Improvement of precipitable water vapour and water vapour mixing ratio profile in atmospheric infrared sounder retrievals: differential linear adjustment model

Knowledge of both the horizontal and vertical distributions of atmospheric water vapour is pivotal to understand the Earth’s radiative balance. The Atmospheric Infrared Sounder (AIRS) provides an opportunity to monitor both precipitable water vapour (PWV) measurements and water vapour mixing ratio (MR) profiles. In this study, we incorporate a differential linear adjust model (DLAM) by using the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Reanalysis (ERA-Interim) to adjust the AIRS water vapour retrievals. The performance of the differential linear adjustment model (DLAM) is assessed by comparing AIRS water vapour retrievals with spatio-temporally synchronized radiosonde (RS) observations. Taking RS data as the reference, RMS of the DLAM-adjusted PWV is reduced by about 16%, and that of water vapour MR is decreased by 17% to 8% from 1000 to 500 hPa. We also find that the DLAM appears to be affected by the generally larger uncertainties of the a priori water vapour MR at upper tropospheric levels, and the DLAM-derived water vapour MR improvement is generally greater from 1000 to 850 hPa than 700 to 500 hPa. Moreover, the potential reason for the effectiveness of the DLAM on AIRS water vapour retrievals adjustment may be the deviation of the differential water vapour information derived by the differential process is significantly reduced in the incorporated model.