Validation of Humidity Profiles Obtained from Saphir, On-Board Megha-Tropiques

The Megha-Tropiques, an Indo French mission, with four on-board payloads was launched in October 2011 to improve our understanding on hydrological cycle and radiation budget. The SAPHIR (Sondeur Atmosphérique du Profil d'Humidite Intertropicale par Radiométrie), a sounder for profiling humidity, is a key payload and expected to play a major role in fulfilling the mission objectives. The present article focuses on the evaluation of SAPHIR-derived humidity profiles against a variety of reference datasets, like measurements from GPS radiosondes and groundbased microwave radiometer, reanalysis datasets and satellite retrievals. The data collected during July- November 2012 were employed to validate humidity profiles. A variety of colocations (matching the sampling volumes of radiosonde and SAPHIR) were employed for the validation against radiosondes, launched from Gadanki. The bias (SAPHIR-derived RH - reference RH) and the rms error are found to be small for near-nadir measurements than those far away from the nadir. Further, the bias shows a clear height dependence with positive (negative) bias dominating in the lowest (uppermost) layers. The RH bias and rms errors are small (within 15%) in the middle layers, altitudes at which the sensitivity of SAPHIR channels is high. The comparisons with ECMWF interim reanalysis (ERA) and advanced infrared sounder (AIRS) data, used to extend the evaluation of SAPHIR data to the entire tropics, reveal strikingly similar spatial and vertical structure in RH bias. The vertical structure of RH bias is somewhat similar to that obtained with the radiosonde. Large biases are seen in regions adjacent to South America and Africa in the latitude band of 20°-30°S and large negative bias is seen along the Intertropical Convergence Zone. Possible reasons for such large biases in those regions are discussed.