1 RTTOV-gb v 1 . 0-Updates on sensors , absorption models , 1 uncertainty , and availability 2

This paper describes the first official release (v1.0) of RTTOV-gb. RTTOV-gb is a FORTRAN 90 18 code developed by adapting the atmospheric radiative transfer code RTTOV, focused on satellite observing 19 geometry, to the ground-based observing geometry. RTTOV-gb is designed to simulate ground-based upward20 looking microwave radiometer (MWR) observations of atmospheric downwelling natural radiation in the 21 frequency range from 22 to 150 GHz. Given an atmospheric profile of temperature, water vapour and, 22 optionally, cloud liquid water content, and together with a viewing geometry, RTTOV-gb computes the bottom 23 of atmosphere radiances and brightness temperatures in each of the channels of the sensor being simulated. In 24 addition, it provides the sensitivity of observations to the atmospheric thermodynamical state, i.e. the Jacobians. 25 Therefeore, RTTOV-gb represents the forward model needed to assimilate ground-based MWR data into 26 numerical weather prediction models, which is currently pursued internationally by several weather services. 27 RTTOV-gb is fully described in a previous paper (De Angelis et al., 2016), while several updates are described 28 here. In particular, two new MWR types and a new parameterization for atmospheric absorption model have 29 been introduced since the first paper. In addition, estimates of the uncertainty associated with the absorption 30 model and with the fast parameterization are given here. Brightness temperatures (TB) computed with RTTOV31 gb v1.0 from radiosonde profiles have been compared with ground-based MWR observations at six channels 32 (23.8, 31.4, 72.5, 83.5, 90.0, and 150.0 GHz). The comparison shows statistics within the expected accuracy. 33 RTTOV-gb is now available to licensed users free of charge from the Numerical Weather Prediction Satellite 34 Application Facility (NWP SAF) website, after registration. Coefficients for four MWR instrument types and 35 two absorption model flavors are also freely available from the RTTOV-gb support website. 36 37 Geosci. Model Dev. Discuss., https://doi.org/10.5194/gmd-2018-285 Manuscript under review for journal Geosci. Model Dev. Discussion started: 21 January 2019 c © Author(s) 2019. CC BY 4.0 License.