The technique of data assimilation gives us an opportunity to further our understanding of important photochemical processes in the Martian atmosphere, through the creation of a reanalysis product that can be used to investigate the temporal and spatial agreement between model and observations and determine any possible causes of identified differences. In this study [1], we have assimilated, for the first time, total ozone retrievals into a Mars Global Circulation model (GCM) to study the ozone cycle

Holmes, J.A.

Lefèvre, F.

Lewis, S.R.

Patel, M.R.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsOn the assimilation of Martian total ozone retrievalsConference or Workshop ItemHow to cite:Holmes, J.A.; Lewis, S.R.; Patel, M.R. and Lefe`vre, F. (2018). On the assimilation of Martian total ozoneretrievals. In: Mars Atmosphere Data Assimilation (MADA) workshop, 29-31 Aug 2018, Savoie Technolac, LeBourget-du-Lac, France.For guidance on citations see FAQs.c© 2018 The AuthorsVersion: Version of RecordCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukMADA2018 workshop     29–31 August 2018, Savoie Technolac, Le Bourget-du-Lac, France Mars Atmosphere Data Assimilation workshop, Abstract #09 (CC BY 4.0) ON THE ASSIMILATION OF MARTIAN TOTAL OZONE RETRIEVALS J. A. Holmes1, S. R. Lewis1, M. R. Patel1,2 and F. Lefèvre,3    1School of Physical Sciences, The Open University, Milton Keynes, UK (james.holmes@open.ac.uk), 2Space Sci-ence and Technology Department, STFC, RAL, UK. 3LATMOS, CNRS/Université Pierre et Marie Curie, France. Introduction:  The technique of data assimilation gives us an opportunity to further our understanding of important photochemical processes in the Martian atmosphere, through the creation of a reanalysis prod-uct that can be used to investigate the temporal and spatial agreement between model and observations and determine any possible causes of identified dif-ferences. In this study [1], we have assimilated, for the first time, total ozone retrievals into a Mars Global Circulation model (GCM) to study the ozone cycle. Model setup and assimilation: We use the UK version of the Laboratoire de Météorologie Dy-namique (LMD) Mars GCM. This 4D-model uses the physical parameterisations [2] and LMD photochem-ical module [3] shared with a recent version of the LMD Mars GCM coupled to a UK-only spectral dy-namical core and semi-Lagrangian advection scheme [4]. It has been developed in a collaboration between the LMD, the Open University, the University of Ox-ford and the Instituto de Astrofisica de Andalucia. For the case study detailed here, the model was run at T31 resolution in the horizontal, corresponding to a reso-lution of 5° latitude by 5° longitude, with 35 vertical levels in the range 0–105 km. Future simulations are planned at higher model resolutions, with the param-eters defining the assimilation technique potentially needing to be optimised as a consequence. The Mars GCM includes the latest sub-models to provide the most realistic modelling of the planetary boundary layer and water and dust cycles [5,6,7]. The photo-chemical module provides multiple photolytic and chemical reactions with up-to-date reaction rates be-tween 16 advected species including carbon dioxide, water vapour and ozone. It also includes heterogene-ous processes removing odd hydrogen radicals, a pro-cess which has been shown to improve the agreement between models and observations [8]. The assimilation is performed using a form of the Analysis Correction scheme [9] converted to martian conditions and has been shown in the past to be a com-putationally inexpensive and robust method [10]. Us-ing this methodology, observations of short-lived (and long-lived) species can be supplemented by knowledge of the transport and atmospheric chemis-try from a Mars GCM. SPICAM retrievals:  The ozone retrievals that have currently been assimilated are displayed in Fig-ure 1a) and come from the SPICAM instrument on Mars Express. Maximum total ozone values are seen around northern hemisphere spring equinox at north-ern polar latitudes, as a result of polar night conditions in the recent past and a lack of odd hydrogen species. The SPICAM instrument is able to observe large ozone abundance before water vapour and associated odd hydrogen species are transported into the north-ern polar regions resulting in destruction of ozone. Figure 1: SPICAM nadir retrievals for LS = 341° MY 26 to LS = 69° MY 28 as a function of latitude and solar longitude displaying a) the total ozone abun-dance and b) number of retrievals. The number of SPICAM retrievals are binned every 5° LS and 5° lat-itude. For the first assimilation of ozone for Mars, the dynamically determined wind field from the Mars GCM is used to transport ozone and all other chemical species, but future investigations could potentially also assimilate thermal profiles alongside ozone re-trievals. This highlights another benefit of data assim-ilation, in that data sparse regions are potentially in-fluenced by SPICAM retrievals of total ozone being advected into the local unobserved region. Results & Discussion:  To investigate the effect of total ozone assimilation on the seasonal cycle, we ran a ‘control’ simulation with no assimilation and a second simulation in which total ozone retrievals from SPICAM were assimilated (hereafter the SPICAM ozone reanalysis). Differences in the modelled and MADA2018 workshop     29–31 August 2018, Savoie Technolac, Le Bourget-du-Lac, France Mars Atmosphere Data Assimilation workshop, Abstract #09 (CC BY 4.0) observed ozone cycle (Figure 2b) are efficiently iden-tified by the assimilation process, with the largest to-tal ozone percentage differences identified between 45°S–10°S from LS = 135–180° and at northern polar (60°N–90°N) latitudes from LS = 150–195°, at the on-set of northern polar winter. An asymmetry in the magnitude of total ozone percentage differences in northern fall (LS = 150–195°) and early northern spring (LS = 0–30°) at northern polar latitudes is also identified. Through investigation of the water cycle, underestimated amounts of total ozone in the control simulation were found to be due to excessive north-ward transport of water vapour west of the Tharsis re-gion and over Arabia Terra throughout northern polar winter resulting in increased increments in total ozone necessary in the SPICAM ozone reanalysis. Neither modelling biases in water vapour or heterogeneous processes on water ice clouds can explain outstanding differences in total ozone between the SPICAM ozone reanalysis and the control simulation at northern polar latitudes from LS = 150–195°. In this region in partic-ular, investigation of the vertical profiles of water va-pour/ice and ozone are needed to try and explain the difference in total ozone between the control simula-tion and SPICAM ozone reanalysis and determine whether it is a further unidentified process which is not adequately represented. Figure 2: (a) Assimilated zonally averaged ozone col-umn at 12 p.m. everywhere and (b) the percentage dif-ference between the SPICAM ozone reanalysis and control simulation for MY 27 and start of MY 28. Shading indicates regions where there are no nearby SPICAM retrievals at the corresponding time and space. The assimilation of SPICAM retrievals can also pro-vide potential information on the polar maximum, with chemical changes as a result of the assimilation process shown to affect the southern polar high-alti-tude ozone layer. For a more realistic study of the po-lar dynamics, assimilation of total ozone, and ideally water vapour too, would provide a more accurate ozone spatial distribution. How best to combine two datasets that show a distinct anti-correlation with one another is currently open for debate. In the future, observations from NOMAD [11] on the ExoMars TGO spacecraft, will provide a more comprehensive dataset to assimilate, with the SPICAM observational dataset extremely useful also for cross-validation purposes. Assimilation of vertical profiles of ozone and water vapour/ice alongside col-umn retrievals, such as will be possible from the NO-MAD instrument on the ExoMars TGO, will also be of great use to further study the origin of differences in total ozone between the SPICAM ozone reanalysis and global circulation models. There is also the possi-bility of assimilating multiple different ozone datasets at the same time, but how best to optimise this reanal-ysis when they are at different spatial and temporal resolutions needs to be considered carefully. Moving forward, the chemical assimilation is be-ing tested for other chemical species and ozone da-tasets, with each one posing a unique challenge be-cause of the high variability in their spatial and tem-poral distribution when compared to one another. For instance, water vapour is far less localised and reac-tive than ozone abundance, and so different parame-ters for the assimilation technique have been found to be optimum for this particular chemical species. References: [1] Holmes, J. A. et al., Icarus 302, 308-318, 2018.[2] Forget, F. et al., JGR 104, 24155-24175, 1999.[3] Lefèvre, F. et al., JGR 109, E07004, 2004.[4] Newman, C. E. et al., JGR 107, 5123, 2002.[5] Colaïtis, A. et al., JGR (Planets) 118, 1468-1487,2013.[6] Navarro, T. et al., JGR (Planets) 119, 1479-1495,2014.[7] Madeleine, J.-B. et al., JGR (Planets) 116,E11010, 2011.[8] Lefèvre, F. et al., Nature 454, 971-975, 2008.[9] Lorenc, A. C. et al., QJRMS 117, 59-89, 1991.[10] Lewis, S. R. et al., Icarus 192, 327-347, 2007.[11] Patel, M. R. et al., Appl. Opt. 56 (10), 2771-2782,2017.Acknowledgements: All authors acknowledge support as part of the project UPWARDS-633127, funded by the European Union Horizon 2020 Pro-gramme. JAH, SRL and MRP also acknowledge the support of the UK Space Agency/STFC. We are grate-ful for an ongoing collaboration with François For-get and co-workers at LMD. 

On the assimilation of Martian total ozone retrievals

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On the assimilation of Martian total ozone retrievals

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