# Spatiotemporal Assessment and Correction of Gridded Precipitation Products in North Western Morocco
## 2 Materials and Methods
### The Gridded Precipitation Products  
In order to estimate and spatialize precipitation amounts, satellites are equipped with specialized sensors that capture and measure various properties of clouds and precipitation systems from space. These measurements are then processed using sophisticated algorithms to estimate precipitation rates and distributions over vast areas. However, it is important to validate and calibrate these estimates in relation to ground observations and potentially correct their biases [39].  
ERA5 is the latest version of atmospheric reanalysis data provided by the European Centre for Medium-Range Weather Forecasts (ECMWF). It covers the period from 1950 onwards and it replaced the previous version, ERA-Interim (spanning 1979 onwards), that was implemented in 2006 and covered the period from 1979 [14]. ERA5 was developed based on the integrated forecasting system Cy41r2, which has been in operation since 2016, benefiting from years of work in model physics, core dynamics, and data assimilation. The ERA5 dataset has a spatial resolution of 0.25\({}^{\circ}\) and a temporal resolution of 1 h. In contrast, ERA5-Land focuses specifically on land data and provides a more detailed characterization of land conditions with an enhanced resolution of 0.1\({}^{\circ}\). The main difference between ERA5 and ERA5-Land is that the latter is influenced only by ERA5's lower atmospheric weather field, with an additional correction for evaporation rate to improve accessibility [40]. In this study, the advanced ERA5 dataset is used.  
Our analysis also utilized three versions of the IMERG V06 products. IMERG is a collection of satellite-based precipitation data provided by NASA, offering a fine spatial resolution of 0.1\({}^{\circ}\) and a temporal resolution of 0.5 h [13]. It builds upon the previous TMPA services, covering a wider spatial range from 60\({}^{\circ}\) N to 60\({}^{\circ}\) S. The IMERG product consists of three runs: Early (ER), Late (LR), and Final (FR). The ER and LR are near-real-time products released 4 h and 12 h after observations, respectively. They differ in the morphing technique used: forward morphing for ER and both forward and backward morphing for LR. The FR, which is the research-level product, is calibrated using monthly GPCC (Global Precipitation Climatology Centre) data, providing more accurate precipitation information [7]. The precipitation estimates from these satellite data encompass various forms of precipitation, including rain, drizzle, snow, graupel, and hail [41].  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Station Name** & **Longitude W** & **Latitude N** & **Z(m)** & **Missing Values (\%)** \\ \hline Aguibat Ezziar & \(-6.54\) & 33.91 & 81 & 0.11 \\ [PERSON] & \(-6.76\) & 33.56 & 163 & 0.08 \\ SMBA Dam & \(-6.75\) & 33.94 & 67 & 1.31 \\ [PERSON] & \(-6.39\) & 33.71 & 193 & 0.08 \\ [PERSON] & \(-6.26\) & 33.11 & 557 & 2.93 \\ [PERSON] & \(-6.54\) & 33.76 & 132 & 0.07 \\ [PERSON] & \(-6.60\) & 33.53 & 308 & 0.12 \\ [PERSON] & \(-6.21\) & 33.63 & 263 & 16.61 \\ [PERSON] & \(-6.43\) & 33.58 & 197 & 0.07 \\ [PERSON] & \(-6.63\) & 33.55 & 282 & 0.10 \\ [PERSON] & \(-6.03\) & 33.35 & 687 & 0.12 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Station coordinates used in this study and their rate of missing values.