1.5. The planetary boundary layer

The lowest level of the atmosphere – the bottom layer of the troposphere – called planetary boundary layer (PBL) is directly and strongly influenced by the underlying surface (Stull, 1988). Within the PBL the convective air motions generate intense turbulent mixing. The upper boundary of PBL is a statically stable layer (temperature inversion). Interactions between the atmosphere and the surface take place in the PBL. Timescale of atmospheric response to surface forcing is an hour or less. Atmospheric variables (wind speed, temperature, water vapour content etc.) show great variability and fluctuation and the vertical mixing is strong.

The structure of PBL varies with season, weather condition and time of day. The depth of the PBL ranges from tens of meters in case of strongly stable stratification, to a few thousand meters in very unstable condition; it is lower at night and winter and higher in day-time and summer.

The planetary boundary layer

Figure 1.11: The structure of the planetary boundary layer

Daly variation of PBL shows a typical pattern during pleasant weather condition (Fig 1.11.) The lowest (about 10%) part of PBL is called surface layer. The thickness of this layer is typically 10–30 m at night, and 50–100 m in day-time. Exchange processes between the atmosphere and the surface (vegetation) are realized here by the turbulent fluxes[17] of heat, momentum, water and air pollutants. After sunrise, a convective mixed layer growing rapidly due to the intensive turbulence. This layer is capped by a stable entrainment zone. Near sunset, the mixed layer collapses and in its place a nocturnal boundary layer is formed. The bottom part of this layer is stabilized by the nigh time radiative cooling of the surface. Above this stable zone a residual layer can be found.

Planetary boundary layer has great importance in dispersion, dilution and deposition of air pollutants.

References

Anfossi D. and Sandroni S.. 1993. Surface Ozone at Mid Latitudes in the Past Century In: Il Nuovo Cimento. No. 17. Vo 2. 199-207.

Holland H.D.. 2006. The oxygenation of the atmosphere and oceans. Phil. Trans. R. Soc. B.. Vo. 361. 903-915.

IPCC, 2001: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Houghton J.T., Ding Y., Griggs D.J., Noguer M., van der Linden P.J., Dai X., Maskell K., and Johnson C.A.. (eds.). Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. 881 pp. ISBN 0521 80767 0.

May L.. 2010. Atomism before Dalton. In: Atoms in chemistry: from Dalton's predecessors to complex atoms and beyond. Book Series: ACS Symposium Series. Vo. 1044. 21-33.

Pöschl U.. 2005. Atmospheric Aerosols: Composition, Transformation, Climate and Health Effects In: Angewandte Chemie International Edition. Vo. 44. 7520-7540.

Stull R.B.. 1988. An Introduction to Boundary Layer Meteorology (Atmospheric Sciences Library). Kluwer Academic Publishers, Dordrecht. 669 pp. ISBN 90-277-2768-6.



[17] Turbulent flux: transport of a quantity by quasi-random eddies. Turbulent fluxes play important role in the surface layer.