First Results of Total Column Water Vapor over Asia from the Geostationary Environment Monitoring Spectrometer (GEMS): Retrievals and evaluation

Water vapor is the most abundant greenhouse gas, playing a significant role in both weather and climate. Despite its importance, water vapor exhibits high spatiotemporal variability, which makes it difficult to understand its physical and chemical processes. To address this problem, we developed a retrieval algorithm for Total Column Water Vapor (TCWV) using visible blue spectra measured by the Geostationary Environment Monitoring Spectrometer (GEMS). The algorithm follows a traditional two-step approach: spectral fitting in the retrieval window of 435.0 – 467.0 nm to obtain slant columns, and calculation of the Air Mass Factor (AMF) to convert slant column densities (SCD) into vertical column densities (VCD). The retrieved results successfully detect the flow of water vapor and atmospheric river events over East Asia.

We evaluated the GEMS TCWV product using a 2-year dataset (data period: March 2021 - February 2023) by validating against AERONET level 1.5 precipitable water datasets from the AERONET Version 3 algorithm and intercomparing with TROPOMI (S5P-PAL) TCWV products. Evaluation results show good correlations with both ground-based measurements (r = 0.96, MBE = -1.7 kg m^-2) and satellite measurements (r = 0.9, MBE = -3.4 kg m^-2). Correlations typically drop in summer months and over the ocean. The GEMS TCWV product is expected to not only improve our understanding of the water vapor diurnal cycle over East Asia but also benefit the study of atmospheric river events.