A global time series of total columnar water vapour from combined data of the
Medium Resolution Imaging Spectrometer (MERIS) onboard ESA's Environmental
Satellite (ENVISAT) and the Special Sensor Microwave/Imager (SSM/I) onboard
the satellite series of the US Defense Meteorological Satellite Program (DMSP)
is presented. The unique data set, generated in the framework of the ESA Data
User Element (DUE) GlobVapour project, combines atmospheric water vapour
observations over land and ocean, derived from measurements in the near-
infrared and the microwave range, respectively. Daily composites and monthly
means of total columnar water vapour are available as global maps on
rectangular latitude–longitude grids with a spatial resolution of 0.05° ×
0.05° over land and 0.5° × 0.5° over ocean for the years 2003 to 2008. The
data are stored in NetCDF files and is fully compliant with the NetCDF Climate
Forecast convention. Through the combination of high-quality microwave
observations and near-infrared observations over ocean and land surfaces,
respectively, the data set provides global coverage. The combination of both
products is carried out such that the individual properties of the microwave
and near-infrared products, in particular their uncertainties, are not
modified by the merging process and are therefore well defined. Due to the
global coverage and the provided uncertainty estimates this data set is
potentially of high value for climate research. The SSM/I-MERIS TCWV data set
is freely available via the GlobVapour project web page (www.globvapour.info)
with associated doi:10.5676/DFE/WV_COMB/FP. In this paper, the details of the
data set generation, i.e. the satellite data used, the retrieval techniques
and merging approaches, are presented. The derived level 3 products are
compared to global radiosonde data from the GCOS upper air network (GUAN),
showing a high agreement with a root-mean-square deviation of roughly 4.4 kg
m−2 and a small wet bias well below 1 kg m−2. Furthermore, the data set is
shown to be free of seasonal biases. The consistency of the MERIS and SSM/I
retrievals is demonstrated by applying the MERIS retrieval to sun glint areas
over ocean