## 2 Rainfall Data Source
### Weather Radar  
#### 2.2.1 Weather Radar Type  
Based on their frequency bands, weather radars are divided into three types: S, C, and X bands. Each type has its characteristics, advantages, and disadvantages [45]. Currently, weather radars in Indonesia are dominated by C-band types and several X-band ones. These have been adjusted to account for diverse topographical conditions [46].  
* The S-band weather radar operates at a 2.7-2.9 GHz frequency and has an 8-15 cm wavelength. Since attenuation has little effect on this type of radar, it has a range of up to 300 km. However, due to its wide beam width, its quantitative precipitation estimation (QPE) range only approaches 200 km [45]. While the S-band weather radar has advantages, due to several factors it may not be as suitable as the C-band radar if used in Indonesia. The S-band works at low frequencies with large lambda, so the penetration is deep because the attenuation effect is negligible. However, with a large lambda, the resolution and accuracy of the measurement results are low. Therefore,  
Figure 2: The most commonly operated rain gauges in Indonesia.  
while using the S-band radar has some benefits, the C-band radar remains the best option for weather monitoring and forecasting in Indonesia.
* The C-band weather radar operates at a 5.6-5.65 GHz frequency with a 4-8 cm wavelength. It can detect rainfall up to a distance of 200 km. The signal attenuation received is significantly stronger than that of the S-band radar, limiting the QPE range to 100-150 km [45]. The C-band weather radar is widely used in Indonesia because it effectively detects and monitors the types of precipitation commonly found in the region, such as convective storms, tropical cyclones, and heavy rainfall events [47, 48]. For many years, the C-band radar has also been extensively used in Indonesia, and the country has established a reliable network of C-band radar stations [34]. The data from these radar stations are used by the Indonesian Meteorology, Climatology, and Geophysics Agency (BMKG) to provide accurate weather forecasts and early warnings of severe weather events to the public [49].
* The X-band weather radar functions at a 9.3-9.5 GHz frequency and has the smallest wavelength of 2.5-4 cm. It is more sensitive to hydrometeors than the S-band or C-band weather radars. The X-band radar measurement range can extend up to 50 km. Signal attenuation caused by rain is the strongest in the X-band radar compared with that of the S-band and C-band radars, greatly limiting QPE. Accurate QPE is usually achieved at 30 km [45]. The X-band weather radar is not commonly used for weather monitoring in Indonesia due to several factors, such as its limited range and susceptibility to attenuation. Because the X-band radar has a much shorter range than other radar frequencies, it is less practical for weather monitoring over large areas, such as the Indonesian archipelago [35]. Another limitation of the X-band radar is its susceptibility to attenuation, which occurs when the radar signal is absorbed or scattered by particles in the atmosphere. As mentioned, attenuation can result in critical data loss and erroneous forecasts, especially in high-precipitation regions, such as Indonesia [50].