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Results from the outputs of a climate model are also used in this paper. The simulation is used to show the heavy rainfall that hit Karangkobar area in Banjarnegara around the time
of the landslide event. The model used for the simulation of the atmospheric condition is the quasi-uniform Conformal-Cubic Atmospheric Model, known as C-CAM McGregor Dix,
2008. C-CAM is a hydrostatic model with two-time-level semi-implicit time differencing, employing semi-Lagrangian advection with bicubic horizontal interpolation on unstaggered
grid. It can be used to simulate a specific region with high resolution for example: 5 km x 5 km, while a coarser grids are used outside that region.
2.2 Method
The brightness variations obtained from the sensors of MTSAT are converted to the equivalent blackbody temperatures on the top of clouds using a calibration scale provided by
the Japan Meteorological Agency JMA as the owner of the satellite. A two-dimensional threshold diagram 2d-THR from Suseno Yamada 2012 is used to classify the cloud type.
Using this method the cumulonimbus cloud is identified. Cumulonimbus is a cloud type that is associated to a heavy rainfall.
3. Results and discussion
The rainfall time series from CMORPH in Banjarnegara area leading to the landslide on 12 December 2014 is shown in Figure 1. The CMORPH data are in 3-hour interval. It can be
seen from that figure that there had been heavy rainfall in Banjarnegara area days before the landslide disaster struck the Karangkobar sub-district, in particular on the 4th and 11th
December 2014. This is in agreement with Sipayung et al. 2014 that landslide is triggered by a threshold of rainfall accumulation 15 days and 3 days before the event.
Figure 2 shows cloud-top temperature in Kelvin of cumulonimbus over Java on 11 December 2014 at 9 LT. The darker the cloud is the lower the temperature and the denser
the cloud. A denser cumulonimbus usually brings intense rainfall. The cumulonimbus cloud Figure 2 and rainfall timeseries Figure 1 over Banjarnegara on 11 December 2014 occurred
around the same time. A series of cumulonimbus clouds were observed above the Banjarnegara area shown in black dot in the figure before the landslide event on 12
December 2014. The cumulonimbus clouds covered most of Central Java, including Banjarnegara several days before the event, such as that on 11 December 2014.
Consequently, prolonged and heavy rainfall occurred over those areas. If this happens on a land that is geologically unstable, such as unprotected slope, it would likely trigger a
landslide.
Figure 1. Rainfall records mmhour in Banjarnegara area leading to the landslide on 12 December 2014. Data are from CMORPH with 3-hour interval.
Meteorological analysis of the Banjarnegara Landslide on 12 December 2014
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Figure 2. Cloud-top temperature K of cumulonimbus over Java on 11 December 2014 at 9 LT. The black dot indicates Banjarnegara area.
The prolonged coverage of cumulonimbus over Central Java was made possible by the low-level 850 hPa wind around Java. Figure 3 shows the low-level wind over Java on 11
December 2014 at 7 LT. The horizontal wind from the south coast of Java was directed towards the land including the Banjarnegara area black dot in Figure 3. This wind kept
pushing water vapor from the Indian Ocean towards the area. This condition sustained the cumulonimbus clouds over most Central Java.
The meteorological condition over Java was simulated using C-CAM model with two resolutions. One has 8 km resolution with a domain covering most Java Island, the other is at
finer resolution of 1 km over Karangkobar Banjarnegara area, as shown in Figure 4. The 8- km simulation shows the rainfall coming from the north coast of Central Java Figure 4
bottom panel. At finer resolution of 1 km Figure 4 top panel, a heavy rainfall is seen around Karangkobar area. This rainfall simulation using a climate model shows the usefulness of such
model in replicating the observed meteorological condition.
Early warning system is one way to mitigate the impact of natural disaster such as landslide. This can be carried out using precision weather forecast. In this context, precision
means the forecast is available at more frequent interval such as every one hour and at a fine spatial resolution such 5 km x 5 km area. Such system has been provided by the
National Institute of Aeronautics and Space LAPAN, and can be accessed through http:sadewa. sains.lapan.go.id. By identifying areas that areas that are prone to landslide,
the weather forecast can be used to give warning to people living on that particular areas.
Figure 3. The horizontal wind ms at 850 hPa level above Java on 12 December 2014 at 00 UT or 07 LT.
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Figure 4. Rainfall mm and horizontal wind ms simulation over Karangkobar area at 1-km resolution top panel and over Java at 8-km resolution bottom panel for 12
December 2012 at 9 pm.
4. Conclusion and remarks
