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Abstract. We investigate the retrieval of terrestrial precipitable water columns using a new spectral fitting method applied to Global Ozone Monitoring Experiment (GOME) data. The method is an optical absorption spectroscopy technique and employs a new approach to the opacity sampling of absorption line spectra which we apply to a littlestudied visible band between 585 and 600 nm. The GOMEretrieved columns are compared with data from the European Center for Medium-Range Weather Forecasts for different orbits and show good agreement. The new retrieval algorithm is sensitive to the temperature and pressure dependence of absorption lines in general and may be easily applied to spectra of trace gases other than water vapor.

[1] An accurate knowledge of the 3-D water vapor (WV) field is still limited, because of the limited capabilities of sensors in the past to cover the whole Earth's surface and the lower part of the troposphere, as well as to measure over reasonably long time series. We show here water vapor total column retrieved from seven years of Global Ozone Monitoring Experiment (GOME) measurements collected from August 1995 until August 2002. Our aim is two-fold: (1) to evaluate the accuracy and the limitations of the GOME water vapor total column and (2) to demonstrate its potential for climate studies. The column retrieval makes use of two innovative techniques operating in tandem, namely the University of Graz empirical air mass factor ratioing technique (IGAM) and the Spectral Structure Parameterization (SSP) retrieval method. The GOME instrument and its follow-up instruments (SCIAMACHY and GOME-2), using these algorithms, have the capability to cover nearly the whole globe in cloud-free situations and collect robust WV total column information over more than 3 decades. In this work we evaluate the results for the first 7 years against independent in situ measurements from the operational WMO radiosonde network, against high spatial resolution water vapor columns from MERIS (the Medium Resolution Imaging Spectrometer on EnviSat) and also compare with ERA40 model results. The GOME water vapor total column exhibits a bias of less than 2.5% with an uncertainty of around 5 mm for collocated measurements against radiosonde and MERIS measurements. Spatial patterns and trends in the global distribution of WV total column fields from GOME against re-analysis model results are well correlated with temperature in the tropics, and exhibit a lesser degree of correlation in the extra tropics. Cloud-free total columns from GOME can be systematically lower by up to 5 mm in the sub-tropics with respect to the all-sky case. In contrast, the impact of the diurnal cycle on the monthly mean values is found to be very small.

Abstract. This study examines two key parameters of the hydrological cycle, water vapor (WV) and precipitation rates (PR), as modelled by the chemistry transport model MATCH (Model of Atmospheric Transport and Chemistry) driven by National Centers for Environmental Prediction (NCEP) reanalysis data (NRA). For model output evaluation we primarily employ WV total column data from the Global Ozone Monitoring Experiment (GOME) on ERS-2, which is the only instrument capable measuring WV on a global scale and over all surface types with a substantial data record from 1995 to the present. We find that MATCH and NRA WV and PR distributions are closely related, but that significant regional differences in both parameters exist in magnitude and distribution patterns when compared to the observations. We also find that WV residual patterns between model and observations show remarkable similarities to residuals observed in the PR when comparing MATCH and NRA output to observations comprised by the Global Precipitation Climatology Project (GPCP). We conclude that deficiencies in model parameters shared by MATCH and NRA, like in the surface evaporation rates and regional transport patterns, are likely to lead to the observed differences. Monthly average regional differences between MATCH modelled WV columns and the observations can be as large as 2 cm, based on the analysis of three years. Differences in the global mean WV values are, however, below 0.1 cm. Regional differences in the PR between MATCH and GPCP can be above 0.5 cm per day and MATCH computes on average a higher PR than what has been observed. The lower water vapor content of MATCH is related to shorter model WV residence times by up to 1 day as compared to the observations. We find that MATCH has problems in modelling the WV content in regions of strong Correspondence to: R. Lang (lang@mpch-mainz.mpg.de) upward convection like, for example, along the Inter Tropical Convergence Zone, where it appears to be generally too dry as compared to the observations. We discuss possible causes for this bias and demonstrate the value of the GOME WV record for model evaluation.