In a companion article (Paper I), a water-vapour retrieval algorithm has been developed using microwave sounding observations. An operational chain derived from this algorithm is being used for the Megha-Tropiques mission, launched in autumn 2011. The water vapour is retrieved for clear and cloudy scenes, excluding precipitation cases, over ocean and land surfaces. By-products are also calculated by the algorithm, including surface temperature and microwave emissivities over land.
Evaluation of the water-vapour products is the main objective of this article. The algorithm is tested using the HSB/AMSR-E (resp. MHS/AMSU-A) instruments on board the AQUA (resp. MetOp) platform, over the Tropics between ±30° in latitude. Results are compared with European Centre for Medium-Range Weather Forecasts (ECMWF) analyses and data from radiosondes. The root-mean-square error for the total column water vapour is ∼5 kg m2 for both clear and cloudy scenes, compared with radiosondes over land. The atmospheric water-vapour profile is retrieved for six atmospheric layers and the root-mean-square error is estimated to be lower than 20% in relative humidity, even for the lowest atmospheric layer over land. A posteriori validation tests on the brightness temperatures indicate an overall positive impact of the retrievals relative to a priori ECMWF analyses. Copyright © 2012 Royal Meteorological Society