Comparison of ERA5-Land reanalysis data with surface observations over Iran

Reanalysis data have been considered as an important source of atmospheric information in a variety of applications such as climate studies, hydrological modeling and numerical weather prediction. Evaluating the effectiveness of the reanalysis products in each area before use is of great importance. With the advent of advanced reanalysis such as ERA5 and ERA5-Land, the interest of many researchers in using these data sources has increased.     To date, several studies have been conducted in the country to statistically compare the reanalysis products with other meteorological data sources, each of which has its limitations and does not provide a comprehensive evaluation of the reanalysis data across the region. In other words, most of these studies are related to the evaluation of one of the meteorological variables such as precipitation and have been done in a specific location of the country or have used a limited number of ground stations in statistical comparison.     In this work, the quality of 2m temperature, surface pressure, 10m wind speed and dew point temperature of ERA5-Land are evaluated temporally and spatially over Iran. For this purpose, sub-daily observations of 406 synoptic stations from 1999 to 2019 were used. The bilinear method was used to spatially interpolate the meteorological values obtained from ERA5-Land at the station locations. After preparing the ERA5-Land sub-daily time series and the corresponding actual observations, the error statistics required to evaluate the ERA5-Land data were calculated. Statistical comparisons between ERA5-Land products and ground observations of 2m temperature, surface pressure, 10m wind speed and dew point temperature parameters are done with a 3-hour temporal resolution.     In the whole region, ERA5-Land products and local measurements of 2m temperature, surface pressure, 10m wind speed and dew point temperature showed agreement about 0.97, 0.98, 0.49 and 0.88, respectively. Also, compared to the actual observations, the mean RMSE for the above ERA5-Land data products achieved 2.87°C, 19.42 hPa, 2.52 m/s and 4.12°C, respectively.     The study of bias values showed that in the region of Iran, ERA5-Land, on average, estimates all the studied variables less than the observed value. In addition, the study of the relationship between height difference of ERA5-Land grid points and station height with estimated error values showed that with increasing altitude difference, the size of negative bias and root mean square error of 2m temperature and the surface pressure of ERA5-Land increases significantly. Therefore, it is suggested eliminating the existing systematic errors in the area before applying this data.