# Studies of General Precipitation Features with TRMM PR Data: An Extensive Overview
## 2 The Reliability Evaluation of PR Data by Comparisons with Ground-Based Instruments
### Comparisons of Rainfall Rate with Rain Gauges  
Observations of rain gauges can be also used as references to compare with rainfall measurements of PR. Key information of the studies on comparisons of rainfall rate derived from PR with rain gauges is listed in Table 5.  
In general, PR and the rain gauges gave consistent rainfall measurements on the whole, but discrepancies had been noted in early studies [32; 47; 48; 49]. Recently, there were more studies on PR rainfall measurements by comparisons of TRMM Multi-satellite Precipitation Analysis (TMPA) products 3B42 or 3B43 with rain gauge data over different regions across the world. The results showed good agreement and high correlation between the two datasets, with some small differences. Most of the studies implied that there was overestimation for light rainfall and/or underestimation for heavy rainfall [50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60], but few of them gave an opposite conclusion [61]. Some studies concluded that the consistency between TMPA products and rain gauge measurements was related to the altitude and the topography [54; 59; 61; 62; 63; 64; 65]. More attention had been paid to statistics of the results in previous studies on rainfall measurements of PR and rain gauges. However, the QPE algorithms of PR should also be carefully analyzed under different situations, and this issue is worth studying before the usual comparisons. For instance, the drop size distribution (DSD) used in PR rainfall products might be not suitable for some regions, topography or seasons. [PERSON] et al. [66] used a 2D-video disdrometer to measure the DSD and establish the Z-\(R\) relationship in Crete island of Greece, and then they found the coefficients of the local Z-\(R\) power laws were different from the values used in PR algorithms and consequently there was an overestimation of rainfall rate by PR algorithms for light to moderate stratiform rain. In addition, the improvement of PR products may affect the results of QPE. [PERSON] et al. [46] analyzed the error structure of PR QPE products of version 6 and version 7 based on GR products, and the results indicated that version 7 was in closer agreement than version 6. The bias of the rain rate estimates was improved from an underestimation of \(-23\%\) of version 6 to \(-18\%\) of version 7. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Reference** & **Period** & **Sites** & **Products (Version) of PR** & **Temporal Scale** & **Conclusion** & **Study Region** \\ \hline \multirow{4}{*}{[PERSON] et al. [87]} & \multirow{4}{*}{1999-2010} & 88 gauges in network of the USDA-ARS & \multirow{4}{*}{2A5 (version)} & \multirow{4}{*}{1 minute} & very good agreement with low \\  & & & & & & bus values and high correlation \\  & & & & & & coefficients \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Key information of the studies on QPE comparison of PR with rain gauges.