An assessment of SAPHIR calibration using quality tropical soundings

International audienceThe SAPHIR sounder (Sondeur Atmosphérique du Profil d'Humidité Intertropicale par Radiométrie) onboard the Megha-Tropiques (MT) platform is a cross track multi-channel microwave humidity sounder with 6 channels near the 183.31GHz water vapor absorption line, a maximum scan angle of 42.96° (resulting in a maximum incidence angle of 50.7°), a 1700 km wide swath and a footprint resolution of 10 km at nadir. SAPHIR L1A2 brightness temperature (BT) observations have been compared to brightness temperatures (BT) simulated by the radiative transfer model (RTM) RTTOV-10, using in situ measurements from radiosondes as input. Selected radiosonde humidity observations from the CINDY/DYNAMO campaign (September 2011 to March 2012) were spatio-temporally co-located with MT overpasses. Although several sonde systems were used during the campaign, all of the sites selected for this study used the Vaïsala RS92-SGPD system and were chosen in order to avoid discrepancies in data quality and biases. In order to interpret the results of the comparison between the sensor data and the RTM simulations, uncertainties associated with the data processing must be propagated throughout the evaluation. The magnitude of the bias was found to be dependent on the observing channel, increasing from 0.12 K for the 183.31 ± 0.2 GHz channel to 2.3 K for the 183.31 ± 11 GHz channel. Uncertainties and errors that could impact the BT biases were investigated. These can be linked to the RTM input and design, the radiosonde observations, the chosen methodology of comparison and the SAPHIR instrument itself