Developing Historical Gridded Climate Datasets for Vietnam Using Reanalysis Data and Station Observations Since the 1940

Accurate and high-resolution gridded climate data are crucial for scientific applications, yet such data have long faced limitations due to sparse and uneven station networks. In this study, we address this gap for Vietnam by developing the Vietnam Historical Gridded Climate (VnHGC) dataset, which for the first time combines an expanded suite of historical station observations, including newly digitized and qualitycontrolled records with state-ofthe- art reanalysis products through Optimal Interpolation (OI). Two products were constructed: a monthly dataset for 1940–1953 with a horizontal resolution of 25 km based on the background field of ERA5, and a daily dataset for 1961–1980 with a horizontal resolution of 10 km based on the background field of ERA5-LAND. Historical observations were biascorrected and assimilated into the background fields to maximize local representativeness, especially in data-poor regions. In general, evaluation against independent station data shows that VnHGC substantially reduces systematic biases and better resolves regional patterns of temperature and precipitation, with the greatest improvements in northern and mountainous areas where reanalyses typically underperform. These improvements are especially pronounced in the daily dataset (1961–1980), reflecting the combined effects of higher temporal resolution and greater station availability.