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86 CHAPTER 2. ATMOSPHERE AND REMOTE SENSING 2.5.15 Satellite observations of the global water vapor distribution Participating scientist Thomas Wagner, Steffen Beirle, Michael Grzegorski Abstract From modern UV/vis satellite instruments, the integrated water vapor concentration (often refered to as vertical column density or total column precipitable water) can be observed on a global scale. In contrast to previous satellite observations in the infrared and microwave spectral range our observations include both ocean and land surfaces with similar sensitivity. Figure 2.49: Global anomalies of the water vapor distribution during the strong ENSO event in 1997/98. Background Atmospheric water vapor is the most important greenhouse gas contributing about 2/3 of the natural greenhouse effect. In contrast to other greenhouse gases like CO2 and CH4 it has a much higher temporal and spatial variability. The corect understanding and assessment of atmospheric water vapor with respect to the Earths energy budget is further complicated by its role in cloud formation and transport of latent heat. Today, many details of how the hydrological cycle reacts to climate change (water vapor feedback) are still not understood. Especially for the tropics, which contribute strongest to the water vapor greenhouse effect, the strength of the water vapor feedback is under intense debate. In our studies we investigate the dependence of the global distribution of water vapor on surface temperature. Particular studies focussed on the response to the strong ENSO in 1997/98 and on the trends from 1996-2003. Funding See satellite group overview. Methods and results The GOME instrument is one of several instruments aboard the European research satellite ERS-2. It consists of a set of four spectrometers that simultaneously measure sunlight reflected from the Earth’s atmosphere and ground in total of 4096 spectral channels covering the wavelength range between 240 and 790 nm with moderate spectral resolutions. The satellite operates in a nearly polar, sun-synchronous orbit at an altitude of 780 km with an equator crossing time at approximately 10:30 local time. The Earth’s surface is totally covered within 3 days, and poleward from about 70 latitude within 1 day. Our water vapor algorithm is based on Differential Optical Absorption Spectroscopy (DOAS) performed in the wavelength interval 611-673 nm. It consists of three basic steps (described in detail in Wagner et al. [2003]): in the first step, the spectral DOAS fitting is caried out, taking into consideration cross sections of O2 and O4 in addition to that of water vapor. From the DOAS analysis, the water vapor slant column density (the concentration integrated along the light path) is derived. In the second step, the water vapor slant column density is corected for the non-linearity arising from the fact that the fine structure water vapor absorption lines are not spectrally resolved by the GOME instrument. In the last step, the corected water vapor slant column density is divided by a ’measured’ air mass factor which is derived from the simultaneously retrieved O4 or O2 absorptions [Wagner et al., 2003, 2005b,b]. Outlook/Future work The work will be continued by applying the method to new satellite instruments like SCIAMACHY and the GOME- 2-series. It can be expected that the time series can be continued until 2020. Main publication Wagner et al. [2003], Wagner et al. [2005a], Wagner et al. [2005b]
2.5. SATELLITE GROUP 87 2.5.16 Analysis of MAXDOAS observations in various observing geometries Participating scientist Thomas Wagner, Nicole Bobrowski, Barbara Dix, Erna Frins, Ossama Ibrahim, Roman Sinreich Abstract MAX-DOAS measurements were caried out at various locations including ship borne measurements. Sophisticated analysis strategies were developed which can yield vertical profiles not only of the trace gases of interest, but also of aerosol properties. In addition, a new tomographic MAX-DOAS technique was developed which allows to determined three-dimensional trace gas fields. Figure 2.50: General dependence of the measured O4 absorption on various parameters of MAXDOAS observations. From MAXDOAS O4 measurements, the atmospheric aerosol properties can be derived (Wagner et al., 2004) Background The Multi AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) technique allows to separate the absorptions taken place at different altitudes in the atmosphere by observing scattered sun light from a variety of viewing directions. This is possible because for most measurement conditions, the observed light is scattered in the free troposphere. Thus air masses located close to the ground are traversed on a slant absorption path determined by the viewing direction; in contrast, stratospheric air masses are traversed on a slant absorption path determined by solar zenith angle. In addition to the partial columns of various trace gases (like NO2, BrO, HCHO, etc.) also information on the aerosol profile can be retrieved (see figure). Funding See satellite group overview. Methods and results MAX-DOAS observations were caried out during coordinated compar- ison campaigns at Milano (September 2003) and Cabauw (July 2005). In addition, permanent instruments are installed at Paramaribo (Suriname) since 2002. MAXDOAS observations were also caried out during several cruises of the German research vessel Polarstern. Thes observations were used for the validation of the SCIAMACHY instrument on board of the European research satellite ENVISAT. In addition to the classical MAX- DOAS geometry, also MAX-DOAS observations of sun-illuminated targets were proposed (Frins et al., 2005). Thus measurements over well defined light paths are possible which will allow tomographic inversion of horizontal gradients. Outlook/Future work The full potential of the various MAXDOAS geometries will be further examined. Inversion schemes for trace gas and aerosol profiles will be optimised. Main publication Wagner et al. [2004]
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Contents 1 Introduction/Overview of
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CONTENTS 7 3.2.1 Glaciological pilo
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