Retrieval of daytime total columnar water vapour from MODIS measurements over land surfaces

Abstract

A retrieval of total column water vapour (TCWV) from MODIS (Moderate- resolution Imaging Spectroradiometer) measurements is presented. The algorithm is adapted from a retrieval for MERIS (Medium Resolution Imaging Spectrometer) from Lindstrot et al. (2012). It obtains the TCWV for cloud-free scenes above land at spatial resolution of 1 km×1 km and provides uncertainties on a pixel- by-pixel basis. The algorithm has been extended by introducing correction coefficients for the transmittance calculation within the forward operator. With that a wet bias of the MODIS algorithm against ARMMicrowave Radiometer data has been eliminated. An extensive validation against other ground-based measurements (GNSS-water vapour stations, GUAN Radiosondes) on a global scale reveals a bias between −0.8 and −1.6mm and root mean square deviations between 0.9 and 1.9 mm. This is an improvement in comparison to the operational TCWV Level 2 product (bias between −1.9 and −3.2mm and root mean square deviations between 1.9 and 2.7 mm).