Variability and Maintenance of Turbulence in the Very Stable Boundary Layer

Mahrt, L.

doi:10.1007/s10546-009-9463-6

Cited by 91 publications

(69 citation statements)

References 56 publications

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“…Figure 7 shows several time series covering a strongly stable night punctuated by a mixing burst, which is consistent with the description of intermittent turbulence given by Van de Wiel et al (2002). The bulk Richardson number (Ri b ; Glickman, 2000) shows that the stability in the lowest 7.5 m of the air column increases after sunset and climbs into the strongly stable regime, Ri b > 1 (Mahrt, 2010). A burst of mixing occurs shortly after midnight, as suggested by an abrupt drop in the bulk Richardson number, but also visible as a sudden increase in wind speed and a cessation of net cooling at 2 m a.g.l.…”

Section: Hourly Variations In H E Related To Mixingsupporting

confidence: 74%

Improved mixing height monitoring through a combination of lidar and radon measurements

et al. 2013

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Surface-based radon (<sup>222</sup>Rn) measurements can be combined with lidar backscatter to obtain a higher quality time series of mixing height within the planetary boundary layer (PBL) than is possible from lidar alone, and a more quantitative measure of mixing height than is possible from only radon. The reason why lidar measurements are improved is that there are times when lidar signals are ambiguous, and reliably attributing the mixing height to the correct aerosol layer presents a challenge. By combining lidar with a mixing length scale derived from a time series of radon concentration, automated and robust attribution is possible during the morning transition. <br><br> Radon measurements provide mixing information during the night, but concentrations also depend on the strength of surface emissions. After processing radon in combination with lidar, we obtain nightly measurements of radon emissions and are able to normalise the mixing length scale for changing emissions. After calibration with lidar, the radon-derived equivalent mixing height agrees with other measures of mixing on daily and hourly timescales and is a potential method for studying intermittent mixing in nocturnal boundary layers

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Section: Hourly Variations In H E Related To Mixingsupporting

confidence: 74%

Improved mixing height monitoring through a combination of lidar and radon measurements

et al. 2013

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“…Nappo 1991;Mahrt et al 2001;Acevedo and Fitzjarrald 2003;Mahrt 2010a). Together with propagating solitary and internal gravity waves that can transport remotely-generated energy horizontally and vertically into the local area, these "submeso" disturbances can sporadically initiate strong turbulent mixing despite the stable stratification (Hunt et al 1985;King et al 1987;Poulos et al 2002;Sun et al 2004;Mahrt 2010b). In fact, at very low wind speeds, it may be that turbulence is generated predominantly by the submeso motions, and its relationship to the mean flow becomes weak (Mahrt 2011;Sun et al 2012).…”

Section: Turbulent Mixing In the Nocturnal Stable Boundary Layer (Sbl)mentioning

confidence: 99%

“…We follow Mahrt and Vickers (2006) and Banta et al (2007) and others in rejecting the use of the Monin-Obukhov parameter (z/L) as a bulk stability measure, due to its uncertainty and self-correlation problems at high stabilities (when turbulent fluxes are close to zero). As with Mahrt (2010b), we wish to relate the outcomes of turbulent mixing to a bulk stability measure that is independent of turbulence quantities. We adopt the tracer difference C = C(z m ) − C s across the layer z as an unbiased measure of the layer-integrated outcome of the turbulent mixing process.…”

Section: A Framework For the Bulk Characterization Of Mixingmentioning

confidence: 99%

Bulk Mixing and Decoupling of the Nocturnal Stable Boundary Layer Characterized Using a Ubiquitous Natural Tracer

Williams

Chambers

Griffiths

2013

Boundary-Layer Meteorol

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Vertical mixing of the nocturnal stable boundary layer (SBL) over a complex land surface is investigated for a range of stabilities, using a decoupling index (0 < D rb < 1) based on the 2-50 m bulk gradient of the ubiquitous natural trace gas radon-222. The relationship between D rb and the bulk Richardson number (R ib ) exhibits three broad regions: (1) a well-mixed region (D rb ≈ 0.05) in weakly stable conditions (R ib < 0.03); (2) a steeply increasing region (0.05 < D rb < 0.9) for "transitional" stabilities (0.03 < R ib < 1); and (3) a decoupled region (D rb ≈ 0.9-1.0) in very stable conditions (R ib > 1). D rb exhibits a large variability within individual R ib bins, however, due to a range of competing processes influencing bulk mixing under different conditions. To explore these processes in R ib -D rb space, we perform a bivariate analysis of the bulk thermodynamic gradients, various indicators of external influences, and key turbulence quantities at 10 and 50 m. Strong and consistent patterns are found, and five distinct regions in R ib -D rb space are identified and associated with archetypal stable boundary-layer regimes. Results demonstrate that the introduction of a scalar decoupling index yields valuable information about turbulent mixing in the SBL that cannot be gained directly from a single bulk thermodynamic stability parameter. A significant part of the high variability observed in turbulence statistics during very stable conditions is attributable to changes in the degree of decoupling of the SBL from the residual layer above. When examined in R ib -D rb space, it is seen that very different turbulence regimes can occur for the same value of R ib , depending on the particular combination of values for the bulk temperature gradient and wind shear, together with external factors. Extremely low turbulent variances and fluxes are found at 50 m height when R ib > 1 and D rb ≈ 1 (fully decoupled). These "quiescent" cases tend to occur when geostrophic forcing is very weak and subsidence is present, but are not associated with the largest bulk temperature gradients. Humidity and net radiation data indicate the presence of low cloud, patchy fog or dew, any of which may aid decoupling in these cases by preventing temperature gradients from increasing sufficiently to favour gravity wave activity. The largest temperature gradients in our dataset are actually associated with smaller values of the decoupling index (D rb < 0.7), indicating the presence of mixing. Strong evidence is seen from enhanced turbulence levels, fluxes and submeso activity at 50 m, as well as high temperature variances and heat flux intermittencies at 10 m, suggesting this region of the R ib -D rb distribution can be identified as a top-down mixing regime. This may indicate an important role for gravity waves and other wave-like phenomena in providing the energy required for sporadic mixing at this complex terrain site.

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“…Such nonstationarity can be internally forced through interactions of the turbulence with the mean flow (van de Wiel et al 2012a) or forced by submeso motions (Mahrt 2010b). Acevedo et al (2014) find that the submeso motions introduce a site dependency for the relationship between the turbulence and the mean flow for stable conditions.…”

Section: Introductionmentioning

confidence: 99%

Dependence of Turbulent Velocities on Wind Speed and Stratification

Mahrt

Sun

Stauffer

2014

Boundary-Layer Meteorol

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We examine the dependence of several turbulence quantities on the wind speed and stability using nocturnal data from the Shallow Cold Pool Experiment. The turbulent quantities (velocities) are defined in terms of the standard deviation of the horizontal and vertical velocity fluctuations, two different calculations of the friction velocity, and two turbulent velocities based on the heat flux. The dependence of the turbulent velocities on the wind speed shows a transition between the weak-wind regime of small slope and the stronger wind regime of larger slope, as found in previous studies. This transition occurs for all of the turbulent velocities examined and occurs for a wide range of averaging times. Although this study concentrates primarily on data over a flat surface above the valley, the transition also occurs at the other 18 stations that have non-zero local slopes up to about 10 %. At the same time, the relationship between the turbulence and the wind speed cannot be universal because of the influence of stratification and site-dependent non-stationarity in the weak-wind regime. The wind speed of the transition increases with increasing stratification at a rate that is an order of magnitude slower than that predicted by a constant transition bulk Richardson number. For the weakest winds, the impact of stratification is unexpectedly small.

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Variability and Maintenance of Turbulence in the Very Stable Boundary Layer

Cited by 91 publications

References 56 publications

Improved mixing height monitoring through a combination of lidar and radon measurements

Improved mixing height monitoring through a combination of lidar and radon measurements

Bulk Mixing and Decoupling of the Nocturnal Stable Boundary Layer Characterized Using a Ubiquitous Natural Tracer

Dependence of Turbulent Velocities on Wind Speed and Stratification

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