The Megha‐Tropiques mission: day 1 algorithms

Abstract

The Megha-Tropiques mission is a mission jointly built by the Centre National d'Études Spatiales and the Indian Space Research Organisation launched in October 2011. ‘Megha’ means cloud in Sanskrit and ‘Tropiques’ is the French word for tropics. The major innovation of Megha-Tropiques is to bring together a suite of complementary instruments on a dedicated orbit that strongly improves the sampling of the water cycle elements. Indeed, the low inclination on the equator (20°) combined with the elevated height of the orbit (867 km) provides unique observing capabilities with up to 6 over-passes per day for the best case. The following payloads are embarked on board: SCARAB, a wide band instrument for inferring longwave and shortwave outgoing fluxes at the top of the atmosphere (cross track scanning, ∼ 40 km resolution at nadir); SAPHIR, a microwave sounder for water vapour sounding: six channels in the WV absorption band at 183.31 GHz (cross track, resolution ∼ 10 km); MADRAS, a microwave imager for precipitation: channels at 18, 23, 37, 89 and 157 GHz, H and V polarisations (conical swath, resolution <10 km to 40 km). A fourth instrument, a GPS-ROSA receiver, completes the mission. The scientific objectives of the mission deal with the water and energy cycle in the intertropical region. To achieve these objectives, a suite of algorithms has been developed based on the mission payloads and the available ancillary observations. This Special Issue presents the day 1 Megha-Tropiques retrievals and approaches with the emphasis on the quantification of the uncertainties associated with the products. The first series of papers by Brogniez et al., Aires et al. and Bernado et al. showcase the importance of the microwave payloads combination of SAPHIR and MADRAS for the estimation of the humidity profile. The instantaneous retrieval of surface rainfall is shown to be improved by using a renewed rainfall detection scheme (Kacimi et al.) and the uncertainty associated with the Bayesian approach to the estimation of the surface rain flux has been quantified over West Africa by Kirstetter et al. The integrated products that are envisioned for the scientific exploitation of the mission concern the accumulated daily rain amount and the composite life cycle of mesoscale convective systems. In both cases, the MT data are combined with the infrared geostationary imagery. Fiolleau and Roca, using the TRMM data as a surrogate, propose a first tropical wide composite of the rainfall evolution during the life cycle of convective systems and underline the improvement in sampling brought by the Megha-Tropiques mission. Chambon et al. show that the main source of uncertainty for the daily rain amount lies in the intermediate estimates of the instantaneous rain rates and promote a fully developed error model that includes the calibration, sampling and algorithm terms. Specific methodological developments for the validation of such products are summarized by Gosset et al., who emphasize the need for integrated hydrological validation in complement of the rainfall evaluation. While the Special Issue is mainly directed towards presenting the French team developments, a final paper by Gohil et al., highlights the Indian algorithms scenario. This Special Issue of Quarterly Journal of the Royal Meteorological Society brings to the future users of the MT data and products a suite of reference papers that should be useful for their further scientific investigations.