Characteristics of Submeso Winds in the Stable Boundary Layer

Mahrt, L.

doi:10.1007/s10546-008-9336-4

Cited by 75 publications

(54 citation statements)

References 28 publications

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“…The observed mixing events can be generated at the surface or result from downward bursting of turbulence (Nappo and Chimonas 1992). The nature of these transient events are site dependent (Mahrt 2009) and preclude simple universal theories. The mixing events in near-calm flow are less frequent than in most intermittency studies in the literature (Katul et al 1994), yet can still dominate the averaged fluxes.…”

Section: Introductionmentioning

confidence: 99%

The Near-Calm Stable Boundary Layer

Mahrt

2011

Boundary-Layer Meteorol

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For the near-calm stable boundary layer, nominally 2-m mean wind speed <0.5 m s −1 , the time-average turbulent flux is dominated by infrequent mixing events. These events are related to accelerations associated with wave-like motions and other more complex small-scale motions. In this regime, the relationship between the fluxes and the weak mean flow breaks down. Such near-calm conditions are common at some sites. For very weak winds and strong stratification, the characteristics of the fluctuating quantities change slowly with increasing scale and the separation between the turbulence and non-turbulent motions can become ambiguous. Therefore, a new analysis strategy is developed based on the scale dependence of selected flow characteristics, such as the ratio of the fluctuating potential energy to the kinetic energy. In contrast to more developed turbulence, correlations between fluctuating quantities are small, and a significant heat flux is sometimes carried by very weak vertical motions with large temperature fluctuations. The relation of the flux events to small-scale increases of wind speed is examined. Large remaining uncertainties are noted.

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Section: Introductionmentioning

confidence: 99%

The Near-Calm Stable Boundary Layer

Mahrt

2011

Boundary-Layer Meteorol

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“…The variance explained by the regression (r 2 ) increases slightly with increasing λ due to a decrease in the random sampling error. The change in C H with flux-averaging timescale is small, decreasing only 6 % for a change in λ from 30 min to 2 h. The r 2 decreases slightly with increasing τ because an increasing perturbation timescale increases the possibility that larger-scale, nonturbulent sub-mesoscale motions, which may not be related to the local mean wind speed or temperature gradient, will be included in the calculated flux, especially in stable conditions (e.g., Smedman, 1988;Vickers and Mahrt, 2006;Mahrt, 2009). C H increases with increasing perturbation timescale because of the systematic flux loss when using too small a value for τ to calculate the heat flux.…”

Section: Vickers and C K Thomas: Turbulence In Very Weak Wind Comentioning

confidence: 99%

Observations of the scale-dependent turbulence and evaluation of the flux–gradient relationship for sensible heat for a closed Douglas-fir canopy in very weak wind conditions

Vickers

Thomas

2014

Atmos. Chem. Phys.

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Abstract.Observations of the scale-dependent turbulent fluxes, variances, and the bulk transfer parameterization for sensible heat above, within, and beneath a tall closed Douglas-fir canopy in very weak winds are examined. The daytime sub-canopy vertical velocity spectra exhibit a double-peak structure with peaks at timescales of 0.8 s and 51.2 s. A double-peak structure is also observed in the daytime sub-canopy heat flux co-spectra. The daytime momentum flux co-spectra in the upper bole space and in the sub-canopy are characterized by a relatively large crosswind component, likely due to the extremely light and variable winds, such that the definition of a mean wind direction, and subsequent partitioning of the momentum flux into along-and cross-wind components, has little physical meaning. Positive values of both momentum flux components in the sub-canopy contribute to upward transfer of momentum, consistent with the observed sub-canopy secondary wind speed maximum. For the smallest resolved scales in the canopy at nighttime, we find increasing vertical velocity variance with decreasing timescale, consistent with very small eddies possibly generated by wake shedding from the canopy elements that transport momentum, but not heat. Unusually large values of the velocity aspect ratio within the canopy were observed, consistent with enhanced suppression of the horizontal wind components compared to the vertical by the very dense canopy.The flux-gradient approach for sensible heat flux is found to be valid for the sub-canopy and above-canopy layers when considered separately in spite of the very small fluxes on the order of a few W m −2 in the sub-canopy. However, singlesource approaches that ignore the canopy fail because they make the heat flux appear to be counter-gradient when in fact it is aligned with the local temperature gradient in both the sub-canopy and above-canopy layers. While sub-canopy Stanton numbers agreed well with values typically reported in the literature, our estimates for the above-canopy Stanton number were much larger, which likely leads to underestimated modeled sensible heat fluxes above dark warm closed canopies.

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“…Processos não turbulentos na escala de submeso (Mahrt, 2009) O objetivo deste estudo é analisar a influência de obstruções na escala de movimentos de submeso na Camada Limite Convectiva (CLC) e comprovar a dependência local do submeso com tais obstruções. Para isso foram analisadas oito estações pertencentes ao Sítio de Pedras Altas-RS, região do pampa gaúcho, onde a Energia Cinética Turbulenta (ECT) horizontal presente em cada estação foi decomposta em várias outras escalas através da técnica da Decompoisição em Multiresolução (DM) (Howell e Mahrt, 1997).…”

Section: Introductionunclassified