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Background and motivations

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Joint GODAE OceanView - WGNE workshop on Short- to Medium-range coupled prediction for the atmosphere-wave-sea-ice-ocean: Status, needs and challenges

Numerical weather prediction (NWP) has continuously improved over the past 25 years as the period of skilful forecasts has doubled. The greatest improvements in skill have come from improved initial conditions, remote sensing and more optimal methods for data assimilation. Performance gains have also come from developments in models and physics parameterizations and from the technology needed to resolve more and parameterise less of the turbulent spectrum. Most NWP forecasts still rely on persistent boundary conditions (e.g., SST), although some operational centers have used partially coupled systems for years. One center has an operational coupled atmosphere-wave model to account for surface roughness, another center has an operational coupled regional ocean-ice-atmospheric model, and a third has found that using a coupled ocean-atmospheric model improves hurricane forecasting, particularly intensity forecasts. No systematic account has yet been made within data assimilation of forecast errors in ocean/wave/sea-ice/land-surface boundary conditions that are in part introduced by (and co-vary with) errors in the estimated atmospheric fluxes. NWP to a very large degree overcomes this limitation through data assimilation and a high density of atmospheric observations. The benefits to NWP of coupling the atmosphere and ocean will therefore likely be a) in extreme events which are rare and short lived and b) within the atmospheric boundary layer which has less memory and is therefore at the margins of standard statistical methods. Since the majority of applications lie in both of these areas, the potential benefits are greater. Resolving shorter spatial scales in SST adds realistic energy cascades to shorter scales in the atmosphere, just as adding higher resolution orography over land does. It is also worthwhile noting that NWP is part of an earth system; had NWP begun with skilful ocean, wave and other component models available, coupled prediction would already be routine. Several vision papers have begun to outline the way forward (e.g., Brunet et al., 2010; Brassington, 2010) as well as a series of workshops (e.g., ECMWF, 2008).

Developments in coupled modelling have been led by those teams pursuing longer range climate and seasonal forecasts in which the atmospheric state transitions from an initial value problem to a mixed initial and boundary value problem. Therefore, the heavy lifting of developing couplers and earth system modelling has had to be undertaken by teams within these groups. The constraints of integration period (months to millennia) and ensemble forecasting have limited the application of coupled earth-system modelling to relatively coarse resolution models. The development of couplers have not been designed or optimised for operational/time critical applications. Further development is needed to bridge this gap; the entire community are likely to benefit from the rigour and demands of these applications to earth-system software. Ideally, in the future, improvements developed in coupled short-range prediction systems would be transitioned to seasonal forecast and climate models.

The Global Ocean Data Assimilation Experiment (GODAE) (Bell et al., 2010; Schiller and Brassington, 2011) has demonstrated the feasibility of using the Global Ocean Observing System (GOOS) to constrain a meso-scale ocean model and perform routine analyses of the ocean state and circulation. As the result of GODAE and comparable efforts, several agencies and centres support first- or second-generation global and basin-scale pre-operational and operational ocean prediction systems. These systems produce both now-casts and short-range forecasts and provide the first routine estimates of the ocean state. They have sufficient skill to improve a wide range of applications (e.g., defence, search and rescue, oil spills etc). However, the performance of these systems is spatially uneven reflecting both the coverage of GOOS and the dynamical regimes e.g,, coastal vs open ocean, low-latitude vs mid- and high-latitudes, western boundary intensifcation vs eastern boundaries. The performance will rapidly improve as more optimal analysis methods are adopted, elements of GOOS transition to operations and new platforms are added. Unlike the close coupling between winds and waves, the oceans’ inertia and heat capacity produce a circulation that has unique time and space scales and is related more to the integrated time history of surface fluxes of mass, heat and momentum than to an immediate response to the atmospheric weather. Indeed the ocean has an analogue to atmospheric highs and lows: geostrophic turbulence with anticylonic/cyclonic "eddies" (first identified in the 1980's) with spatial scales an order of magnitude shorter (O(100) km)) than the atmosphere and timescales of days to months. Important exceptions apply: for example over the continental shelf and the turbulent surface layer where time and space scales are a blend between the atmosphere and ocean. These regions also correspond to the highest biological and human activity and therefore the majority of applications for ocean prediction. Minimising errors in the applied stress and fluxes will substantially benefit ocean prediction. Further, translating the atmospheric time history of increments from data assimilation into adjustments to the time history of the surface fluxes will further minimise errors in the ocean state.

The availability of GOOS and GODAE high resolution estimates of the ocean state makes possible the development of high resolution coupled prediction systems. Making progress in this field will require coupled infrastructure, coupled modelling, observational requirements (including experimental campaigns) and large and more diverse teams of scientific experts. Progress will be driven by funded programs at institutional level of which there are already systems developed or under development by the US Navy (COAMPS), NOAA (coupled hurricane prediction system, coupled data assimilation), CMC(coupled regional model), ECMWF (coupled atmosphere-wave), Meteorological Office (coupled coastal modelling), Australian Bureau of Meteorology (CLAM) and others. Progress in this field will be accelerated by facilitating greater scientific exchange amongst the community. Two workshops at ECMWF (2008) and at the UK Meteorological Office (2009) have addressed the needs of NWP for coupled atmosphere-ocean systems. This workshop would continue the progress made at these two workshops and provide a unique joint forum for the NWP and ocean forecasting communities.

 

References

Bell, M. J., M. Lefebvre, P.-Y. Le Traon, N. Smith and K. Wilmer-Becker, 2010: GODAE: The global ocean data assimilation experiment, Oceanography, 22(3), 14-21

Brassington, G. B., 2009: Ocean prediction issues related to weather and climate prediction, CAS XV Vision paper (Agenda item 8.5)

Brunet, G., T. Keenan, J. Onvlee, M. Béland, D. Parsons and Jocelyn Mailhot, 2010: The next generation of regional prediction systems for weather, water and environmental applications, CAS XV Vision paper (Agenda item 8.2)

Proceedings of the ECMWF Workshop on Atmosphere-Ocean Interaction, 10-12 Nov 2008 (http://www.ecmwf.int/publications/library/do/references/list/28022009)

Proceeding of the Ocean Atmosphere Workshop, UK Met Office, 1-2 Dec 2009 (http://www.ncof.co.uk/modules/documents/documents/OAsummary.pdf)