European Perspective on Geostationary Microwave Sounding

Bizzarro Bizzarri, CNR Istituto di Scienze dell'Atmosfera e del Clima,
Roma, Italy

Phone +39.06.4426.1604, Fax +39.06.4423.7615, E-mail bibizzar@tin.it

# Abstract

The first European undertaking in space meteorology was Meteosat,
launched in the geostationary orbit on 23 November 1977. Since then,
seven flight models have been placed in orbit, Meteosat-7 due to be in
use up to about 2004. Meteosat Second Generation (MSG) has been launched
on 28 August 2002. The MSG programme will provide four flight models, to
be in use up to about 2017. Planning for an MSG-follow-on programme,
possibly Meteosat Third Generation, has already initiated. After a
one-year preparatory work, the EUMETSAT 1^st^ Post-MSG User Consultation
Workshop took place on 13-15 November 2001. Follow-on industrial studies
are now being organised in cooperation with ESA.

A number of user requirements clearly call for an improved VIS/IR
imager, which is certainly feasible with current technology. Also it
seems feasible to meet a number of requirements calling for a
high-spectral-resolution IR sounder. A major requirement, to have good
wind profiles, can only indirectly be addressed by improving the imagery
and sounding missions. Another major requirement is for [frequent
observation of precipitation]{.underline}. This requirement can
partially be addressed by improving the VIR/IR imagery and the IR
sounding missions, but clearly calls for [MW radiometry]{.underline}. In
addition, there is a strong requirement to have frequent
temperature/humidity sounding in overcast areas, which also calls for MW
radiometry. However, it is currently felt that affordability and
programmatic constraints are too tight to enable a MW radiometer being
baselined for an operational Meteosat Third Generation proper.

Studies on MW sounding from geostationary orbit have therefore started
as a side-activity to post-MSG planning. A project named GOMAS
(*Geostationary Observatory for Microwave Atmospheric Sounding*) has
been defined (in cooperation with US partners) and proposed to ESA as an
Earth Explorer Opportunity Mission. The proposal was not selected
because of incompatibility between the technology readiness and the
schedule constraint, but ESA has accepted a recommendation to pursue
some technological activity in the field. Meanwhile, the Italian Space
Agency (ASI) has approved a 3-year study aimed at preparing a proposal
for a demonstration mission to be implemented as a national or
multi-lateral programme. Convergence between GOMAS and the U.S. activity
on GEM (Geostationary Microwave Observatory) is being pursued.

The principle of GEM and GOMAS is to infer precipitation from
[absorption bands]{.underline} rather than atmospheric windows. Oxygen
and water vapour bands at different frequencies are used to retrieve
temperature and humidity profiles. The retrievals at different
frequencies are differently affected by liquid and ice water, drop size
and shape, and precipitation; therefore, in principle, simultaneous
retrieval of temperature/humidity profile, cloud ice/liquid water
columnar content or gross profile and precipitation is possible. Being
in absorption bands, the observation is equally available over sea and
land.

European requirements for GOMAS and U.S. requirements for GEM are
slightly different. Better s.s.p. resolutions are required to compensate
for the relatively high latitudes of Europe (the baseline antenna
diameters are 3-m for GOMAS, 2-m for GEM). The variability of
precipitation types is higher for Europe, since non-convective rain and
snowfall are dominant and convective rain also is well represented:
therefore, the spectral range of the radiometer must be such as to
ensure widest applicability. All-weather temperature and humidity
profiles are important over (the generally cloudy) Europe: therefore,
the number of channels, their bandwidths and the radiometric accuracy
must be conveniently stringent. The final outcome is that, whereas GEM
could be attempted to be compatible with a large multi-purpose satellite
of a GOES+ series, GOMAS is more likely to require a free-flyer.

The current preliminary concept of GOMAS, scaled from GEM and based on
the latest performances in MW and Sub-mm technology, will be presented.
A 3-m antenna and five bands are considered (54, 118, 183, 380 and 425
GHz), for a total of about 40 channels. The radiometric performance
needed for profiling each 15 min is achieved if the scanned area is
limited to about 1/12 of the Earth disk. If USA/Europe cooperation
materialises, the demonstration mission would be organised so as to move
the scanned area towards any part of the disk and periodically migrate
the satellite s.s.p. between America and Europe. The mass estimate of a
GOMAS free-flyer is 860 kg at launch, 430 kg "dry". The data rate is 128
kbps, compatible with direct reception at low-cost MSG local stations.
The satellite would allow simultaneous retrieval of:

-   temperature profile with resolution 30 km at s.s.p.

-   humidity profile with resolution 20 km at s.s.p.

-   cloud liquid/ice water columnar content and gross profile with
    resolution 20 km at s.s.p.

-   precipitation rate with resolution 10 km at s.s.p.

***each 15 minutes, over 1/12 of the disk covering sea and land.***