*3.4. Evaluation Using Error Metric*

The MC RMSEs for LTwd, LTd and LTn of each product are illustrated in Figure 6(a1–a5). For MC, TRMM3B42, TRMM3B42RT, PERSIANN, PERSIANN-CCS, and MERRA-2, RMSEs for annual LTwd, LTd and LTn were lowest (<20.00 mm/yr for LTwd; <10.00 mm/yr for LTd and LTn); this indicates that the accuracy of the five products, especially TRMM3B42, in detecting annual LTs is better. Except for PERSIANN-CCS and MERRA-2, the slightly smaller differences in annual RMSE for LTd and LTn from the remaining products suggests a comparable accuracy at daytime and nighttime. For LTwd, LTd, and LTn, the largest RMSEs for each product occurred in summer and the smallest occurred in winter, due to their larger and smaller portion of annual MC precipitation, respectively. In each season, the minimum RMSE of the three LTs generally came from TRMM3B42 and MERRA-2, however, larger RMSEs were frequently found for GSMaP-RNL, GSMaP-RNLG, JRA-55, ERA-5, NCEP1, and NCEP2. Comparing RMSEs of LTd and LTn for each product in each season, LTd for most of the products exhibited larger and smaller values in summer and the other three seasons, respectively; it should be noted that differences between LTd and LTn were not evident, excluding PERSIANN-CCS, which had an absolute difference > 2 mm/yr in winter.

**Figure 6.** MC root mean square error (RMSE) derived from the selected 12 precipitation products (**a1**–**a5**), RMSE-based optimal products (OPs) for MC and ten WRRs (**b1**–**b5**), and number of cases corresponding to RMSE-based OPs for annual or seasonal scale in ten WRRs (**c1**–**c5**). In figures (**b1**–**b5**), the number of each box represents RMSEs (mm/yr) of OP, which are labelled with different colors. The number of figures (**c1**–**c5**) indicates the amount of a certain OP.

Figure 6(b1–b5) illustrate the RMSE-based OPs of LTs for MC and WRRs. In general, the MC RMSE-based Ops for the three LTs for annual, spring (excluding LTwd) and summer were TRMM3B42, while MERRA-2 was the autumn OP (excluding LTn) and winter OP. At the annual scale, 13 cases had an RMSE-based OP of TRMM3B42, generally in southern WRRs, YRB, and HuRB; the remaining four WRRs had MERRA-2 as their OP (Figure 6(b1,c1)). In spring, the RMSE-based OP for the three LTs in northern WRRs and LTwd in southern WRRs was MERRA-2, corresponding to 21 cases; eight cases had TRMM3B42 and PERSIANN as their OP, and these mainly appeared in LTd and LTn of southern WRRs (Figure 6(b2,c2)). During summer (Figure 6(b3,c3)), with three exceptions, all cases had TRMM3B42 (19 cases) and MERRA-2 (eight cases) as their OPs. In autumn, TRMM3B42 was the OP in seven cases mainly in southern WRRs (except for LTwd and LTd in YZRB and SERB), followed by MERRA-2, which was the OP in 21 cases (Figure 6(b4,c4)). With the exceptions of LTwd and LTd in YZRB and LTwd in SWRB, MERRA-2 was the OP in 27 cases in winter (Figure 6(b5,c5).
