## 2 Rainfall Data Source
### Weather Radar
#### 2.2.2 Weather Radar Product  
Weather radar has product features that can be utilized as needed. Weather radar's standard image products include the following:  
* Plan Position Indicator (PPI) This product is displayed as a cone-shaped sector projected onto a two-dimensional (2D) map. The weather radar antenna rotates to scan radial tracks. The distance from the radar and height above the ground is depicted as concentric circles [51]. Since the PPI requires an elevation angle, the beam height increases as the distance grows. The heights of displayed data differ depending on the distance. To obtain maximum results from PPI products in Indonesia, it is necessary to pay attention to height, angle, and distance when scanning weather radar [52]. This is important because PPI products can experience coalescence with strong echo clutter at close distances at low elevations, which results in the product having a high level of uncertainty. The PPI display has a high level of uncertainty, which can affect rainfall estimation [53]. * Constant Altitude PPI The constant altitude plan position indicator (CAPPI) is a slice of volume data at a constant altitude. The elevation scan data values at the lower and upper levels are linearly interpolated to estimate the data at the CAPPI altitude. The advantage of this product is that it reduces ground clutter around the radar location. The amount of ground clutter around the radar decreases substantially with increasing altitude. A distance of 3.5 km is found to be the most suitable and sufficient application of the CAPPI to avoid ground clutter [54]. The application of CAPPI products in Indonesia needs to consider the distance between the radar and the area being observed. A study in Indonesia [55] showed that a CAPPI applied at a distance of more than 50 km can produce poor-quality data, considering that long distances can reduce data accuracy [56]. * Column Maximum  
Column maximum (CMAX) is a product that shows the maximum data value in a particular location. The displayed value is the maximum value of the observed volume scanning data. A CMAX product converts a set of polar volumes to Cartesian volumes and displays the maximum value for each vertical column. Physically, CMAX represents the maximum amount of possible rainfall in an area [57]. Meteorologists can view the estimated worst-case scenario overall by using the CMAX product instead of comparing several two-dimensional images from multiple layers and three-dimensional products. This tool helps in the observation of high-intensity rainfall from convective clouds, which often occurs in Indonesia [47]. Several studies in Indonesia have shown that the use of CMAX produces a more representative product in estimating rainfall [52, 55]. A comparison of the mechanisms and results of radar image products is shown in Figure 3 and Table 1.  
Estimating rainfall using radar in Indonesia is a complex process influenced by factors such as topography, distance from the radar, and Z-R relationships. Topography affects the propagation of radar signals, especially in mountainous areas, which can cause radar shadows and bias rainfall estimates [61]. Distance is also important because increasing the distance from the radar can reduce the estimation accuracy due to signal attenuation and divergence [62]. The Z-R relationship, which relates radar reflectivity to rainfall intensity, is often adapted to local conditions to improve estimation accuracy [63]. Dual-polarization radar has been introduced, producing significant improvements in rainfall estimation using additional information such as differential reflectivity (ZDR) and phase differential ratio (KDP) which can increase precision and reduce errors in rainfall estimates in various topographic conditions and distances [64]. Figure 3: Comparison of radar imagery products and illustrations of the PPI, CAPPI, and CMAX products: (**a**) radar imagery of PPI dBZ products at an elevation of 0.5\({}^{\circ}\), (**b**) CAPPI dBZ products at 0.5 km, and (**c**) CMAX products (modified from [58]).