Estimating confidence intervals around relative humidity profiles from satellite observations: Application to the SAPHIR sounder

Abstract : A novel scheme for the estimation of layer-averaged relative humidity (RH) profiles from space-borne observations in the 183.31GHz line is presented. Named ARPIA for Atmospheric Relative humidity Profiles Including Analysis of confidence intervals, it provides for each vector of observations the parameters of the distribution of the RH instead of its expectation as usually done by the current methods. The profiles are composed of 6 layers distributed between 100 and 950hPa. The approach combines the 6 channels of the SAPHIR instrument onboard the Megha-Tropiques satellite and the Generalized Additive Model for Location, Scale and Shape (GAMLSS) to infer the parametric distributions, assuming that they follow a Gaussian law. The knowledge of the conditional uncertainty is an asset in the evaluation using radiosounding profiles of RH with a dedicated bayesian method. Taking the uncertainties into account in both the ARPIA estimates and the in situ measurements yields to have biases, root-mean-square and correlation coefficients in the range -0.56% - 9.79%, 1.58% - 13.32% and 0.55 - 0.98 respectively, the largest biases being obtained over the continent, in the mid-tropospheric layers.