ESA/EUMETSAT EPS-MetOp RAO
Faisabilité d'assimilation de colonnes troposphériques d'ozone mesurées
par EPS.
Feasibility of assimilating ozone tropospheric columns measured by EPS
Description
The Clime project is principal investigator of the project 3029 under the
ESA-Eumetsat "EPS MetOp" Research Announcement of Opportunity (RAO).
The objectives of the project are to assess the feasibility of assimilating
the tropospheric soundings of the IASI and GOME2 sensors onboard the ESA's
EPS/Metop satellite, launched in October 1996.
The project is mainly interested in ozone measurements provided by IASI: 0-6km
column, tropospheric profiles, to be assimilated in the Polyphemus platform
for regional air quality forecast.
Achievements
- Due to the late availability of IASI level 2 data, simulated data have been used in
a preliminary phase:
- Using a reference simulation of the atmosphere as input (domain: Europe,
period: July 2001);
- Radiative transfer modelled by LBLRTM code, convolved with the IASI
instrument transfer function;
- The obtained spectra are inverted using the SA-NN code (thanks IPSL/SA)
to yield 0-6km ozone column with similar characteristics as the expected
IASI columns.
- The simulated columns are assimilated using Optimal Interpolation within
a perturbated run.
- The experiments show that assimilating columns has the potential to
improve the forecast of the boundary layer ozone, despite the fact that
the column mainly contains free troposphere ozone.
- Assimilation experiments
have been carried out on NO2 columns measured by the OMI sensor onboard NASA's Aura
satellite. On the one hand, NO2 assimilation is expected to have a greater impact
on air quality forecast due to (1) its high concentrations in the boundary layer
as compared to higher altitudes and (2) the better accuracy of NO2 satellite measurements,
as compared to O3. On the other hand NO2 is hard to predict due to its short
lifetime.
- The performances of Polyphemus with respect to NO2 forecast are assessed
and comply with state-of-the-art. A significant bias
is observed between satellite and model columns.
- Assimilation of NO2 columns, taking into account a seasonally constant bias, has
shown significant improvements brought to air quality forecat.
-
IASI level 2 data (O3 columns) are finally considered for assimilation. A first comparison
has been performed with ozone sonde data to assess both the quality of the satellite
measurement and the quality of the model, especially in higher altitudes. It appears
that the model significantly underestimates ozone concentrations in the free troposphere,
leading to a large systematic bias between model and satellite columns. The assimilation
experiments are currently pending: either the assimilation algorithm is adapted to
account for an altitude dependent biais; or it is better to consider improving the
model.
