Fourth Study Conference on BALTEX Scala Cinema Gudhjem

- 5 -
Remote Sensing of Atmospheric Properties Above the Baltic Region
Jürgen Fischer, Peter Albert and Rene Preusker
Institut für Weltraumwissenschaften, Freie Universität Berlin, Germany
Carl-Heinrich Becker Weg 6-10
D-12165 Berlin
e-mail: anja.hünerbein@wew.fu-berlin.de
1. Introduction
The observation of water vapour and cloud properties from
the polar orbiting satellites ENVISAT, TERRA and AQUA
has improved the understanding of their spatial and temporal
variability. The columnar water vapour are retrieved from
measurements in the near-infrared with an accuracy which
has been not reached before the satellite instruments MODIS
(Moderate Resolution Imaging Spectroradiometer) and
MERIS (Medium Resolution Imaging Spectrometer) has
been launched in 1999 and 2002. The unique spectral
channels within the oxygen A-band absorption of MERIS
are suited to improve the retrieval of cloud top pressure.
MODIS has dedicated channels within the CO2 absorption
band at 15 µm which also leads to a more precise retrieval of
the cloud top pressure. Both, MODIS and MERIS
measurements are used to observe water vapour and cloud
properties above the Baltic region.
2. Results
The algorithms for the retrieval of the atmospheric
properties are validated for MODIS and MERIS
measurements. Figure 1 exemplarily shows a scatter plot of
columnar water vapour measured by GPS stations vs.
MERIS measurements. The measurements are taken over
Germany between October 2002 and September 2003. Both,
the rms of 0.17g/cm 2 and a bias of 0.03g/cm 2 , indicate that
the MERIS water vapour retrieval above land surfaces is of
high quality. The accuracy of the MODIS water retrieval is
in the same range as shown for MERIS. Small scale
variations of the water vapour fields can be observed with
the Full Resolution (300*260 m 2 ) MERIS images (Figure 2).
MODIS and MERIS data are used to estimate daily and
monthly means of the water vapour fields above Europe.
Figure 1. Scatter plot of columnar water vapour measured
by GPS stations vs. MERIS measurements. Measurements
are taken over Germany between October 2002 and
September 2003 .
The cloud top pressure is more difficult to validate, because
there are only a few independent measurements, such as
from cloud radars, are available. The accuracy of the cloud
top pressure retrieval which can be achieved from MODIS
Figure 2. Integrated water vapour taken from a Full
Resolution MERIS image (12th of August 2003).
and MERIS measurements depends on the cloud type and
the existence of multi-layer clouds. A comparison of
MODIS and MERIS cloud top pressure retrievals are
shown in figure 3. MODIS observes more high clouds
which is caused by the measurements in the thermal
infrared which are more sensitive to high thin clouds,
which are more difficult to detect from MERIS
measurements in the near infrared.
Observations from the satellite instruments MERIS and
MODIS will help to study the atmospheric variability and
contribute to the aims of GEWEX.
Figure 3. Scatter diagram of the median MODIS and the
median MERIS cloud top heights for all considered
scenes.
References
Albert, P., R. Bennartz, R. Preusker, R. Leinweber, J.
Fischer: Remote Sensing of atmospheric water vapor
using the Moderate Resolution Imaging Spectrometer
(MODIS), Geophysical Research Letters, in press,
2004
Preusker, R. J. Fischer, P. Albert, R. Bennartz, and L.
Schüller: Cloud-top pressure retrieval using the
oxygen A-band in the IRS_3 MOS instrument, IJRS,
excepted, 2004.