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66 CHAPTER 2. ATMOSPHERE AND REMOTE SENSING
2.4.8 Satellite observations of global water vapor trends 1996 - 2003
Thomas Wagner
Abstract From modern UV/vis satellite instruments, the integrated water vapor concentration
(often referred 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.
Surface temperature trend 1996 – 2002 Trend of the H2O VCD 1996 – 2002
Figure 2.35: Global trend patterns of yearly averaged total column precipitable water and surface
temperature (from http://www.giss.nasa.gov/data/update/gistemp/). The trends of the total column
precipitable water are expressed as relative trends per year. Dark blue color indicates areas without
data.
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 correct 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. Especially in the tropics
and the southern hemisphere many similarities
between the trend patterns of the total column
precipitable water and the temperature are
found. Over the northern hemispheric continents
also opposite trends occur.
Methods and results Our water vapor algorithm
is based on Differential Optical Absorp-
tion 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 carried 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 corrected 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 corrected
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., 2006a].
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. [2006e]