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| Titel |
Shear Capacity as Prognostic of Nocturnal Boundary Layer Regimes |
| VerfasserIn |
Ivo van Hooijdonk, Judith Donda, Fred Bosveld, Arnold Moene, Herman Clercx, Bas van de Wiel |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250105024
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| Publikation (Nr.) |
 EGU/EGU2015-4468.pdf |
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| Zusammenfassung |
| After sunset the surface temperature can drop rapidly in some nights and may lead to
ground frost. This sudden drop is closely related to the occurrence of fundamentally
different behaviour of turbulence in the nocturnal boundary layer. Recent theoretical
findings predict the appearance of two different regimes: the continuously turbulent
(weakly stable) boundary layer and the relatively ’quiet’ (very stable) boundary
layer. Field observations from a large number of nights (approx. 4500 in total) are
analysed using an ensemble averaging technique. The observations support the
existence of these two fundamentally different regimes: weakly stable (turbulent) nights
rapidly reach a steady state (within 2-3 hours). In contrast, very stable nights reach a
steady state much later after a transition period (2-6 hours). During this period
turbulence is weak and non-stationary. To characterise the regime a new parameter is
introduced: the Shear Capacity. This parameter compares the actual shear after
sunset with the minimum shear needed to sustain continuous turbulence. In turn, the
minimum shear is dictated by the heat flux demand at the surface (net radiative
cooling), so that the Shear Capacity combines flow information with knowledge
on the boundary condition. It is shown that the Shear Capacity enables prediction
of the flow regimes. The prognostic strength of this non-dimensional parameter
appears to outperform the traditional ones like z/L and Ri as regime indicator. |
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