Validation of MERIS near IR water vapour retrievals using MWR and GPS measurements

In this paper, the reduced resolution (RR) MEdium Resolution Imaging Spectrometer (MERIS) near infrared (IR) water vapour (PWV) products above land surfaces produced at the Freie Universität Berlin (FUB) are examined through inter-comparisons with contemporaneous microwave radiometer (MWR) and GPS data sets. The intercomparison of MWR and MERIS water vapour retrievals showed a good correlation of 0.99 with a standard deviation of 1.0 mm, and an overall slight wet bias of MERIS near IR water vapour was observed. Unfortunately, this inter-comparison was limited by the number of samples. Moreover, a spatio-temporal comparison between MERIS and GPS water vapour products was performed using data covering all of Germany for one year from October 2002 to September 2003. MERIS water vapour values appeared to be slightly greater than GPS values under either very dry (PWV<5 mm) or very wet (PWV>25 mm) conditions. However, MERIS water vapour agreed well with GPS water vapour retrievals under moderate conditions with a standard deviation of 1.3 mm and a mean difference of -0.4 mm.