Water vapor measured by ground-based FTIR technique
Water vapor is the dominant greenhouse gas in the atmosphere, and in particular its
concentration, evolution and transport in the upper troposphere and lower stratosphere are in
the focus of scientific interest. Atmospheric water vapor can be measured in-situ or remotesensed by microwave, Lidar and FTIR experiments from different platforms. Currently there is
no single observation technique that produces continuous and precise water vapor profiles
from the troposphere up to the mesosphere. Figure 1 shows the ground-based FTIR
experiment at the Izaña Observatory (28°N, 16°W, 2370 m a.s.l.). The technique consists in
analysing the water vapor signatures of the measured highly-resolved solar absorption
spectra (Fig. 2). FTIR measurements nicely complement in-situ or satellite based instruments:
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The FTIR measurement can be taken with no extra cost (e.g. for balloons) whenever
weather conditions permit. Especially short term variations can thus be observed.
The FTIR measurement is locally well defined. Locations of sonde measurements are
determined by local winds, satellite measurements measure a mean over a large area,
depending on their ground pixel size.
It can distinguish between different water vapor isotopologues (H2O, H218O, and HDO),
which allows the study of transport processes.
Total water vapor
Figure 3 shows an example of continuous measurements of total water vapor above Ny
Ålesund. Even for very stable weather conditions during the polar summer, the variation of the
total water vapor is quite high. Figure 4 shows an example of FTIR measurements in lunar
and in solar absorption. Lunar absorption measurements are necessary for measurements
during polar winter, when there is no sun above the horizon for several months. Lunar
absorption measurements can partially fill in the gap (Palm et al., in preparation).
Upper tropospheric water vapor
The problem when aiming on the vertical distribution of water vapor are the strong
interferences of the large lower tropspheric water vapor amounts with the signals coming from
higher altitudes. To keep these interferences as small as possible, it is necessary to optimize
the retrieval approaches. In this context an innovative retrieval approach was developed. The
innovations are the transformation of the inversion problem on a logarithmic scale and the
joint retrieval of temperature profiles (Hase et al., 2004; Schneider et al., 2006a) and
constraining the H2O and HDO profiles against each other (optimal estimation of δD(*) profiles;
Schneider et al (2006b)).
A comparison of simultaneously performed FTIR and Vaisala radiosonde (RS80)
measurements confirms the good precision of the so-retrieved upper tropospheric H2O
amounts (Fig. 5). We get a correlation coefficient of 0.81 between the data of both techniques.
Figure 6 depicts a time series of H2O partial column amounts detected by FTIR instruments
between 1999 and 2005 above the Izaña Observatory.
A detection of middle tropospheric δD values is even more challenging than the detection of
upper tropospheric H2O amounts. The reason is that the natural δD variabilities are very low.
While H2O and HDO have a typical 1σ variability of 100%, the δD 1σ variability is only around
5-10%. Only the new retrieval approach provides the required high precision. Figure 7 shows
the evolution of the middle tropospheric δD values above Izaña from January 2005 to March
2006. It provides information about the origin of the detected airmass. If it originates from a
region with a statically instable troposphere like the tropics, the value is relatively high.
the δD value is the relative difference of the actual HDO/H 2O ratio to a standard HDO/H2O
ratio RSMOW = 3.1152 x 10-4; δD = 1000 x [ (HDO/H2O) / RSMOW – 1 ]
(*)
Figure Captions:
Figure 1: FTIR experiment at Izaña. Center: housing of the experiment; Left: solar tracker on
the roof of the housing; Right: entrance optic of the spectrometer.
Figure 2: Mid-infrared measurement and zoom into an H2O and HDO spectral microwindow.
Figure 3: Short term variation of water vapor column amounts obtained by continuous
measurements of FTIR spectra during May 1999.
Figure 4: Seasonal cycle of water vapor column amounts above Ny Ålesund, Spitsbergen,
obtained by solar and lunar absorption measurements.
Figure 5: Correlation between upper tropospheric (7.5-10.0 km) water vapor amounts
measured at Izaña by the Vaisala radiosonde (RS80) and by the ground-based FTIR
spectrometer (Bruker IFS 120M).
Figure 6: Time series of water vapor above Izaña. Black circles: measurements of
spectrometer Bruker IFS 120M; red crosses: measurements of spectrometer Bruker IFS
125HR. Please note that the water vapor amounts above 8 km are only detectable for
moderate water vapor slant column amounts (moderately saturated lines).
Figure 7: 15-months record of δD above Izaña.
References:
Hase, F., J.W. Hannigan, M.T. Coffey, A. Goldman, M. Höpfner, N.B. Jones, C.P. Rinsland,
S.W. Wood: Intercomparison of retrieval codes used for the analysis of high-resolution,
ground-based FTIR measurements, J. Quant. Spectrosc. Radiat. Transfer, 87, 25-52, 2004.
Palm, M., C. Melsheimer, S. Noël, J. Notholt, J. Burrows, and O. Schrems:
Total water vapour above Ny Alesund, Spitsbergen: a multisensor intercomparison.
Manuscript in preparation.
Schneider, M., F. Hase, T. Blumenstock: Water vapour profiles by ground-based FTIR
spectroscopy: study for an optimised retrieval and its validation, Atmos. Chem. Phys., 6, 811830, 2006a.
Schneider, M., F. Hase, T. Blumenstock: Ground-based remote sensing of HDO/H2O ratio
profiles: introduction and validation of an innovative retrieval approach, Atmos. Chem. Phys.,
6, 4705-4722, 2006b.