Non-Negligible Urbanization Effects on Trend Estimates of Total and Extreme Precipitation in Northwest China

Quantifying and removing urbanization-induced biases in existing precipitation datasets is critical for climate change detection, model assessment, and attribution studies in Northwest China (NWC). The precipitation observational stations of NWC were divided into rural (reference) stations and urban stations using the percentage of urban areas calculated from the land use/land cover (LULC) satellite data of the European Space Agency (ESA) Climate Change Initiative (CCI) Land Cover project. The annual extreme precipitation index series for urban stations (all stations) and rural stations from 1961 to 2022 were calculated based on the categorization of meteorological stations, and the urbanization effects and their contributions to precipitation index series were quantitatively evaluated through estimating trends in the difference series between all stations and the rural stations. The results showed that the urbanization effect varies among different regions and indices. The R10mm, R95pTOT, R99pTOT, and PRCPTOT indices in the sampled urban areas of NWC exhibited statistically significant negative urbanization effects, reaching −0.075 days decade−1, −0.038 % decade−1, −0.024 % decade−1, and −0.035 % decade−1, respectively. However, the R95pTOT, SDII, CDD, and CWD indices at the urban station of the largest city, Urumqi, have been significantly positively affected by urbanization, which is inconsistent with the sampled urban areas of NWC, where the urbanization effect reached 0.069 % decade−1, 0.054 mm·d−1 decade−1, 2.319 days decade−1, and 0.112 days decade−1, respectively. Our analysis shows that the previously reported regional increase in total precipitation and extremes has been underestimated due to the negative urbanization effects in the precipitation data series of urban stations.