
2
) over the lake surface and are designated
as northwest quadrant (NW: 0°–1°S, 32°E 33°E),
southwest quadrant (SW: 1°–2°S, 32°E–33°E), north-
east quadrant (NE: 0°–1°S, 33°E–34°E), and southeast
quadrant (SE: 1°–2°S, 33°E34°E). We use 3-h rainfall
totals to derive the diurnal cycles since the TRMM es-
timates were not available at any finer temporal reso-
lution during this study. While the simulated rainfall
patterns during each of the 3-h intervals for November
during the three years are slightly different, the diurnal
cycles are quite similar. For example, the diurnal cycles
of rainfall over all four quadrants are characterized by
nocturnal peaks (between midnight and early morning
hours), with rainfall drastically diminishing over the en-
tire lake surface thereafter.
These results are consistent with those of previous
studies by Datta (1981), Ba and Nicholson (1998), and
Song et al. (2002), all of which showed that the diurnal
cycle of rainfall over a limited sample of lake island
FIG. 8. Same as in Fig. 7, but for November 2000.
3598 MONTHLY WEATHER REVIEW VOLUME 134
stations and lakeshore stations over different parts of
the lake is characterized by nocturnal peaks. Evidently,
the overlake rainfall is mainly experienced during late
night into early morning hours when there is uplift or
rising motion over the relatively warmer lake surface
associated with land breeze circulation. During the day,
there is sinking motion and flow divergence (Fig. 6)
over the lake surface due to reversal in the thermal
gradient between the lake and surrounding land areas
(lake breeze circulation). The lowest rainfall amounts
are simulated over the SE quadrants for all three years.
The 850-hPa (⬃300 m above lake surface elevation)
simulated mean temperature gradient between the lake
and land at 15 LST is in the order of 6°C, while at 3 LST
it is much weaker, about 3.5°C. This could be due to the
fact that nocturnal convection leads to rain-cooled air
over the lake, which minimizes the lake–land tempera-
ture gradient.
It is evident from the simulated results that there is
reasonable agreement between the model and TRMM
estimates over southwestern and northwestern quad-
rants (Figs. 7c, 8c, and 9c) for all the November months
in 1998, 2000, and 2002. At the same time, the simulated
diurnal cycle over the southeastern sector of the lake is
not consistent with TRMM estimates during all three
years. Overall, the simulated diurnal rainfall variability
is relatively more consistent with TRMM satellite esti-
mates in 2000 (near-normal season) than during the
1998 and 2002 seasons. This is possibly a manifestation
that large-scale climate anomalies (El Niño and La
Niña events) are superimposed even in the diurnal vari-
ability of the lake basin climate and weather patterns.
c. Contribution of large-scale moisture to the lake
basin rainfall variability
A suite of sensitivity experiments (described in sec-
tion 2c) were performed by systematically reducing the
amount of large-scale moisture advected into the lake
basin through the four lateral boundaries. Figure 10
shows the response of the simulated lake basin rainfall
FIG. 9. Same as in Fig. 7, but for November 2002.
DECEMBER 2006 ANYAH ET AL. 3599
to changes in large-scale moisture advected into the
interior domain via the eastern lateral boundary lo-
cated across the western Indian Ocean. This experi-
ment was specifically designed to attempt to quantify
some of the evidence shown in previous observational/
empirical studies that rainfall variability over eastern
Africa during the short rains season is significantly in-
fluenced by the SST gradients (moisture anomalies)
over the equatorial Indian Ocean (Saji et al. 1999; Mu-
tai and Ward 2000). Figures 10a–c show simulated rain-
fall variability in November 2000 when lateral bound-
ary moisture forcing was reduced by 20%, 50%, and
80%, respectively. To avoid any instabilities/inconsis-
tencies in both the dynamics and physics of the model
and uncertainties in interpreting the model results, the
lateral boundary moisture (mixing ratio) was reduced
(dried) before being interpolated onto the model grids
in the interior domain.
It is evident from our results that the simulated rain-
fall amount reduces dramatically as the amount of
moisture entering the interior domain through the east-
ern boundary is systematically reduced. As would be
expected, it is also apparent that the eastern side of
the lake exhibits more sensitivity to the moisture re-
duction (changes). This is also consistent with the fact
that the prevailing monsoonal flow over the lake basin
is easterly most of the year. When large-scale mois-
ture over the eastern boundary is reduced by half, the
corresponding decrease in the simulated rainfall com-
pared to the control was quite dramatic over the entire
lake basin. The overlake averaged rainfall also gener-
ally reduces significantly (by about 50%) compared
with the control (Table 1). With the large-scale mois-
ture entering the interior domain (lake basin) through
the eastern boundary reduced by 80% (Fig. 10c), the
reduction in the simulated rainfall amount is quite dra-
matic over the entire lake basin, except over a small
region in the western-to-southwestern sector of the lake
surface.
When large-scale moisture entering the western
boundary of our domain is reduced by 50% (Qw-50;
Fig. 11b), the corresponding reduction in the simulated
rainfall is negligible compared to the control experi-
ment. However, a rather surprising feature is the slight
increase in the amount of area-averaged simulated rain-
fall over the lake surface, which possibly indicates the
nonlinear response and feedback between the local and
large-scale moisture sources over the lake basin. Simi-
larly, when large-scale moisture entering the southern
boundary is reduced by 50% (Qs-50; Fig. 11c), the re-
FIG. 10. Simulated rainfall (mm) over Lake Victoria basin with
large moisture through the eastern boundary reduced by (a) 20%,
(b) 50%, and (c) 80%.
TABLE 1. Response of lake basin rainfall to large-scale moisture
anomaly.
Expt Qw-50 Qe-50 Qn-50 Qs-50 Qa-50 CTRL
Overlake
rainfall
(mm)
338.1 120.2 321.2 315.3 120.9 328.4
⌬%⫹3⫺63 ⫺2.3 ⫺3.9 ⫺63 0
3600 MONTHLY WEATHER REVIEW VOLUME 134
duction in the total overlake rainfall is negligible
(⬃4%). Our results also show that the simulated rain-
fall averaged over the lake surface in the Qn-50 experi-
ment is only less than the control by about 3%, almost
similar to the Qs-50 experiment (Table 1), although the
reduction in the simulated rainfall amount seems to be
confined over the northern parts of the lake catchment.
However, the dominant impact of the large-scale mois-
ture entering the eastern boundary is demonstrated in
the results of Qa-50 (i.e., large-scale moisture via all
four lateral boundaries is reduced by 50%; Table 1),
where the corresponding changes in the rainfall pattern
and amounts over the entire basin are more similar to
those of the Qe-50 experiment (Fig. 10b).
d. Effects of lower-boundary forcing on Lake
Victoria basin climate variability
To isolate and understand some of the mechanisms
associated with interactions between topographic and
lake-induced circulations and their impacts on lake ba-
sin climate variability, we performed three experiments
where
(i) the high terrain all around the lake basin was
smoothed leaving maximum terrain height at 1300
m (TPALL), which is just above the lake surface
elevation;
(ii) only the terrain between the lake and Indian
Ocean (TPEA) was smoothed, as in (i);
(iii) effects of changes in the lake surface characteris-
tics on lake basin climate were investigated by re-
placing the lake surface with marsh/swamp (LBOG).
Our analysis of the response/sensitivity of lake basin
climate to the changes in lower-boundary forcing (de-
scribed in the list above) is based mainly on simulated
rainfall and vertical (omega) velocity. Unfortunately,
while the simulated overlake rainfall in the control run
is compared with TRMM satellite estimates, no obser-
FIG. 11. Simulated monthly mean rainfall over the lake basin in November (1998–2002) with large-scale moisture advected across
different boundaries reduced by 50% in the (a) control, (b) northern boundary, (c) southern boundary, and (d) western boundary
experiments.
DECEMBER 2006 ANYAH ET AL. 3601
vations of the vertical velocity or related fields were
available during this study to evaluate the simulated
fields. Figures 12a,b show the simulated mean vertical
velocity (November 2000) at 3 LST and 15 LST in the
TPALL experiment. Figure 12a shows that the western
sector of the lake is characterized by strong upward
motion, with maximum vertical velocity located ap-
proximately over the center of the lake (33°E). The
rising motion extends over a very deep column, from
the lake surface (⬃900 hPa) to around 300 hPa. This
upward motion is apparently associated with very deep
convection and produces significant amounts of rainfall
over the western sector as consistent with the control
simulations shown in Fig. 3. The eastern side of the lake
is, however, characterized by net subsidence. This flow
pattern is consistent with the expected diurnal circula-
tion and rainfall variability over the lake basin associ-
ated with lake- and land- breeze circulations (Figs. 6, 7,
8, and 9). Nevertheless, the region of the strongest ris-
ing motion is located over the center of the lake. Thus
in the absence of strong downslope winds over the high-
lands east of the lake (due to smoothed terrain), the
eastern branch of the land breeze is weakened and does
not extend farther to the west of the lake surface. Con-
versely, the western branch becomes relatively stronger
and extends more offshore into the interior of the lake
than in the control. This is consistent with the vertical
velocity difference (TPALL ⫺CTRL) presented in Fig.
12b that shows net subsidence over the western sector
of the lake and net uplift over the center of the lake.
The likely mechanism that contributes to the above
vertical flow characteristics can be explained as follows:
Smooth topography east of the lake leads to weaker
downslope flow. With weaker downslope winds, less
cooler air from the mountain tops is transported down
the valley to help in enhancing the lake–land thermal
gradient (i.e., weaker land breeze circulation) as com-
pared to the control. When the eastern branch of the
FIG. 12. Vertical (omega) velocity profiles in the TPALLexperiments
at 0300 LST for (a) TPALL and (b) TPALL ⫺CTRL.
FIG. 13. Same as Fig. 12, but at 1500 LST.
3602 MONTHLY WEATHER REVIEW VOLUME 134
land breeze is relatively weaker than normal, the west-
ern branch of the nocturnal circulation then becomes
relatively stronger and penetrates into the interior of
the lake as shown in Fig. 12a.
In the late afternoon (Fig. 13a), smooth topography
east of the lake catchment (TPALL) results in much
weaker upward motion. The rising motion (collocated
with the apparent lake breeze front) is also located
around 34.5°E, close to the lakeshore. A rather unex-
pected feature is the subsidence over the highlands far-
ther east of the lake (around 35°E). This possibly sug-
gests that the late afternoon thunderstorms experi-
enced over the NKK highlands (located about 70 km
east of Lake Victoria; Fig. 1) are significantly influ-
enced by a combination of orographic lifting of the
moisture embedded in the prevailing monsoon circula-
tions and moisture transport from Lake Victoria via
lake breeze circulation. The vertical velocity difference
(TPALL ⫺CTRL) in Fig. 13b also show that there is
net subsidence over the highlands east of the lake
(around 35°E) and net upward motion along the east-
ern lakeshore.
In the early morning (3 LST; Fig. 14a), when the
nocturnal (land breeze) circulation is expected to be at
its peak, the TPEA simulations show rising motion over
the center of the lake. However, the upward motion is
weak (only reaching a maximum of about 0.1 m s
⫺1
at
the center of the lake). Also, the subsidence (sinking
motion) to the east of the lake is very weak. The dif-
ference between TPEA and CTRL experiments (Fig.
14b) indicates net upward motion over the center of the
lake and net subsidence (sinking motion) over the west-
ern rim of the lake. The results also suggest that the
horizontal branch of land breeze circulation at the sur-
face originating from the western side of the lake is
relatively stronger than it is in the control run. Two
possible physical mechanisms may be responsible for
the simulated vertical velocity pattern. First, the steep
topography east of the lake influences the strength of
FIG. 14. Same as Fig. 12, but for TPEA experiment.
FIG. 15. Same as Fig. 12, but for TPEA experiment at 1500 LST.
DECEMBER 2006 ANYAH ET AL. 3603
the eastern branch of the nocturnal circulation (land
breeze). The cold downslope (katabatic) flow helps to
decrease the surrounding air temperature as it speeds
down the topography. This enhances the land–lake
thermal gradient and in turn intensifies the land breeze
circulation.
In the late afternoon (Fig. 15a), the ascending motion
to east of the lake is relatively weaker and more con-
fined to the shoreline (34°E) in the TPEA compared
with the control run. The difference between the verti-
cal velocities in TPEA ⫺CTRL shown in Fig. 15b also
indicates that there is net upward motion confined
along the eastern rim of the lake and net subsidence
(sinking motion) located slightly inland (between 34.5°
and 35.5°E). Possible mechanisms associated with the
differences in vertical motion between TPEA and
CTRL simulations could be explained as follows: The
reduced terrain height over the highlands east of the
lake results in weak upward motion (in the absence of
elevated heating). However, given the relatively weak
wind speeds over the lake (westerly winds), the lake
breeze front does not appear to extend farther inland
east of the lake as in the control but remains confined
along the lake perimeter. Furthermore, the lower the
terrain height over the east of the lake, the more pen-
etrative the prevailing easterlies into the lake basin,
leading to flow convergence closer to the lakeshore as
opposed to the control run. This implies that the el-
evated heating over the NNK highlands (Fig. 1) mostly
triggers strong upslope flow during the day. This may
play two major roles with respect to the strength, hori-
zontal extent, and convective activity associated with
the lake breeze circulation. First, the elevated heating
over the highlands to the east of the lake triggers strong
upslope flow, which then creates favorable conditions
for entraining (inducing) flow from the lake and thus
makes the horizontal branch of the lake breeze extend
farther inland than it would otherwise be. However,
FIG. 16. Same as Fig. 12, but for LBOG experiment. FIG. 17. Same as Fig. 12, but for LBOG experiment at 1500 LST.
3604 MONTHLY WEATHER REVIEW VOLUME 134
this still depends on the magnitude of the lake–land
thermal gradient. Second, due to orographic lifting and
its influence on the horizontal extent of lake breeze
explained above, the region of strong vertical motion
(lake breeze front) forms more inland in the control
case than in the case with less steep topography that
does not generate strong upslope winds.
e. Effects of changes in the physical characteristics
of Lake Victoria on basinwide climate variability
The impact of changes in the lake surface conditions
on the basinwide rainfall variability is examined by re-
placing the lake with bog/marsh (swamp; LBOG ex-
periment). Given the recent invasion of Lake Victoria
by the water hyacinth weed, this experiment tests a
realistic scenario of the lake surface conditions.
The simulated response of the lake basin circulation
at 0300 LST is characterized by weak upward motion
over the western sector of the lake (Fig. 16a) that ex-
tends from around the lake surface to about a 400-hPa
level. On the other hand, a relatively strong subsidence
is simulated to the east of the lake, extending from the
surface up to around 500 hPa. The LBOG minus CTRL
simulations show a net sinking motion over the center
of the lake (Fig. 16b) and extend over a very deep layer
(900–200 hPa). This is consistent with the fact that in
the LBOG experiment, less heat is retained during the
day compared to the CTRL experiment because of
changes in surface albedo, surface roughness, and ther-
mal capacity. Consequently, at night, the lake–land
thermal contrast that triggers the nocturnal circulation
is significantly suppressed. Hence, the nocturnal circu-
lation that is normally characterized by convergence
over the western sector of the lake (control) is sup-
pressed remarkably.
In the late afternoon, the upward motion to the east
of the lake is enhanced in the LBOG experiment (Fig.
17a) compared to the control simulations. This is pos-
sibly due to the fact that unlike the dynamic lake, bog/
marsh conditions trigger stronger evaporation since
they do not retain most of the heat received from solar
radiation. This leads to stronger evaporative cooling
that consequently creates a sufficient land–lake thermal
gradient, in turn driving a lake-breeze-like circulation.
This is consistent with results (see Fig. 20) showing that
the mean monthly evapotranspiration (mm) averaged
over the lake surface in LBOG is less than in CTRL by
almost 40 mm over the western sector of the lake. This
can be attributed to the overall changes in the surface
roughness, albedo, and thermal capacity of the lake
(marsh).
f. Anomalous rainfall response to lower-boundary
forcing
The simulated rainfall differences between LBOG,
TPALL, TPEA, and CTRL in November 2000 are
shown in Fig. 18. In Fig. 18a, the LBOG minus CTRL
FIG. 18. Sensitivity of Lake Victoria basin rainfall to surface
boundary conditions for (a) LBOG ⫺CTRL, (b) TPALL ⫺
CTRL, and (c) TPEA ⫺CTRL in November 2000.
DECEMBER 2006 ANYAH ET AL. 3605
is characterized by rainfall deficit over the western half
of the lake and rainfall surplus over the eastern shore-
line of the lake. However, immediately outside the lake
perimeter to the east, there is little or no difference
between the LBOG and CTRL simulations. The most
striking feature is that over the immediate land areas
east of the lake, the simulated amount of rainfall is
almost twice the amount in the control simulation.
Three possible mechanisms could be responsible for
the increase (decrease) in the rainfall amount simulated
over the eastern (western) shoreline when the lake is
replaced with bog/marsh. First, the significant reduction
in the simulated rainfall over the western sector could
be attributed to the anomalous subsidence over the
lake surface associated with the nocturnal circulation
pattern shown in Fig. 16. The subsidence of motion
over the lake during late night through early morning
hours could be due to the fact that the bog/marsh has
relatively lower thermal capacity. This means that the
“lake”will cool faster at night, significantly suppressing
nocturnal circulation as a consequence of a weaker
land–lake thermal gradient.
The second mechanism may be attributed to in-
creased evaporation over the lake surface during the
day. This leads to evaporative cooling, thus creating a
sufficient thermal gradient with the surrounding land
areas, which in turn drives a relatively strong circula-
tion (lake breeze like) directed toward the warmer sur-
rounding. However, this circulation is not as strong as
the lake breeze circulation simulated in the control and
thus has limited horizontal extent. This explains why
the approximate location of the lake breeze front (also
collocated with the region of maximum precipitation) is
close to the eastern shoreline in the LBOG simulations,
while in the CTRL it is located farther inland.
The third mechanism that may limit (enhance) the
penetration of the horizontal branch of the lake breeze
circulation inland in the LBOG experiment is due to
increased (reduced) nocturnal convection. Intense noc-
turnal convection leads to more precipitation over the
lake surface (western sector), thus triggering outflow
from the lake due to the rain-cooled overlake air. This
in return determines how far the horizontal branch of
the lake breeze circulation will penetrate inland in the
late afternoon.
It is also evident in Figs. 18b,c that when the maxi-
mum terrain height all around Lake Victoria is
smoothed (TPALL), less rainfall is simulated over the
eastern border and northern and northwestern sectors
of the lake compared to the control. On the other hand,
relatively more rainfall is simulated over the western
border of the lake, but confined just along the rim of
the lake (32°E). As shown earlier in section 3c, the
eastern sides of the lake catchment tend to benefit a lot
from moisture entering the lake basin through the east-
ern lateral boundary (western Indian Ocean). How-
FIG. 19. Differences in the distribution of simulated rainfall across the Lake Victoria
basin between CTRL and TPALL, TPEA, and LBOG experiments.
3606 MONTHLY WEATHER REVIEW VOLUME 134
ever, it is also evident from the vertical velocity fields
shown earlier in Figs. 11–14 that orographic lifting over
the eastern side of the lake may also help to organize
and enhance convection. The mountain breeze (up-
slope winds) to the east of the lake creates favorable
conditions for the interactions between large-scale
moisture transported via the prevailing easterly mon-
soons and moisture from the lake transported via the
land–lake breeze circulations. This interaction is signifi-
cantly suppressed when the terrain height is reduced.
However, over the western sector of the lake, the re-
duced terrain height to the east of the lake basin results
in relatively more large-scale moisture penetrating into
the lake basin, especially during nocturnal land breeze
circulations, and thus enhances precipitation over the
western half of the lake.
The rainfall anomalies associated with land surface
forcing are also exhibited in the horizontal (west–east)
cross section of rainfall distribution over the lake basin
(Fig. 19). It is apparent that the steep topography to the
east of the lake does influence the amount of rainfall
simulated over the entire basin, whereas the topogra-
phy to the west side of the lake only imposes a negli-
gible impact on the diurnal variability of rainfall over
the lake basin, at least during the short rains season.
Comparisons among LBOG, TPEA, TPALL, and
CTRL confirm that lake surface conditions significantly
influence the distribution and pattern over the lake ba-
sin. This is also manifested in the simulated evapotrans-
piration differences between LBOG and CTRL runs
(Fig. 20), where it can be seen that the mean monthly
evapotranspiration (mm) averaged over the lake sur-
face in LBOG is less than CTRL by almost 40 mm over
the western sector of the lake. This can be attributed to
the overall changes in the surface roughness, albedo,
and thermal capacity of the lake (marsh). Furthermore,
though the LBOG case experiences stronger evapora-
tion during the day, the nighttime component is signifi-
cantly suppressed. Thus, on the mean, the evapotrans-
piration in the control is relatively larger than in the
LBOG (Fig. 20).
4. Summary and conclusions
In this study, the downscaling ability of a fully
coupled RegCM3–POM system to reproduce the mul-
tiscale variability of Lake Victoria basin climate and the
associated physical mechanisms has been demon-
FIG. 20. Difference in mean evapotranspiration rate (mm) in LBOG ⫺CTRL in
November 2000.
DECEMBER 2006 ANYAH ET AL. 3607
strated. In general, the mean monthly rainfall simulated
over the lake basin, particularly over the lake surface, is
shown to reasonably agree with the satellite estimates
(TRMM data). The simulated diurnal cycle of rainfall
over the four quadrants of the lake shows a coherent
pattern with the TRMM diurnal rainfall fluctuations.
Rainfall peaks occur between midnight and early morn-
ing hours, and thereafter, a general decline in both
simulated and TRMM rainfall is witnessed over the
lake and areas within the immediate catchment. How-
ever, the simulated diurnal cycle tends to have midnight
through early morning rainfall peak a little earlier than
in the TRMM estimates.
Two mechanisms can be inferred from our model
results regarding interactions between topographic and
lake-induced circulations and the consequent impact on
lake basin rainfall variability:
(i) The steep topography east of Lake Victoria gener-
ates very strong downslope (katabatic) winds at
night since the air over the mountain top is rela-
tively colder than the air down the valley. As the
colder air from the mountain tops mixes with air
down the valley and air over land areas adjacent to
the lake, the thermal gradient between the land
surface and the lake is enhanced. Consequently, a
relatively stronger land breeze circulation is gener-
ated. The stronger the land breeze circulation, the
more favorable it is for strong convection and pre-
cipitation to develop over the central and western
sectors of the lake. The opposite is the case when
the terrain height is smoothed, as is also clearly
evident in our simulations.
(ii) The horizontal extent of the late afternoon lake
breeze circulation is also affected by steep topog-
raphy east of the lake. Due to high elevation, the
mountain tops heat faster during the day than the
surroundings. This creates a thermal low that in
turn induces significant upslope (anabatic) winds
on both sides of the mountain. The stronger the
upslope flow on the lee side of the mountains east
of the lake, the more this flow would tend to pull
(entrain) the lake breeze front farther inland. Hence,
when the terrain height is reduced/smoothed, the
horizontal extent of the lake breeze circulation also
reduces. This leads to significant reduction in the
simulated afternoon rainfall associated with the lake
breeze circulation in areas far away from the lake.
Thus, the simulated rainfall tends to be confined
along the eastern shoreline, but does not extend
farther inland compared to the control simulation.
The simulated response of the nocturnal circulation
to changes in the physical characteristics of the lake
(i.e., replacing lake with marsh) is characterized by
weak upward motion over the western sector of the
lake. This is consistent with the fact that when the lake
is replaced with marsh (or water hyacinth), it retains
less heat during the day due to changes in surface al-
bedo, surface roughness, and thermal capacity. Conse-
quently, at night, the lake–land thermal contrast that
triggers the nocturnal circulation is significantly sup-
pressed. Conversely, in the late afternoon, the upward
motion to the east of the lake (near the shore) is more
enhanced compared to the control simulations, possibly
due to the fact that the marsh conditions trigger stron-
ger evaporation since they do not retain most of the
heat received from solar radiation. This leads to stron-
ger evaporative cooling that consequently creates a suf-
ficient land–lake thermal gradient, in turn driving a
lake-breeze-like circulation.
The apparent role of the large-scale moisture trans-
ported via the prevailing easterly monsoons in enhanc-
ing precipitation over the lake basin is also clearly
manifested in our simulations. This is evident in the
simulated rainfall patterns and by the amounts in the
runs with large-scale moisture entering the four lateral
boundaries of the interior domain (Lake Victoria ba-
sin) systematically reducing or increasing. A more strik-
ing result is that large-scale moisture advected through
the eastern boundary (located across the equatorial
western Indian Ocean) enhances overlake rainfall, es-
pecially over the surrounding land areas to the east and
southeast of the lake. However, a rather surprising re-
sult is that there is negligible influence on the basinwide
(particularly overlake) rainfall variability because of
changes in large-scale moisture entering the lake basin
domain through the western, northern, and southern
boundaries.
Acknowledgments. The valuable comments made by
Jared Bowden, Robert Mera, Neil Davis, and Baris
Onol on the original manuscript are highly appreciated.
We would also like to thank the three anonymous re-
viewers for their insightful comments that led to signifi-
cant improvement of the original manuscript. This re-
search was supported by NSF Grant ATM-0438116.
The model experiments were performed on the Na-
tional Center for Atmospheric Research (NCAR) su-
percomputers and at the North Carolina State Univer-
sity High Performance Center. NCAR is sponsored by
the National Science Foundation.
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