TROPOMI/S5P Total Column Water Vapor Validation against AERONET ground-based measurements

Abstract

Abstract. Water vapor plays a very important role on the greenhouse effect, rendering it an atmospheric constituent that requires continuous and global monitoring by different types of remote sensing instruments. The TROPOMI/S5P Total Column Water Vapor (TCWV) is a new product retrieved from the blue wavelength band (435 –455 nm), using an algorithm that was originally developed for the GOME-2/Metop sensors. For the purposes of this work, 2.5 years of continuous satellite observations at high spatial resolution are validated against co-located (in space and in time) precipitable water Level 2.0 (quality-assured) ground-based measurements from the NASA AERONET (AErosol RObotic NETwork). The network uses CIMEL sunphotometers located at approximately 1300 stations globally to monitor precipitable water among other products. The two datasets, satellite and ground-based, were co-located and the percentage differences of the comparisons were calculated and statistically analyzed. The correlation coefficient of the two products is found to be 0.9 and the mean bias of the relative percentage differences is of the order of only -3 % for the mid-latitudes and the tropics (±60°). The effect of various influence quantities, such as air mass factor, solar zenith angle, clouds and albedo are also presented and discussed. It was found that the cloud properties affect the validation results, leading the TCWV to a dry bias of -19 % for low cloudiness (CTP > 800 hPa). The cloud albedo introduces a wet bias of 10 % when the cloud albedo is below 0.3 and a dry bias up to -20 % when the clouds are more reflective. Overall, the TROPOMI/S5P TCWV product, on a global scale and for moderate albedo and cloudiness, agrees well at -4.0 ± 4.3 % with the ground-truth.