# Comparison of GPM IMERG and TRMM 3B43 Products over Cyprus
## 4 Results and Discussion
### Influence of Elevation on Satellite Precipitation Products  
To analyze further the influence of elevation and satellite precipitation products, we grouped all the rain gauge stations into three categories according to their elevation (0-300 m, 300-600 m, \(>\)600 m), and compared the evaluation metrics across the different elevation ranges. The annual rBias results for the both GPM and TRMM products are presented in Tables 5 and 6, respectively. Minimum and maximum respective values are provided in brackets along with the corresponding number of satellite products cell for each elevation category. Results as presented in Tables 5 and 6, respectively, portray that, on an annual scale, both of the two SP products overestimate the precipitation below an altitude of 300 m, with IMERG presenting the largest overestimation (mean annual RB values: RBIMERG = 123.8%, RB3B43 = 107.7%). On the contrary, both SP products underestimate precipitation with increasing elevation, with 3B43v7 displaying a more apparent underestimation than IMERG, with RBIMERG = 93.0%, RB3B43 = 78.0% and RBIMERG = 70.3%, RB3B43 = 65.6% for elevation ranges between 300 and 600 m and \(>\)600 m, respectively. Regarding the Pearson correlation coefficient, the performance for 3B43 v7 was slightly better than IMERG at all of the categories, with \(r\) values increasing with elevation. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **TRMM 3B43** & **GPM IMERG** \\ \hline R (correlation coefficient) & 0.81 (0.46–0.99) & 0.82 (0.37–0.99) \\ \hline Mean Error (Bias) & \(-\)30.5 (\(-\)219.3–77.0) & \(-\)17.9 (\(-\)324.4–132.2) \\ \hline Relative Bias & \(-\)3.0 (\(-\)38.6–35.0) & 3.1 (\(-\)45.6–58.7) \\ \hline RMSE & 94.7 (37.5–234.8) & 108.8 (32.3–335.0) \\ \hline Mean Absolute Error & 83.4 (30.2–219.3) & 96.1 (25.2–324.4) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Performance evaluation metrics of annual data from 3B43V7 and IMERG and (the range of values is given in parentheses). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **All Annual** & **2014** & **2015** & **2016** & **2017** & **2018** \\ \hline \multirow{2}{*}{\% of gauge} & 123.8 & 133.8 & 133.8 & 127.3 & 118.1 & 119.6 \\  & (90.9–158.7) & (93.6–234.0) & (86.5–185.2) & (79.7–175.1) & (76.4–196.8) & (77.7–200.2) \\ \hline \multirow{2}{*}{correlation} & 0.90 & \multicolumn{6}{c}{Elevation: 0–300 m (30 cells)} \\  & (0.78–0.95) & \multicolumn{6}{c}{} \\ \hline \multirow{2}{*}{\% of gauge} & 93.0 & 96.3 & 107.1 & 89.8 & 93.7 & 81.9 \\  & (68.5–131.1) & (68.8–144.7) & (74.5–145.2) & (58.5–127.9) & (68.5–145.4) & (52.6–143.4) \\ \hline \multirow{2}{*}{correlation} & 0.91 & \multicolumn{6}{c}{Elevation: 300–600 m (19 cells)} \\  & (0.85–0.96) & \multicolumn{6}{c}{} \\ \hline \multirow{2}{*}{\% of gauge} & 70.3 & 77.9 & 82.7 & 65.8 & 68.8 & 58.3 \\  & (54.4–81.3) & (61.8–109.4) & (64.9–101.6) & (50.3–92.9) & (50.3–85.8) & (44.0–72.8) \\ \hline \multirow{2}{*}{correlation} & 0.92 & \multicolumn{6}{c}{Elevation: \(>\)600 m (12 cells)} \\  & (0.88–0.94) & \multicolumn{6}{c}{} \\ \hline \end{tabular}
\end{table} 
Table 5: Annual rBias and Pearson correlation coefficient performance for GPM data according to elevation (the range of values is given in parentheses). More evaluation metrics (Bias, RMSE, MAE) were used to evaluate the performance of GPM and TRMM monthly data according to elevation (Table 7). Overall, the performance of both satellite products metrics (Bias, RMSE, MAE) were worst in higher altitude areas than in lower altitude areas. Regarding Bias, findings established that TRMM performed better in the elevation range 0-300 m, while GPM exhibited lower bias values in higher altitudes.