Validation of brightness temperatures observed by SAPHIR instrument onboard megha-tropiques satellite

The SAPHIR (Sondeur Atmosphérique du Profil d'Humidité Intertropicale par Radiométrie) instrument onboard Meghatropiques observes the tropical atmosphere since fall 2011 with 6 channels near the 183 GHz water vapour absorption band. It is a cross track scanning radiometer with a footprint resolution of 10km at nadir. This work investigates the concordance of SAPHIR observed brightness temperatures with regards to brightness temperatures modelled from radiosonde observations performed during a dedicated Megha-Tropiques campaign (Ouagadougou, Burkina Faso, june, july 2012) and the Cindy Dynamo campaign (Equatorial Indian ocean, October 2011 to March 2012). This database includes 13 sites and several thousands Vaïsala RS92 radiosonde profiles performed at 3 continental sites (Ouagadougou, Nairobi, Darwin), 7 tropical islands and onboard 3 scientific ships (R/V Mirai, R/V Baruna, R/V Roger Revelle). The water vapour observations from the dedicated campaign in Burkina Faso and those performed during Cindy Dynamo campaign were subject to the same quality control procedure with the ASPEN software (Ciesielski et al. 2011) and a correction algorithm which was developped by the GRUAN community and accounts for time lag and radiative bias. The quality checked and bias corrected water vapour profiles are then used to simulate brightness temperatures with RTTOV 10 radiative transfer model (Matricardi et al., 2004). The RTTOV simulations are then compared to collocated SAPHIR observations at the closest pixel in a 90 minutes interval around radiosounding launch time. For each SAPHIR channel, the bias distribution is investigated in order to identify specific features of relative humidity profiles which correspond to extreme brightness temperature biases (edges of the PDF). The mean bias and dispersion observed in the modelled brightness temperatures are also discussed with regards to instrumental accuracies (both satellite and radiosonde) and site types