1. . Assimilation of MIPAS-ENVISAT chemical constituents during a major EPP-NOx event over Antarctic winter 2003. A.Robichaud 1, R. Ménard 1, Yves Rochon 1, Yan Yang 1, S.Chabrillat 3, J. de Grandpré 1 and C. Charette 2 1. Science & Technology Branch (Environment Canada), 2. Meteorological Research Division, 3. Belgium Institute for Space Aeronomy GOALS Evaluate the impact of assimilating chemical constituents of MIPAS/ENVISAT (O3,NO2 and HNO3) Evaluate the success of our stratospheric assimilation system in capturing a major EPP-NOx episode Demonstrate that an assimilation system could be option and can reproduce to a large degree some phenomena even if they are absent in the modeling system INTRODUCTION The stratospheric chemical constituents retrieved from MIPAS (Michelson Interferometer for Passive Atmospheric Sounder) instrument has been assimilated using the Canadian 3D-VAR system in a new coupled dynamical-chemical stratospheric model (GEM-BACH). The data products for MIPAS onboard the ENVISAT satellite, one of the most largest observation platform ever launched to space, include temperature and various gas vertical profiles from limb sounding in the mid-infrared part of the spectrum. The period of study is austral winter 2003 where a considerable flux of NOx was reported to descend to the stratosphere linked with an EPP (Energetic Particle precipitation) event. Such phenomenon can modify substantially the NOx budget of upper stratosphere and participate in catalytic processes controlling ozone of polar regions. The focus of the study is to show how the assimilation system can handle the phenomenon given that there is no provision for the model to simulate EPP (or similar geomagnetic events) and its indirect effects in the stratosphere. The mismatch between observation and model is likely to be related with high amounts of NOx being continuously transported downward from the lower thermosphere during May to August 2003 linked with geomagnetic/EPP event (Funke et al., 2005, Stiller et al., 2005, Randall et al., 2007). Modeling such phenomenon would require reproducing physical and chemical processes from the surface to the thermosphere, even perhaps to the exosphere which is quite costly. Assimilation instead provides an interesting option to capture phenomena that cannot be taken into account without prohibitive costs. MODEL, ASSIMILATION AND OBSERVATION SYSTEMS USED IN THIS STUDY: -GEM-BACH: Global Environmental Multi-scale model (stratospheric version) coupled with the Belgium Atmospheric Chemistry model. The Belgium photochemical module was developed at BIRA Institute and includes 57 chemical species which interact through 143 gas-phase reactions, 48 photolysis reactions and 9 heterogeneous reactions. The heterogeneous chemistry is parametrized using a simple function of temperature and a prescribed climatology sulfate area densities. The model has no provision for complicated processes such as ion-cluster chemistry. The model GEM is a global non-hydrostatic grid point model developed by Coté et al., 1998 for the purpose of environmental prediction and uses a semi-implicit scheme and a two-time level semi-Lagrangian advection scheme. -3DVAR-CHEM: A new version of the 3DVAR originally developed at CMC-MRB by Gauthier et al., (1999) has been upgraded to take into account chemical assimilation. When performing assimilation the FGAT (First Guess at Appropriate Time) method was utilized. The background error statistics used were obtained applying the Hollingsworth and Lonnberg (1987) methodology whereas the spatial correlations are determined from 6-hours differences method (so-called “Quick Covariance”, Polavarapu et al., 2005). -MIPAS/ENVISAT OBSERVATIONS: The MIPAS_ESA offline data products used in this study are the version 4.61 retrieved by the operational OFM/ORM MIPAS level 2 data processor. OBSERVATIONAL EVIDENCE AND DESCRIPTION OF THE PROBLEM FIGURE 1 A FIGURE 1B IMPACT OF ASSIMILATION OF CHEMICAL CONSTITUENTS Figure 7. (left). Verification OMP (Observation minus Precition) for 6 hours forecast of NO2 interpolated at the point of observation for the South Pole region (-60 S to -90S). Solid line is for bias (systematic error) and dotted lines for standard deviation of OMP (random error). Black lines are for when the assimilation is ON and red curves for the case of assimilation OFF. Note the large improvement for the case when assimilation is present for both systematic and random errors. Right: same but for HNO3 except that green curves represent the case when the assimilation is OFF. In the upper stratosphere (1.0-10 mbs) the bias without assimilation is large and positive indicating undcerestimation linked with the EPP event. In the lower stratosphere, the bias changes sign and becomes large and negative. This is likely due to underestimation of the denitrification process, a known weakness of the model. When we take into account EPP event such as the one of austral winter 2003, the impact of assimilation is large on NO2 and HNO3 (black curves) suggesting a significant influence on the ozone budget of the upper stratosphere. SUMMARY AND CONCLUSIONS Univariate assimilation of MIPAS-ESA chemical constituants greatly improves the quality of forecats and analysis of all assimilated constituants. Use of H-L method for observation and background error variances has significantly improved the quality of assimilation. Since model has no provision for EPP or other related geomagnetic events, it strongly under estimates values of HNO3 and NO2 during Antarctic winter 2003. However, assimilation can partly handle the phenomenon. Since mesospheric NOX descent occurs relatively often and penetrates upper atmosphere, stratospheric or mesospheric modellers are trying to make effort to account for this phenomenon in upper atmospheric models and studies. It has a deep influence on the NOx (and likely on the ozone) budget of the upper stratosphere. However, assimilation of observations such as MIPAS/ENVISAT provides an interesting option which permits to avoid prohibitive costs of modeling from surface to the exosphere to take into account geomagnetic events and their influence on the stratosphere. Assimilation in our particular case also corrects known model deficiencies such as underestimation of denitrification at the South Pole, problems of J-value of the photochemical module, etc. REFERENCES Coté J et al., 1998. The Operational CMC-MRB Global Environmental Multiscale (GEM) Model. Part I: Design Considerations and formulation. Monthly Weather Review, 126, 1373-1395, 1998. Funke B., M. Lopez-Puertas and S. Gil-Lopez, 2005. Downward transport of upper atmospheric NOx into the polar stratosphere and lower mesosphere during the Antarctic 2003 and Arctic 2002/2003 winters. Journal of Geophysical Research, vol. 110, D24308, 2005. Gauthier, P. Charette C., Fillion L., Koclas P. and Laroche S., 1999. Implementation of a 3D Variational data assimilation system at the Canadian Meteorological Centre. Part I: The Global Analysis. Atmosphere Ocean, vol. XXXVII, no. 2, pp. 103-156. June 1999. Hollingsworth A. and Lönnberg P., 1986. The statistical structure of short-range forecast errors as determined from radiosonde data. Part I. The wind field. Tellus, 38A, 111-136. Polavarapu S., Ren S., Rochon Y., et al., 2005. Data assimilation with the Canadian Middle Atmosphere Model. Atmos-Ocean, 43, 77-100, 2005. Randall, C.E., V.L. Harvey, C.S. Singleton, S. M. Bailey, P.F. Bernath, M. Codrescu, H. Nakajima and J.M. Russell III, 2007. Energetic particle precipitation effects on the Southern Hemisphere stratosphere in 1992-2005. Journal of Geophysical Research, vol. 112, D08308, 2007. Siskind D.E., G.E. Nedoluha, C.E. Randall, M. Fromm and J.M. Russell III, 2000. An assessment of Southern Hemisphere stratospheric NOx enhancements due to transport from the upper atmosphere. Geophys. Res. Lett., 27, 339-332. Stiller G.P., Gizaw Menggistu Tsidu, von Calrmann T., Glatthor N., Hopfner M., Kellmann S., Linden A., Ruhnke R. H. Fischer, M. Lopez-Puertas, Funke B., and S. Gil-Lopez, 2005. An enhanced HNO3 second maximum in the Antarctic midwinter upper stratosphere 2003, Journal of Geophysical Research, 110, D20303, doi:10.1029/2005JD006011 ACKNOWLEDGMENTS. This study was done under contract ESA/ESTEC No. 18560/04/NL/FF for the European Space Agency. VERIFICATION OMP 6H FORECAST VS OBS (NO2 and HNO3) (with and without assimilation) FIGURE 2 A FIGURE 3 FIGURE 4 FIGURE 5 FIGURE 6 Corresponding author: Alain Robichaud (alain.robichaud@ec.gc.ca) FIGURE 2 B ASSIM ONASSIM OFF ASSIM ON ASSIM OFF Figure 1. A. Potential temperature versus time for NO2 (left top) and CH4 (left bottom) (from Funke et al., 2005) revealing the Figure 2. Model vmr (green) vs MIPAS observation (blue) as a function of pressure in the South Pole Figure 3. Scatter plot for NO2 vs CH4 for model and observations in the Southern descent of air from mesosphere bringing air rich in NO2 and poor in CH4. Figure 1B. Pressure versus time for HNO3 vmr region for NO2 (left) and HNO3 (right) shorthly after a mesospheric intrusion of NOx following an Hemisphere during the second half of August 2003 near the top of the upper stratos-(from Stiller et al., 2005). The NO2 is converted into N2O5 and eventually in HNO3 creating an anomalous high values in the EPP-NOx event. The large mismatch between model and observations is due to the fact that EPP sphere. Deviation of linear relationship NO2 (or NOx) vs CH4 indicaates an EPP upper stratosphere. Note that coincident high values of NO2, HNO3 and low values of CH4 rules out the possibility that the events are not taken into account in our modelling system. event according to Randall et al., 2007. phenomenon is a retrieval artifact. Figure 4. Left: analysis increments showing the impact of assimilation NO2 on a zonal mean vs pressure for Aug 11 (top) and Aug 31 (bottom) 2003. Right: same but with no assimilation. Note that the signature of mesospheric descent is indicated by the circle (anamalous average analysis increment). Figure 5. Left: analysis increments showing the impact of assimilation HNO3 on a zonal mean vs pressure for Aug 11 (top) and Aug 31 (bottom) 2003. Right: same but with no assimilation.. A connection between figure 4 and 5 is clearly visible as the NO2 (figure 4) converts into HNO3 between 1 and 10 hPa (via ion-cluster chemistry) so that analysis increments of HNO3 show a maximum just below that of NO2 consistent with a transformation NO2 to HNO3 (figure 5, left panels). Figure 6. Time and zonal averaged analysis increments from assimilating MIPAS- ESA observations (AUG 17-SEP 21 2003). Top left: ozone, top right: NO2, bottom left: HNO3, bottom right: temperature. Some interesting features appear especially in the South Pole region associated with the EPP-NOx event and also near the stratopause level. The latter suggests a problem with the J-value of the photochemical module.