Abstract
Abstract. The main goal of this paper is to validate the total water vapour column (TWVC) measured by the Global Ozone Monitoring Experiment-2 (GOME-2) satellite sensor and generated using the GOME Data Processor (GDP) retrieval algorithm developed by the German Aerospace Centre (DLR). For this purpose, spatially and temporally collocated TWVC data from highly accurate sounding measurements for the period January 2009–May 2014 at six sites are used. These balloon-borne data are provided by the GCOS Reference Upper-Air Network (GRUAN). The correlation between GOME-2 and sounding TWVC data is reasonably good (determination coefficient, R2, of 0.89) when all available radiosondes (1400) are employed in the inter-comparison. When cloud-free cases (544) are selected by means of the satellite cloud fraction (CF < 5%), the correlation exhibits a remarkable improvement (R2 ~ 0.95). Nevertheless, the analysis of the relative differences between GOME-2 and GRUAN data shows a mean absolute bias error (weighted with the combined uncertainty derived from the estimated errors of both data sets) of 15% for all-sky conditions (9% for cloud-free cases). These results evidence a notable bias in the satellite TWVC data against the reference balloon-borne measurements, partially related to the cloudy conditions during the satellite overpass. The detailed analysis of the influence of cloud properties – CF, cloud top albedo (CTA) and cloud top pressure (CTP) – on the satellite-sounding differences reveals, as expected, a large effect of clouds in the GOME-2 TWVC data. For instance, the relative differences exhibit a large negative dependence on CTA, varying from −6 to −23% when CTA rises from 0.3 to 0.8. Furthermore, the satellite-sounding TWVC differences show a strong dependence on the satellite solar zenith angle (SZA) for values above 50°. Hence the smallest relative differences found in this satellite-sounding comparison are achieved for those cloud-free cases with satellite SZA below 50°. Finally, the relative differences also show a negative dependence on the reference TWVC values, e.g. changing from +10% (TWVC below 10 mm) to −10% (TWVC above 40 mm) when cloud-free conditions with SZA below 50° are selected. Overall, relative differences within ±10% with respect to reference sounding data for a large range of TWVC values can be considered as a good result for satellite retrievals.
Highlights
Atmospheric water vapour is a key component for weather and the climate system because it plays a vital role in the formation of clouds and precipitation, the growth of aerosols and significantly contributes to the energy balance of the Earth when acting as a powerful greenhouse gas
This paper focuses on the validation of the total water vapour column (TWVC) data measured by the Global Ozone Monitoring Experiment 2 (GOME-2)/MetOp-A satellite instrument using as reference the balloon-borne data recorded between January 2009 and May 2014 from six GCOS Reference Upper-Air Network (GRUAN) stations
A linear regression analysis between the GRUAN and GOME-2 TWVC data is performed for each GRUAN station and for all stations together in order to analyse their proportionality and similarity
Summary
Atmospheric water vapour is a key component for weather and the climate system because it plays a vital role in the formation of clouds and precipitation, the growth of aerosols and significantly contributes to the energy balance of the Earth when acting as a powerful greenhouse gas. Kaufman and Gao, 1992; Bauer and Schlüssel, 1993; Noël et al, 1999, 2004; Maurellis et al, 2000; Wagner et al, 2006; Li et al, 2006; Deeter, 2007; Lang et al, 2007; Mieruch et al, 2008; Pougatchev et al, 2009) Within this framework, the European satelliteborne atmospheric sensor Global Ozone Monitoring Experiment 2 (GOME-2) aboard the Meteorological Operational satellite program (MetOp-A and MetOp-B) provides the potential for a detailed analysis of the global distribution of the atmospheric water vapour (Grossi et al, 2014). The main objective of MetOp missions is to provide continuous and long-term observations of the most important trace gases, supporting operational meteorology, global weather forecasting and climate monitoring (Edwards et al, 2006). The three MetOp satellites will guarantee continuous TWVC time series using the same sensor (GOME-2) to at least the first half of the 2020s
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