
AMC-DOAS
DOAS
SCIAMACHY
GOME
|
|
Water
Vapour
Retrieval using
AMC-DOAS
Introduction
Retrieval Method
Data Products
Data Access
References
Contact
This is the AMC-DOAS main page.
Introduction
Water vapour is one of the most
important atmosphere constituents. Most of it is located in the
troposphere close to the surface of the Earth. Especially, water vapour
is the major greenhouse gas; without the natural greenhouse effect due
to water vapour life on Earth as we know it would not be possible. The
knowledge about the global distribution of water vapour is a relevant
input quantity for atmospheric models aiming to predict weather or
climate.
The probably most accurate tropospheric water vapour measurements are
performed by radio sondes. However, radio sondes can only provide very
localized snapshots of the atmospheric conditions, and the distribution
of radio sonde measurement sites over the Earth is rather inhomogeneous
and considerably sparse especially over the oceans and in the southern
hemisphere.
Only satellite measurements using remote sensing techniques can provide
water vapour data on the global scale, but they are usually limited in
spatial and/or temporal coverage and resolution. Different sensors
using different spectral regions and viewing geometries and various
retrieval methods have been developed to derive water vapour
concentrations from satellite data.
The Air Mass Corrected
Differential Optical Absorption Spectroscopy (AMC-DOAS) is a
method to retrieve total water vapour column amounts from spectral
measurements in the visible wavelength region around
700 nm. Because data in the visible spectral range are analysed, the
AMC-DOAS
method is only applicable to measurements on the dayside and to
(almost) cloud-free ground scenes. A significant advantage of the
AMC-DOAS method is that the derived water vapour columns do not depend
on additional external information, like a calibration using radio
sonde data which is often used in the microwave spectral region. The
AMC-DOAS water vapour columns therefore provide a completely
independent data set.

Example for
an
Earthshine spectrum measured by the GOME
instrument (channel
4).
The AMC-DOAS fitting window is marked.
(Click to enlarge)
Retrieval Method
The AMC-DOAS algorithm is based on a modified DOAS approach
as it uses the information contained in the differential absorption
structures. The main differences to "standard" DOAS are:
- Saturation effects arising from highly structured
differential
spectral features which are not resolved by the measuring instrument
are accounted for.
- O2 absorption features are fitted in
combination with
H2O to determine an air mass correction factor which
compensates to some degree for insufficient knowledge of the background
atmospheric characteristics, especially cloudiness, and for geometrical
aspects like scan angle dependencies.
Although no clouds are considered explicitly in the retrieval,
the method of air mass correction provides the possibility to retrieve
meaningful H 2O total columns also for partly cloudy scenes.
The size of the air mass correction factor can be used a criterion for
the quality of the H 2O data product.
In the ideal case, i.e. if the atmospheric conditions used in the
model calculations match the real conditions, the air mass correction
factor should be 1.
In the presence of clouds, only the atmosphere above the
clouds can be probed by the instrument, so the effective amount of
both O 2 and H 2O seen by the instrument is smaller
than the
amount used
in
the model calculations. The air mass factor correction would then try
to compensate for this. In this case the correction factor would be
smaller than 1.
If the air mass correction factor deviates too much from 1, this is an
indication that the conditions of the reference atmosphere differ to
much from reality.
In this case the retrieved H 2O columns are considered to be
unrealistic.
In practice, it could be shown that data retrieved with air mass
correction factors smaller than 0.8 are unreliable.
Therefore, these data have been omitted from the distributed data
products.
In addition, measurements performed at high solar zenith angles (larger
than 88 deg) have been excluded.
Details about the AMC-DOAS method can be found e.g. in Noël
et al. (2004).
There is also an Algorithm
Description Document available.
Data Products
The AMC-DOAS retrieval method has been successfully applied to
measurements of GOME (Noël
et al., 1999)
and SCIAMACHY (Noël
et al., 2004) . Because the ground pixel size of
SCIAMACHY is typically much
smaller than for GOME (30 km x 60 km compared to 40 km x 320 km), the
probability for cloud-free scenes is much higher for SCIAMACHY. Global
coverage is achieved for SCIAMACHY nadir measurements in 6 days, for
GOME in 3 days.
SCIAMACHY and GOME H2O
total columns over Europe
for 27 Jan
2003.
The smaller SCIAMACHY ground pixels are marked by boxes.
(Click to enlarge)
AMC-DOAS water vapour results have been compared with various
independent data, including
measurements of the Special Sensor
Microwave Imager (SSM/I) over ocean, assimilated global water vapour
data provided by the European Centre for Medium-Range Weather Forecast
(ECMWF) and in-situ radio sonde data. These intercomparisons revealed
in general a good agreement
between the different data sets, but AMC-DOAS water vapour columns are
typically slightly (~0.2 g/cm 2) lower than e.g. SSM/I and
ECMWF data. This is most likely caused by the exclusion of too cloudy
(and thus moist) scenes in the AMC-DOAS data. In all cases a large
scatter in the data (~0.5 g/cm 2) was observed which can be
mainly attributed to the large spatial and temporal variability of
water vapour (see e.g. Noël
et al., 2005, for details).
Intercomparisons between AMC-DOAS results based on GOME and SCIAMACHY
measurements showed a good agreement (Noël et al., 2006, 2007).
It is
therefore possible to
generate a combined GOME/SCIAMACHY water vapour climatology which is
useful for climatological trend analysis applications.

The
combined GOME (1996-2002) and SCIAMACHY (2003-2006) water vapour data
set (monthly means).
(Click to enlarge)
Data Access
AMC-DOAS
water vapour total column data have been derived for all GOME data from
July 1995 until December 2004 (with reduced coverage after June 2003).
All available
SCIAMACHY data since August have been processed. The analysis is
ongoing, have a look at the
SCIAMACHY DOAS nadir browser for most recent results.
Current data product version is 1.0. Click here
for a list of changes. You may also download the Product Specification Document.
Access to all data is
possible under certain conditions;
click here for details.
References
- S. Noël, M. Buchwitz, H.
Bovensmann,
R.
Hoogen, and J. P. Burrows,
Water vapour retrieval from GOME data, Proc. European Symposium on
Atmospheric
Measurements from Space, vol. 1, 375-381, 1999. (PDF
file, ca. 284 kB)
- S. Noël, M. Buchwitz, H.
Bovensmann,
R.
Hoogen, and J. P.
Burrows, Atmospheric Water Vapor Amounts Retrieved from GOME Satellite
Data,
Geophys.
Res. Lett., vol. 26(13), 1841-1844, 1999. (PDF
file, ca. 211 kB)
- S. Noël, H. Bovensmann, J. P.
Burrows,
Water vapour retrieval
from
GOME data including cloudy scenes, Proc. ENVISAT/ERS Symposium,
Gothenburg,
2000. (PDF
file, ca.1.7 MB)
- S. Noël, M. Buchwitz, H.
Bovensmann,
and
J. P. Burrows,
Retrieval
of Total Water Vapour Column Amounts from GOME/ERS-2 Data, Adv. Space
Res., 29(11), 1697-1702, 2002. (PDF file, ca.
284 kB)
- S. Noël, M. Buchwitz, and J. P.
Burrows,
First retrieval of
global
water vapour column amounts from SCIAMACHY measurements, Atmos. Chem.
Phys., 4, 111-125, 2004. (PDF
file at ACP web site, ca. 22.6 MB)
- S. Noël, M. Buchwitz, H.
Bovensmann,
and
J. P. Burrows,
SCIAMACHY
water vapour retrieval using AMC-DOAS, Proc. ENVISAT Symposium,
Salzburg, Austria, 6-10 September, 2004 (ESA SP-572), 2005. (PDF file, ca. 1.4 MB)
- S. Noël, M. Buchwitz, H.
Bovensmann,
and J. P. Burrows, Validation of SCIAMACHY AMC-DOAS water vapour
columns,
Atmos. Chem. Phys., 5, 1835-1841, 2005.
(PDF
file at ACP web site, ca. 3.6 MB)
- S. Noël, S. Mieruch, M.
Buchwitz, H.
Bovensmann, and J. P. Burrows, GOME and SCIAMACHY global water vapour
columns, Proc. of the First ‘Atmospheric Science Conference’, ESRIN,
Frascati, Italy, 8–12 May 2006 (ESA SP-628), 2006. (PDF file, ca. 1.4 MB)
- S. Noël, S. Mieruch, H.
Bovensmann,
and J.
P. Burrows, A combined
GOME and SCIAMACHY global water vapour data set, Proc. ENVISAT
Symposium, Montreux, Switzerland, 23–27 April 2007 (ESA
SP-636), 2007. (PDF file, ca. 9.6
MB)
- S.
Mieruch, S. Noël, H. Bovensmann and J. P. Burrows,
Analysis of global water vapour trends from satellite measurements in
the visible spectral range, Atmos.
Chem. Phys., 8, 491-504, 2008. (PDF file, ca. 13
MB)
- S.
Noël, S. Mieruch, H. Bovensmann and J. P. Burrows,
Preliminary results of GOME-2 water vapour retrievals and first
applications in polar regions, Atmos. Chem. Phys., 8,
1519-1529, 2008. (PDF file, ca. 9.7
MB)
Contact
If you are interested in more information on
AMC-DOAS water vapour, please contact Stefan
Noël.
Author of this page: Stefan
Noël

©2008
|