Lake and Land Breezes on Lake Towuti and Their Effect
on Convection in Larona Catchment Area, Sulawesi, Indonesia
Findy Renggono, Erwin Mulyana, Tri Handoko Seto, Budi Harsoyo, Djazim Syaifullah
[email protected]
Weather Modification Technology Centre, Agency for the Assessment and Application of Technology, Indonesia
1.
Introduction
Nickel production at the Sorowako plant is
dependent on the availability of water from the lakes
surrrounding Sorowako in Sulawesi. Low water levels
in the lakes feeding the hydro-power stations at
Larona and Balambano could limit the amount of
hydropower more than those which can be
generated. In order to overcome this problem, PT
INCO has been conducted cloud seeding for
precipitation enhancement purposes for the region.
Previous studies found that lake breezes effects
seemed to enhance rainfall during the night along the
shores of Lake Matano (NCAR, 2006), but there are
no further studies regarding lake breezes effect in
this region.
From studies of Lake breezes, it was found
that lake and land breezes could best develop only
when the synoptic or gradient winds are very light to
non existent, and they need partly to mostly clear
skies to allow solar heating to warm the land. Laird
et al. (2001) showed in Michigan Lake that the firstdegree factor influencing the lake breeze is synoptic
rather than local (thermal gradient) and also found
negative correlation between the wind speed of the
opposing synoptic wind and the sea breeze
frequency. Scott and Huff (1996) found that the
presence of Lake Michigan imposed a net cooling of
summer mean temperature as large as 28oC within
80 km of the shoreline. Individual lake-breeze events
could also play an instrumental role in providing cool
temperatures to relieve metropolitan areas in coastal
regions, such as Chicago, Illinois, and Milwaukee,
Wisconsin, during intense heat waves (Kunkel et al.
1996).
The Larona Catchments Area (LCA) is located
at South Sulawesi, Indonesia (Fig. 1). It is composed
of a large body of water, three cascade lakes, and
land. This composition is believed to have prevented
potential cumulus cloud to develop originally within
the LCA. It seems that potential cumulus clouds
which develop into rain cloud or thunderstorm within
the LCA, originally came from outside of the LCA by
way of advection.
The purpose of this study is to investigate the
characteristics of lake breezes and their effects on
convection in the Larona catchments area.
Figure 1. Map of Larona Catchment Area showing the
location C-Band Radar (black-shaded square), and
surface stations (red circle).
Tabel 1. Location of Radar and AWS
Station Name
C-band Radar
Timampu
Plant Site
Tokalimbo
Matano
Nuha
2.
Latitude
2.53S
2.66S
2.57S
2.81S
2.46S
2.45S
Longitude
121.35E
121.43E
121.38E
121.57E
121.22E
121.34E
Data and Method
The LCA as the target area for cloud seeding
program, is about 2,477 km2. There are three lakes in
the LCA, namely the Matano (166 km2), Mahalona
(24 km2) and Towuti(562 km2) lakes. The Map of LCA
can be seen in Figure 1.
This study used a variety of data to examine
Lake Towuti lake breezes. Surface wind
observations, rain gauge and weather radar are
used. In this abstract paper, it is only observation on
March 2010 is shown, but the result from observation
on November 1999 to March 2010 will be presented
during the conference.
Tabel 2. Radar data availability
Hourly surface wind observations were used
to investigate lake breezes events. There are 5
locations of automatic weather stations around the
catchments area (red-circle on Figure 1). The
Latitude and longitude coordinate of each stations
are shown in table 1.
The C-band radar is located at the south part
of Lake Matano (black-square on Figure 1). It is used
to detect the precipitating cloud and their
propagation. The radar observation was only
available during the cloud seeding project, i.e. only
on daylight. Radar data availability is also shown in
Table 2.
3.
Precipitation climatology
The Geographical position of LCA, in which it is
close to the equator and located between Bone Gulf
at west side and Tolo Gulf at east side, causes
unique rain characteristic. Rain might occur in either
wind condition, westerlies or easterlies direction.
Larger water body (3 lakes) inside the catchments
area causes local dynamic circulation which
influences weather pattern and rain occurrence in
this area significantly.
Based on the result of Aldrian et.al. (2003)
which analyzed rainfall data using the REMO model,
this region has rainfall type between the antimonsoonal and monsoonal type, and also has a
strong influence on El-Nino event. This causes the
peak of the rainy season to occur in April-May and
the peak of dry season occurring in August to
October.
Figure 2 shows annual variation of rainfall in the
plant site from 1977 to 2006, located about 7 km
southeast of the radar site. It is shown here that the
peak of rainfall occurs on April (dark shading) and the
dry condition (without shading area) occurring around
Figure 2. Annual Variation of Rainfall at Plant Site
September-October. Rainfall peak reached more
than 400mm/month on 1980-1981, 1991-1995, and
2001-2005, whereas on strong El-Nino year (1997),
rainfall seemed fewer than other years.
4. Case study on 10 March 2010
El Niño phenomenon appeared in weakened
to moderate strength in March 2010. Also the
suppressed convection lessened in Indonesia
(Maritime Continent) in early March. In mid to late
March, convective cloud formations were much
better. Therefore, cumulus clouds could developed
well in the LCA.
There was tropical depression (TD) east of
the Philippine and TC IMANI south west of southern
part of Sumatera. Wind direction was mostly
northeasterlies, while sea level pressure ranged
Figure 3. Surface wind at Timampu on 10 March 2010
12-17 LT (left) and 19-23 LT (right)
Figure 4. Surface wind on 10 March 2010 12-17 LT at
Plantsite (left) and Nuha (right)
about 1012 – 1014mb and 1010 mb, in early March
and mid to late March respectively.
Surface wind observations are plotted at 5
stations. Timampu station, which is located in the
northwestern shore of Lake Towuti, detected
northerlies and southeasterlies wind on 12 -17 LT.
Southeasterly wind is suspected to originate from
Lake Breeze. But in the contrary, land breeze are
found during night time. Figure 3 shows the wind
rose for Timampu station during daylight (12-17LT)
and night time (19-23 LT). The northerly component
Figure 5. Radar reflectivity on 10 March 2010, 11.25 LT.
Cross is radar location, and shaded circle is Timampu
station.
Figure 6. Time-Longitude of radar reflectivity at 2.65oS on
10 March 2010 10.00-18.00. Solid line is Timampu.
during the daylight maybe related to cumulus clouds
which developed southwest side of Timampu station.
Lake breezes are also found at Tokalimbo
station, where wind dominantly blew from the west.
The other three stations (Plantsite, Nuha and
Matano) could not identify Lake Breeze event clearly
due to topographic and weather conditions (Fig. 4).
Figure 5 is the radar reflectivity composite on
11:25 LT. This figure shows that on LCA,
precipitation cloud occurred at west Timampu, west
radar site and north site of Lake Matano. The cloud
propagation is from west to east.
The lake breezes met the warm air and
produce a lake breezes front at the lake shore. Cloud
formed over the Timampu area and surface
observation also detected rainfall on 12 LT.
Temperature drop was also found during this time.
The west-east propagation of cloud over Timampu
station is shown in figure 6.
5. Observation of Lake Breeze on March 2010
During March, potential clouds are more
frequently observed. Radar observation results from
5th to 19th March showed that during those days
potential clouds occurred over Larona catchments
area. Clouds growth are mostly in northeast and
South of East Matano, and also Southeast to West
Towuti, and Sorowako area. During the period of
20th to 31st March, potential clouds started to grow
at daylight in north Matano, northeast Mahalona,
west and southeast of Towuti. The potential clouds
formation started at daylight period. Potential clouds
formation occurred at west, north to northeast of
Matano area and then at north to east of Mahalona
area and also north, west, northeast, and southeast
of the Towuti area. These potential clouds entered
the Larona catchment area at early afternoon.
Surface wind on March 2010 at Timampu shows
that the wind mostly blew from the northwest-north
direction, but during daylight it is was also found that
the south easterlies component was dominant.
Figure 7 shows the time-wind direction for March
Figure 7. Surface wind direction at Timampu on March
2010.
Figure 9. Accumulation of radar reflectivity for March 2010.
Figure 8. Wind Rose for Timampu (upper left), Plantsite
(upper right) and Tokalimbo (lower left) on March 2010,
12-17LT
2010. The wind direction on 09.00-18.00 is from ESE
to SSE.
Windrose diagram at Timampu for March 2010
12-17 LT also shows that wind direction
southeasterly dominant (fig. 8 upper-left). This means
that in the afternoon most of the lake breeze were
found at Timampu.
Tokalimbo has westerlies and northerlies
component (Fig. 8 lower left). Lake breeze at
Tokalimbo was not too strong, as there is a small of
island in the western part of the station. At Plantsite
the wind came dominantly from the north part of the
station. The wind at this station had less impact from
Lake Breeze.
The accumulation of radar reflectivity for March
2010 is shown in figure 9. On March 2010,
precipitation cloud mostly accumulated at the north
side and southwest side part outside the catchments
area. Inside the catchments area there are only few
clouds were found over Lake Towuti and Matano.
The precipitating clouds were found west of Lake
Towuti
6.
Summary and Discussion
The Lake Breezes events on Larona catchment
area were investigated. Preliminary result of one
month surface wind observation found that the
Timampu and Tokalimbo station, which is located at
lake shore of Lake Towuti, are affected by Lake
Breeze. The distribution of precipitating cloud
observed by radar also showed that the cloud
propagation from west to east were blocked by the
lake breezes, and the cloud formed over the lake
shores.
In the near future, the study of lake breeze in
this region will be done by extending the
observational data and also discovering appropriate
method to investigate lake breeze event.
Acknowledgements
This work was carried out as a Weather Modification
Project, funded by the Weather Modification technology
Center, BPPT and PT Valle Inco. The authors would like to
thank Mr. Andi Suntoro of PT. Valle Inco for used of the
radar and Mr. Kaimudin for providing the surface
meteorological data. They thank to Mr. Krisna Aditya for his
careful reading of the original manuscript with constructive
comments.They would also like to thank all UPTHB staff for
data collection and making the climatological data used in
this study available.
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