• Yano, J.-I. & , and P. Bechtold, 2009: Toward physical understanding of cloud entrainment-detrainment processes: Workshop on Concepts for Convective Parameterizations in Large-Scale Models II: Entrainment and Detrainment in Convective Plumes; Prague, Czech Republic, 25–27 March 2009. Eos, Trans. Amer. Geophys. Union, 90, 258, doi:10.1029/2009EO300003.

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  • Yano, J.-I., , J. Quaas, , T. M. Wagner & , and R. S. Plant, 2008: Toward statistical descriptions of convective cloud dynamics: Workshop on Concepts for Convective Parameterizations in Large-Scale Models; Hamburg, Germany, 12–14 February 2008. Eos, Trans. Amer. Geophys. Union, 89, 212, doi:10.1029/2008EO230009.

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  • J.-I. Yano, , , J.-F. Geleyn & , and S. Malinowski, 2010: Challenges for a new generation of regional forecast models. Workshop on Concepts for Convective Parameterizations in Large-Scale Models III: Increasing Resolution and Parameterization; Warsaw, Poland, 17–19 March 2010. Eos, Trans. Amer. Geophys. Union, 91, 232, doi:10.1029/2010EO260003.

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Theoretical and Operational Implications of Atmospheric Convective Organization

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  • 1 GAME/CNRM, Météo-France, and CNRS, Toulouse, France
  • 2 CAS, University of Cambridge, Cambridge, United Kingdom
  • 3 National Institute for Laser, Plasma and Radiation Physics, Bucharest, Romania
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CORRESPONDING AUTHOR: Jun-Ichi Yano, GAME/CNRM, Météo-France, and CNRS, 42 Av. Coriolis, Toulouse, France, E-mail: jun-ichi.yano@zmaw.de

CORRESPONDING AUTHOR: Jun-Ichi Yano, GAME/CNRM, Météo-France, and CNRS, 42 Av. Coriolis, Toulouse, France, E-mail: jun-ichi.yano@zmaw.de

The present workshop constitutes the fourth in the series on “Concepts for Convective Parameterizations in Large-Scale Models.” The purpose of the workshop series has been to discuss the fundamental theoretical issues of convection parameterization with a small number of European scientists. Each workshop chose a specific topic in order to organize it in a focused manner. The first was organized with a focus on statistical cumulus dynamics (Yano et al. 2008), the second with a focus on entrainment and detrainment (Yano and Bechtold 2009), and the third with a focus on the high-resolution limit (Yano et al. 2010). The workshop series has been funded by European Cooperation in the Field of Scientific and Technical Research (COST) Action ES0905 (http://convection.zmaw.de) since 2010.

The focus of this last workshop in the series was to discuss the issues of atmospheric convective organization and implications for parameterization from both theoretical and operational perspectives. The topic was chosen at the end of the previous workshop based on the conclusion that issues of convective organization become more and more acute with increasing resolution of forecast models.

WORKSHOP ON CONCEPTS FOR CONVECTIVE PARAMETERIZATIONS IN LARGE-SCALE MODELS IV: “CONVECTIVE ORGANISATION”

What: Thirty-nine scientists from 15 European countries and Israel met to discuss the fundamental theoretical concerns regarding convection parameterization in synoptic-scale models.

When: 23–25 March 2011

Where: Cambridge, United Kingdom

The workshop started with two presentations from the perspectives of theoretical physics and of meteorological phenomenology, respectively, in order to better understand the mechanisms for convective organization. These two complementary perspectives were emphasized by considering the needs for introducing both a more solid theoretical basis as well as rich phenomenologies into current operational convection parameterizations. With a wide range of participants from operational forecast researchers to theoretical physicists, the workshop was focused on the basic issues as seen from these two complementary perspectives, and on seeking common ground for collaboration and interaction. Participants came from forecast research centers (AEM, BRMI, CHMI, DWD, ECMWF, FMI, Met Office, Météo-France, SMHI, and HMS1) on the one hand, and specialists on plasma physics as well as Hamiltonian dynamics on the other hand.

These two introductory presentations were followed by four on specific issues and concluded with a half-day round table discussion. Enough time was also taken for discussions during the presentations. The following report is a consensus of the participants based on subsequent e-mail discussions.

DIFFERENT TYPES OF ORGANIZATIONS.

From a perspective of theoretical physics, subtle differences between coherency, self-organization, and self-organized criticality (SOC) were emphasized. Coherency is generated by a purely mechanical process. Good examples of coherencies would be vortices generated by inverse cascade in geophysical flows under weak forcing and dissipation. Self-organization suggests important contributions from small-scale fluctuations or more general thermodynamic processes. An equilibrium state is established under balance between thermal forcing and dissipation. The equilibrium is defined by a particular geometry that minimizes rate of entropy production. Alternatively, the system may evolve to a state where the fluctuations have correlations on all scales (the self-organization at criticality), and random external excitation induces fast, avalanche-like responses.

Without doubt moist atmospheric convection is strongly controlled by thermodynamic processes, notably by cloud physics. However, the phenomenological review of convective organization revealed that many of the relevant systems are primarily controlled by mechanical processes. The formation of a supercell, for example, is essentially dictated by the deformation of a horizontal vortex tube by ascending motion. The organizational tendency of a squall line can be interpreted in terms of the budget of a horizontal component of vorticity. Finally, the important role of vortex dynamics for tropical cyclone evolution can hardly be denied.

At the same time, the importance of the self-organizing tendency of clouds can hardly be denied, either. An “archetype” representing convective coherency in a truncated manner would be required in order to better elucidate the interactions between coherency and full cloud physics.

SELF-ORGANIZED CRITICALITY.

The key feature of criticality is that the statistical properties of the fluctuations are related by scaling laws. Since scaling behavior has been observed in association with atmospheric convection, an approach consisting of developing a SOC model may be relevant. SOC has a close analogy to thermodynamic criticality (such as phase transitions). However, as a major difference from a phase transition, the system always remains at criticality by its own self-regulation. ECMWF analysis data appear to offer some support for atmospheric SOC by showing a clear relationship between column-integrated water and precipitation rate. A cellular automaton model, a theoretical model originally developed for crystal growth, has also been introduced at ECMWF in order to mimic SOC in a crude manner.

Dynamic renormalization group (RNG) theory is a powerful method for analyzing SOC. Possibilities for its application to atmospheric convective systems were extensively discussed. A key to this exercise is choosing a relatively simple toy model that is relevant to atmospheric convection but that also fits well into the RNG framework. A sequential formulation was identified as a good candidate.

Unfortunately, in the absence of a simple physical intuition, scaling laws have never been popular as a basis for parameterization in operational forecast models. A key point of an SOC description is that the marginal stability against large-scale convectively coupled instability can provide a quasi-stationary state with the help of avalanche-like small-scale convection events induced by random external forcing, such as boundary layer fluctuations. Experience from other fields (e.g., plasma physics) may be helpful in order to build the necessary physical intuition.

SUMMARY.

Despite its potential importance, we are still far from including convective organization explicitly within parameterization algorithms. As one participant from Météo-France remarked, the operational priorities always remain the closure and entrainment–detrainment problems. No one objected to this remark, except for possibly regarding downdrafts as another priority. The effects of organization may somehow be implicitly taken into account within our current frameworks, but no specific proposal from participants was heard.

It was concluded that more fundamental research into basic formulational aspects (closure, entrainment–detrainment, downdrafts) of convection parameterization is still desperately needed before more advanced aspects, such as convection organization, can be taken into account. For this reason, it was decided that the focus of the next workshop in the series would be on scrutinizing the bells and whistles of current operational convection parameterizations. A particular question to be asked will be whether we should stay with the traditional “bulk” approach or move to a “spectrum” approach. The next workshop is planned for Savona, Italy, for 20–22 March 2012.

REFERENCES

  • Yano, J.-I. & , and P. Bechtold, 2009: Toward physical understanding of cloud entrainment-detrainment processes: Workshop on Concepts for Convective Parameterizations in Large-Scale Models II: Entrainment and Detrainment in Convective Plumes; Prague, Czech Republic, 25–27 March 2009. Eos, Trans. Amer. Geophys. Union, 90, 258, doi:10.1029/2009EO300003.

    • Search Google Scholar
    • Export Citation
  • Yano, J.-I., , J. Quaas, , T. M. Wagner & , and R. S. Plant, 2008: Toward statistical descriptions of convective cloud dynamics: Workshop on Concepts for Convective Parameterizations in Large-Scale Models; Hamburg, Germany, 12–14 February 2008. Eos, Trans. Amer. Geophys. Union, 89, 212, doi:10.1029/2008EO230009.

    • Search Google Scholar
    • Export Citation
  • J.-I. Yano, , , J.-F. Geleyn & , and S. Malinowski, 2010: Challenges for a new generation of regional forecast models. Workshop on Concepts for Convective Parameterizations in Large-Scale Models III: Increasing Resolution and Parameterization; Warsaw, Poland, 17–19 March 2010. Eos, Trans. Amer. Geophys. Union, 91, 232, doi:10.1029/2010EO260003.

    • Search Google Scholar
    • Export Citation
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AEM: Agencia Estatal de Meteorología (Spanish Meteorological Agency); BRMI: Belgium Royal Meteorological Institute; CHMI: Czech Hydrometeorological Institute; DWD: Deutcher Wetter Dienst (German Weather Service); ECMWF: European Centre for Medium-Range Weather Forecasts; FMI: Finish Meteorological Institute; SMHI: Swedish Meteorological and Hydrological Institute; HMS: Hungary Meteorological Service.

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