# 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 Reflectivity with Ground-Based Radars  
Both the PR and GR detected radar reflectivity of rainfall and hence could be compared with each other. Key information of the studies on reflectivity comparison of PR with GR is listed in Table 3.  
There are four primary TRMM Ground Validation (GV) radar sites around the world: Houston, Texas, US (S-band), Melbourne, Florida, US (S-band), Kwajalein, Republic of the Marshall Islands (S-band), and Darwin, Australia (C-band). These famous GV radars had already been corrected and thus could be served as credible references for PR measurements [25; 26]. Before the reflectivity comparison of PR and GR, spatial matching of measurements should be carried out since PR and GR had different view angles, scanning modes, and data resolutions. The matching approaches could be classified into two categories: grid-matching [27; 28] and area-matching [29; 30]. When the reflectivity of calibrated GR was used as the reference, PR reflectivity could be evaluated. It was shown that PR reflectivity generally agreed well with GR reflectivity, but a notable difference was that PR highly undersampled weak echoes below the detection sensitivity of PR [25; 29]. Fortunately, this defect had no substantial effect on the total accumulated rainfall measurements because the weak echoes contributed little to the precipitation amounts. Most studies showed that the attenuation correction of PR performed quite well for stratiform rain but insufficiently for convective rain from studies comparing PR with GR for different rain types and different altitudes (Figure 3) [27; 30; 31]. An exception was a study indicating that the attenuation was accurately corrected for convective rain and was overcorrected for stratiform rain [28], but it was noted that the non-Rayleigh scattering was not taken into account before the comparison. On the other hand, some scholars used the reliable PR reflectivity as the reference to evaluate or calibrate the GR reflectivity. [PERSON] et al. [32; 33; 34; 35] designed range-dependent adjustment methods for the compensation of GR observations in consideration of the increasing sampling volume of the GR with range, and used this method to improve the rainfall estimation of the GR. Some recent studies made comparisons to evaluate the GR reflectivity in East Asia [36; 37; 38; 39; 40], in which stratiform precipitation was generally used to avoid uncertainties of PR products related to convective rain. To analyze in detail the sources and magnitudes of discrepancies between PR and GR reflectivity, it is necessary to adopt more and further simulation studies or dual-polarimetric radar observations for various possible situations like rainfall types, intensities and space-time positions, phase states of drops, topography effect, etc. Early examples are studies of [20; 23]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Reference** & **Period** & **Ground-Based Radar** & **Products (Visional of PR** & \begin{tabular}{c} **Preprocessing before** \\ **Comparison** \\ \end{tabular}  & **Matching Method** & **Study Region** \\ \hline Bolen and & & an-Stund radar of National Center & & & non-Rayleigh scattering & grid-matching and & Melbourne, US \\ [PERSON] [29] & Aug. 1998 & for Atmospheric Research (NCAR), PR data & & correction & area-matching & \\ [PERSON] et al. [52] & 12 Feb. 2002 & a-Stund radar in Kyukko, Cyprus & 2A5 & data removal in shielded or & grid-matching & Cyprus \\ [PERSON] et al. [3] & 3 to 21 Mar. 2003 & a Cabard radar at Shubam in Israel & 2A5 & / & grid-matching & southeast corner of the \\ [PERSON] et al. [5] & Feb. 2002 and 2003 & Cyprus and Shubam in Israel & 2A5 & / & grid-matching & southeast corner of the \\ [PERSON] et al. [50] & Aug. 2006 to Aug. 2010 & & 2A5 (version) & / & area-matching & Korean Peninsula and insbore areas \\ [PERSON] et al. [44] & 2003-2014 & an S-band radar in Nanjing, China & 2A25 (versioned) & removal of precipitation & boundaries; non-Rayleigh & grid-matching & Nanjing, China \\ [PERSON] et al.