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

frm_68@yahoo.com

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 first-degree 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.

2. 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.

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 Latitude Longitude C-band Radar 2.53S 121.35E

Timampu 2.66S 121.43E Plant Site 2.57S 121.38E Tokalimbo 2.81S 121.57E

Matano 2.46S 121.22E Nuha 2.45S 121.34E

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 anti-monsoonal 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

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 2. Annual Variation of Rainfall at Plant Site

Tabel 2. Radar data availability

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

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 4. Surface wind on 10 March 2010 12-17 LT at Plantsite (left) and Nuha (right)

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.

Figure 7. Surface wind direction at Timampu on March 2010.

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. References

Aldrian, E., R. D. Susanto, Identification of three dominant

rainfall regions within Indonesia and their relationship to sea surface temperature, International Journal of Climatology, 23, 1435-1452, DOI: 10.1002/joc.950, 2003

Kunkel, K. E., S. A. Changnon, B. C. Reinke, and R. W. Arritt, The July 1995 heat wave in the Midwest: A climatic perspective and critical weather factors. Bull. Amer. Meteor. Soc., 77, 1507–1518, 1996.

Laird, N.F., Kristovich, D.A.R., Liang, X.Z, Arritt, R.W., Labas, K., Lake Michigan Lake Breezes: Climatology, Local Forcing, and Synoptic Environment, J. Appl. Meteor., 40, 3, 409-424, 2001.

NCAR, Feasibility Study for the Augmentation of Rain in Sulawesi, Final Report, 2006

Scott, R. W., and F. A. Huff, Impacts of the Great Lakes on regional climate conditions. J. Great Lakes Res., 22, 845–863, 1996.

Figure 8. Wind Rose for Timampu (upper left), Plantsite (upper right) and Tokalimbo (lower left) on March 2010, 12-17LT

Figure 9. Accumulation of radar reflectivity for March 2010.