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| Titel |
The structure of the stable boundary layer as from radiosounding profiles and eddy correlation measurements |
| VerfasserIn |
L. Caporaso, A. Riccio, F. Di Giuseppe, F. Tampieri |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250059215
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| Zusammenfassung |
| The planetary boundary layer (BL) can be thought as the region where air masses are strongly
influenced by the surface processes and which interacts through exchange processes with air
masses originating from above. Being strongly coupled to the ground, this layer evolution is
controlled by turbulence, produced through shear and heat flux, which leads to rapid
fluctuations in quantities such as flow velocity,temperature, moisture and to intense vertical
mixing.
At the resolution of atmospheric models employed in operational weather forecast,
turbulent processes are not explicitly resolved and the Monin-Obukhov similarity theory
(MOST) commonly provides the framework for their parametrization. Accordingly to MOST,
temperature and wind profiles below the first model level (in the surface layer) can be
diagnosed by scaling prescribed functions with surface turbulent fluxes of heat and
momentum. The relationship which holds between mean vertical profiles and surface fluxes is
therefore the key element to determine the lower boundary condition for vertical transport in
these models.
Using an extensive set of measurements and a multivariate nonlinear regression algorithm
it will be shown that in the limit of very stable conditions the Monin-Obukhov similarity
theory looses validity. For high atmospheric stability, while it is possible to find values for
surface fluxes of momentum and heat to scale the similarity temperature and wind profiles so
to match observations, these values are not in agreement with the equivalent measured
quantities. In other words surface fluxes are found not to be adequate scaling variables
for the mean profiles as they are not sufficient to completely describe the stable
boundary layer structure. Discrepancies with the MOST theory become more evident
in very stable conditions when the bulk Richardson number (Rb) is larger than
O(2).
The correct characterization of very stable boundary layer is of considerable practical
importance. The absence of significant mixing allows buildup of high concentrations of
contaminants which can be diagnosed directly by the boundary layer height, h. One of
the practical consequences of the outlined finding is therefore that formulations
of h, which mainly relay on surface turbulent fluxes are inaccurate in very stable
conditions. It is then shown as an example that the expression for h suggested by
Zilitinkevich et al which is a function of surface fluxes and atmospheric stability leads to h
estimates in disagreement with the ones obtained by inspecting the mean profiles |
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