1

Assimilation of EPS tropospheric ozone for air quality forecastB. Sportisse, M. Bocquet, V. Mallet, I. Herlin, JP. Berroir, H. Boisgontier

ESA EUMETSAT EPS/MetOp RAO Workshop, May 2006

2

Project objectives

•Applicative context: air quality forecast at European Scale. Air quality -> boundary layer concentrations

•Objective: • to assess the feasibility of assimilating O3 columns and profiles, to be

provided by MetOp (IASI, GOME): numerical experiments to be performed before the launch of MetOp

• to perform experiments with real O3 data when available

• validation and quantification by comparison with forecast obtained without assimilation, or with assimilation of ground stations measurements

3

Outline

•The POLYPHEMUS air quality modelling system

•Preliminary studies:• Relative weight of boundary layer ozone in the IASI 0-6km O3 column

• Sensitivity of boundary layer ozone to IASI O3 column

•Data assimilation feasibility• Based on twin numerical experiments

• Based on simulated IASI columns

•Perspectives: assimilation of real data

4

The Polyphemus air quality modelling system

•Developed by CEREA (joint ENPC-EdF laboratory, Clime team shared between ENPC and INRIA)

•Modular architecture:• Access to raw input data• Input data processing: physical parameterizations, …• Numerical heart: CTM, its linear tangent and adjoint models• High level applications:

– Direct forecast– Sensitivity studies– Impact studies– Ensemble forecast– Data assimilation– …

• Visualisation

5

POLYPHEMUS architecture

6

POLYPHEMUS applications: ozone forecast

European scale run

ECMWF met. Fields

EMEP emissions

Extensively validated

http://www.enpc.fr/cerea/polyphemus

7

POLYPHEMUS applications: ensemble forecast

0 5 10 15 20Hours

30

40

50

60

70

80

90

100

110 O3 vs time for different physical parameterization

Need of better constraining models

Data assimilation

8

Preliminary studies

Is assimilation of EPS O3 information within regional CTMs feasible?

Problem:• EPS O3 mainly representative of upper troposphere

• Air quality application interested in boundary layer ozone

Feasible if:• Boundary layer has a significant weight in the 0-6km column

• Boundary layer ozone, as forecast by Polyphemus, is sensitive to changes in upper tropospheric ozone

9

Relative weight of boundary layer O3

•Computed from a reference situation (validated Polyphemus analysis – July 2001)

•Average contribution of boundary layer 03 to 0-6km column: 14% (more during day because boundary layer is higher)

•Irregularly scattered in space and time

Mean at 0h

Mean at 15h

10

Sensitivity of boundary layer O3

•Experiment: • Consider a reference run

• Apply perturbation to upper tropospheric ozone: at initial date, every 24h

• Run forecast with perturbated O3

• Compare forecast with reference

•Conclusion, from initial perturbation.• Max. sensitivity: 50%

• Max difference with reference observed 27h after simulation

•Conclusion, from cyclic pertubation• Perturbated runs quicly diverges from reference

•Overall conclusion: Yes, upper tropospheric ozone can be used to constrain boundary layer O3

11

Sensitivity of boundary layer ozone

-50% perturbation applied on UT O3

Graph: mean relative difference at ground level between perturbation and reference

12

Sensitivity of boundary layer O3

Same experiment

Relative difference 27h after perturbation

13

Sensitivity of boundary layer O3

Perturbation applied every 24h

14

Feasibility of data assimilation

•Numerical experiments:

•Columns generated from reference situation, 5% perturbation

•Assimilation within perturbated runs:• Method: Optimal Interpolation

• Different choices of model/observation covariance matrices

• Assimilation every 24h

• 2 types of perturbated runs:

– Perturbation of initial condition: assimilation should speed up the convergence to reference

– Pertubation of physical parameterization (Kz): perturbation always different from reference, assimilated run should lie in between.

15

Feasibility of assimilation: perturbation of initial condition

0 20 40 60 80 100 120 140 160 1800

5

10

15

20

Relative difference with reference

Blue: without DA

Others: DA with different values of obs. error.

Quicker convergence to reference

16

Feasibility of assimilation: simulated IASI column

•Context: cooperation with SA (C. Clerbaux)

•Simulation of IASI radiances from:• Polyphemus reference (0-5km)

• UGAMP climatologies (6-60km)

• Interpolation (5-6km)

• LBLRTM code + IASI instrument model

•Inversion of IASI radiances:• Atmosphit model (for evaluation, 2 dates)

• SA NN operational code

17

Simulation of IASI measurements

Example of profiles/inversion error (Atmosphit)

18

Simulation of IASI measurements: issues

•Preparing the assimilation of real data: • Realistic noise (instrument, inversion procedure)

• Set up of observation operator

• Set up of observation covariance matrices

•Influence of a priori information:• 6-60km (O3, T, p)

• LST

•Ongoing, awaiting results

19

Conclusions

•Twin numerical experiments tend to show that assimilation of 0-6km is feasible and can improve the analysis of boundary layer O3, in the case of high quality measurements

•Realistic accounting for instrument and inversion noise: experiment with simulated IASI columns going on

•Waiting for real data…