(10), each operation needs to calculate R i separately, multiply by α i , and then accumulate. However, in equation (11), only rainfall intensity and accumulated rainfall are required, which greatly reduces the amount of calculation and contributes to subsequent large‐scale calculations.
2.4. RESULTS
2.4.1. Overall Performance
Figure 2.3 shows the spatial distribution of KGE' for different precipitation products in China. IMERG_uncal represents the estimated value of IMERG microwave infrared without gauge adjustment, while IMERG_cal combines IMERG_uncal and GPCC gauge analysis on a monthly basis. The comparison between IMERG_cal and IMERG_uncal can clarify two important questions: (1) What is the best quality that can be achieved by pure satellite estimation, and (2) How useful is the rain gauge adjustment?
SM2RAIN has the worst performance compared to other products. About 90% of the rain gauges have a KGE' less than 0.6; this is expected because the rain gauge information is not included. SM2RAIN achieved the highest KGE' value in the central region, which is equivalent to areas with less rainfall in Inner Mongolia. In contrast, satellite products such as IMERG_uncal, PCDR, and CHIRPS perform best in areas with ample precipitation. We assume that the reason could be SM2RAIN relies on soil moisture to estimate precipitation, and in humid areas, the soil is usually saturated and cannot well reflect changes in precipitation. Due to the influence of snow and permafrost, SM2RAIN lacks TP data.
Figure 2.3 Spatial distributions of KGE' on the daily scale from 2000 to 2018 for various precipitation products in China,
Source: Based on Tang et al. (2020), Figure 01B, p 03 / Elsevier. based on Tang et al. (2020).
Unlike other products, PCDR and CHIRPS estimate precipitation mainly using infrared data and show lower accuracy among other products due to the physical limitation of infrared data compared to microwave data. IMERG_cal and GSMaP are the best two in most regions according to metrics. The performance in TP and XJ, where the topography and climate often decrease the quality of satellite precipitation retrieval, is also acceptable. CMORPH is only worse than IMERG_cal and GSMaP. The comparison between IMERG_cal and IMERG_uncal shows that the ground‐based correction of IMERG_cal improves its KGE'. The improvement is particularly significant in TP, XJ, and NE, where IMERG_uncal exhibits degraded quality.
ERA5 has the highest KGE', followed by ERA‐Interim, and finally MERRA2. Due to the rough terrain, reanalysis products are not adequate in western China. As the altitude increases near the TP boundary, the quality of MERRA2 is significantly worse than ERA5 and ERA‐Interim. However, in the low‐lying Sichuan Basin (29°–32°N and 103°–108°E) eastern to TP, MERRA2 shows a normal performance.
Figure 2.4 shows a box plot of five indicators (CC, ME, RMSE, CSI, and KGE'). IMERG_cal has higher median metrics and degree of aggregation than IMERG_uncal, especially for ME. However, the improvement is slight for CSI. This shows that the gauge adjustment is very effective for improving the intensity estimation on a daily scale but only has little contribution to improving event detection. As far as all indicators are concerned, IMERG_cal is superior to all products except the GSMaP. The similarity between the GSMaP algorithm and IMERG is high; therefore, the better performance of GSMaP is due to the difference between the daily correction of GSMaP based on CPC analysis and the monthly correction of IMERG based on GPCC.
Figure 2.4 Boxplots of five metrics at the daily scale from 2000 to 2018 based on more than 2000 rain gauges in China. The bottom and top edges of boxes indicate the 25th and 75th percentiles, respectively. The central black line indicates the median. The dashed lines extend from the interquartile with a length of 1.5 times box width. The dots are outliers.
Source: Based on Tang et al. (2020).
CHIRPS and PCDR are both infrared‐based long‐term climatological data records, but CHIRPS has a higher spatial resolution than PCDR. Since CHIRPS is slightly better than PCDR, as shown in Figure 2.4, it could be a better choice of long‐term satellite data sets than PCDR in China. For reanalysis products, the overestimation is very significant compared with satellite products with respect to the larger ME values, whereas their CSI values are preceded only by GSMaP, indicating that reanalysis products are superior in precipitation occurrence detection. Concerning KGE', reanalysis products perform better than infrared‐based CHIRPS and PCDR and soil‐moisture‐based SM2RAIN but worse than microwave‐infrared combined products, including IMERG_uncal.
Figure 2.5 Taylor diagrams on the daily scale from 2000 to 2018 for different seasons and regions, including whole China, TP, XJ, and NE. “OBS” on the x‐axis represents gauge observations.
Source: Based on Tang et al. (2020).
2.4.2. Regional and Seasonal Characteristics
In this section, two types of diagrams, i.e., Taylor diagrams and performance diagrams, are used to assess the seasonal and regional bias features of various products. Three metrics (CC, standard deviation, and centered root mean square difference [RMSD]) are synthesized in the Taylor diagram. The distance between points and the “observation” point is inversely proportional to the accuracy (Figure 2.5). Four metrics, i.e., POD; CSI; success ratio, which equals one minus FAR; and bias score, which equals POD divided by success ratio, are integrated into the performance diagrams. The distance between points and the top right corner is inversely proportional to the accuracy (Figure 2.6).
Figure 2.6 Performance diagrams on the daily scale from 2000 to 2018 for different seasons and regions, including whole China, TP, XJ, and NE. Blue lines represent bias scores. Green lines represent CSI.
Source: Based on Tang et al. (2020).
GSMaP shows the best performance except during winter in TP, XJ, and NE, as shown in Figure 2.5. IMERG is relatively weak in estimating precipitation under cold climate, e.g., during winter in China and XJ and during spring, autumn, and winter in NE. Summer is the only season in NE when the temperature is relatively high and precipitation
