Wet Path Delay for Satellite Altimetry computed from External Water Vapor Data

Wet Path Delay (WPD) to correct sea level measurements from satellite altimetry is estimated by on-board microwave radiometers (MWR) observations. However, in cases where on-board MWR retrievals are invalid or absent, WPD must be derived from external sources, such as scanning imaging MWR or atmospheric models. Instead of WPD, these alternative sources provide total column water vapor (TCWV) values, introducing the need for converting TCWV into WPD. In its state-of-the-art, this conversion can be performed solely from TCWV or from a combination of TCWV and near surface air temperature. The first approach, which is the focus of this study, is particularly relevant when the external products only provide TCWV. In this context, this paper presents, first, a comprehensive intercomparison of the methods available in the literature and, second, an improved TCWV-WPD conversion. Results show that one of the existing functions underestimates WPD by up to 1.2 cm in regions of high water vapor content, while the other provides accurate WPD values only under specific conditions. This study proposes an updated methodology that yields accurate WPD across the entire TCWV range, highlighting the importance of a reliable TCWV-WPD conversion for accurate sea level estimation when valid MWR observations are unavailable.