Foreword
xi
of all, on the ABL height. The temporal and spatial variations in the ABL height
and the entrainment processes at the ABL upper boundary lead to the penetration of
pollutants from the ABL to the free troposphere, and, vice versa, to the intrusion of
some chemical compounds (e.g., ozone) from the upper atmospheric layers down to
the surface. Physical processes controlling the ABL height and the turbulent entrainment (e.g., Zilitinkevich, 1991; Zilitinkevich et al., 2007a; and references therein)
are, therefore, of crucial importance for the air-pollution applications.
Furthermore, some physical processes at the ABL upper boundary, crucially
important for air pollution modelling, are still insuffi ciently understood, e.g., turbulent entrainment in rapidly deepening convective ABLs and nonsteady interactions
between the stable ABLs and the free fl ow. The latter are comparatively simple at
mid-latitudes where nocturnal stable ABLs develop on the background of almost
neutrally stratifi ed residual layers, whereas at high latitudes, long-lived stable ABLs
develop against very stable stratifi cation typical of the free troposphere, yielding
the formation of strong capping inversions and making the theory much more
complicated (e.g., Zilitinkevich and Esau, 2007).
For short-range dispersion of simple cases or targeted plumes, one classical modeling
approach is based on using the so-called statistical technique or the eddy diffusivity
concept. Several chapters in this book address new developments with these techniques. Therefore, new developments in turbulence theory and ABLs will have a direct
impact on these techniques as well. For instance, one typical long-lasting issue has
been the turbulence closure for very stable stratifi cation (including the turbulent diffusion formulations), whereby the energetics of turbulence is modeled using solely the
turbulent kinetic energy budget equation, leading to a cut off in turbulence at “supercritical” stratifi cation, though observations showed the presence of turbulence in typical atmospheric and oceanic sheared fl ows. The problem was treated heuristically by
prescribing a “minimal diffusivity”—just to avoid the total decay of turbulence. New
insight might come from recent work based on the concept of total turbulent energy and
applicable to “supercritical” fl ows with no cut off (Mauritsen et al., 2007; Zilitinkevich
et al., 2007b; Canuto et al., 2008). Another area of potential development is the generalization of the Monin–Obukhov similarity theory, taking into account the nonlocal
effect of free-fl ow stability on stably stratifi ed ABLs and also nonlocal mixing due to
large-scale, organized eddies in the shear-free convection (Zilitinkevich et al., 2006;
Zilitinkevich and Esau, 2007). Further work is also needed to extend the ABL theory
to the sheared convection and to ABLs over complex and sloping terrains.
During the last decade, meso-scale modeling of pollution dispersion and air
quality employing the integrated modelling approach together with advances in ABL
physics reported above have been developed in both research and operational modes
(see an overview of European models in COST-WMO, 2007).
Short-term pollution episodes occurring during adverse meteorological conditions and causing strong short-term exceedances of air quality standards in ambient
air are presently one of the major concerns for the protection of human health, ecosystems, and building materials, especially in cities. Reliable urban-scale forecasts
of meteorological fi elds are, therefore, of primary importance for urban emergency
management systems, addressing accidental or terrorist releases, and fi res, of chemical, radioactive, or biological substances.
© 2010 by Taylor and Francis Group, LLC
xi
of all, on the ABL height. The temporal and spatial variations in the ABL height
and the entrainment processes at the ABL upper boundary lead to the penetration of
pollutants from the ABL to the free troposphere, and, vice versa, to the intrusion of
some chemical compounds (e.g., ozone) from the upper atmospheric layers down to
the surface. Physical processes controlling the ABL height and the turbulent entrainment (e.g., Zilitinkevich, 1991; Zilitinkevich et al., 2007a; and references therein)
are, therefore, of crucial importance for the air-pollution applications.
Furthermore, some physical processes at the ABL upper boundary, crucially
important for air pollution modelling, are still insuffi ciently understood, e.g., turbulent entrainment in rapidly deepening convective ABLs and nonsteady interactions
between the stable ABLs and the free fl ow. The latter are comparatively simple at
mid-latitudes where nocturnal stable ABLs develop on the background of almost
neutrally stratifi ed residual layers, whereas at high latitudes, long-lived stable ABLs
develop against very stable stratifi cation typical of the free troposphere, yielding
the formation of strong capping inversions and making the theory much more
complicated (e.g., Zilitinkevich and Esau, 2007).
For short-range dispersion of simple cases or targeted plumes, one classical modeling
approach is based on using the so-called statistical technique or the eddy diffusivity
concept. Several chapters in this book address new developments with these techniques. Therefore, new developments in turbulence theory and ABLs will have a direct
impact on these techniques as well. For instance, one typical long-lasting issue has
been the turbulence closure for very stable stratifi cation (including the turbulent diffusion formulations), whereby the energetics of turbulence is modeled using solely the
turbulent kinetic energy budget equation, leading to a cut off in turbulence at “supercritical” stratifi cation, though observations showed the presence of turbulence in typical atmospheric and oceanic sheared fl ows. The problem was treated heuristically by
prescribing a “minimal diffusivity”—just to avoid the total decay of turbulence. New
insight might come from recent work based on the concept of total turbulent energy and
applicable to “supercritical” fl ows with no cut off (Mauritsen et al., 2007; Zilitinkevich
et al., 2007b; Canuto et al., 2008). Another area of potential development is the generalization of the Monin–Obukhov similarity theory, taking into account the nonlocal
effect of free-fl ow stability on stably stratifi ed ABLs and also nonlocal mixing due to
large-scale, organized eddies in the shear-free convection (Zilitinkevich et al., 2006;
Zilitinkevich and Esau, 2007). Further work is also needed to extend the ABL theory
to the sheared convection and to ABLs over complex and sloping terrains.
During the last decade, meso-scale modeling of pollution dispersion and air
quality employing the integrated modelling approach together with advances in ABL
physics reported above have been developed in both research and operational modes
(see an overview of European models in COST-WMO, 2007).
Short-term pollution episodes occurring during adverse meteorological conditions and causing strong short-term exceedances of air quality standards in ambient
air are presently one of the major concerns for the protection of human health, ecosystems, and building materials, especially in cities. Reliable urban-scale forecasts
of meteorological fi elds are, therefore, of primary importance for urban emergency
management systems, addressing accidental or terrorist releases, and fi res, of chemical, radioactive, or biological substances.
© 2010 by Taylor and Francis Group, LLC