Atmospheric Disturbances that Generate Intermittent Turbulence in Nocturnal Boundary Layers

Sun, Jielun; Lenschow, Donald H.; Burns, Sean P.; Banta, Robert M.; Newsom, Rob; Coulter, R. L.; Frasier, Stephen J.; İnce, Türker; Nappo, Carmen J.; Balsley, B. B.; Jensen, Mark J.; Mahrt, L.; Miller, David R.; Skelly, Brian

doi:10.1023/a:1026097926169

Cited by 196 publications

(170 citation statements)

References 35 publications

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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%

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

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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“…This choice was a consequence of data being related to night-time situations. As described in reference [36], for n ight conditions, 5 minutes means capture all turbulent eddies contributions for NBL, for most of the time. Fo r each 5 minutes intervals in the periods presented in Table 1,the described parameters were calculated.…”

Section: Case Studiesmentioning

confidence: 99%

Low Level Jets in the Pantanal Wetland Nocturnal Boundary Layer – Case Studies

Martins¹,

Sá²,

Moraes³

2013

AJEE

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Situated in South America midwest region, Pantanal is a unique biome, alternating dry and flooded periods. An important seasonal variability characteristic fro m Pantanal's energy balance is the occurrence of situations in which sensible heat flu x is positive (bottom-up) during night-time, when the region is flooded enough. In this study it is investigated an interesting aspect of Nocturnal Boundary Layer's (NBL) structure seasonal variability above Pantanal, that is, how Low-Level Jets (LLJ) occurrence and associated turbulent structure. For th is, scale action of different Low-Leve l Jets (LLJ) types on Pantanal's Nocturnal Boundary Layer was investigated through case studies. Six events, distributed the following way, were used: Two events without LLJ's, two events LLJ's weak shear and two events LLJ's strong shear. Fro m the two events without LLJ's, one of them is observed during the dry season and the other, during the flooded one. The same procedure was applied to other events (LLJ's weak shear and LLJ's strong shear). Vertical wind velocity and temperature variance, as well as the covariance among these variables were analyzed and investigated in scale via Wavelet Transform. Remarkab le differences were observed among turbulence in NBL characteristics during the "dry" and "flooded" periods. It was observed that LLJ weak shear acts like a forcing that generates action of top-down mechanisms in the Pantanal's Nocturnal Boundary-layer. On the other hand, LLJ strong shear causes eddies blocking situations with length scales greater than the LLJ height. The decrease of the surface roughness at the flooded season, compared to the dry season, reduces remarkably the temperature variance in the lowest length scales below 350m. Such results are useful for a better understanding of seasonal variability in the Pantanal's Nocturnal Boundary-layer, as well as to other regions with similar environmental characteristics and must be taken into consideration in nu merical simu lations of the flow structure above the region.

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“…The stratified boundary layer is occupied by numerous isolated events (e.g., Anderson 2003;Sun et al 2004;Mahrt 2008). Here, wind changes will be summarized in terms of kurtosis of the velocity components for each time scale, as shown for the Iowa network in Fig.…”

Section: Gradient Kurtosismentioning

confidence: 99%

Characteristics of Submeso Winds in the Stable Boundary Layer

Mahrt

2008

Boundary-Layer Meteorol

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The characteristics of submeso motions in the stable boundary layer are examined using observations from networks of sonic anemometers with network sizes ranging from a few hundred metres to 100 km. This study examines variations on time scales between 1 min and 1 h. The analysis focuses on the behaviour of the spectra of the horizontal kinetic energy, the ratios of the three velocity variances, their kurtosis, the dependence of horizontal variability on time scale, and the inter-relationship between vertical vorticity, horizontal divergence and deformation. Motions on larger time and space scales in the stable boundary layer are found to be nearly two-dimensional horizontal modes although the ratio of the vorticity to the divergence is generally on the order of one and independent of scale. One exception is a small network where stronger horizontal divergence is forced by a decrease in surface roughness. The horizontal variability, averaged over 1 h, appears to be strongly influenced by surface heterogeneity and increases with wind speed. In contrast, the time dependence of the horizontal structure on time scales less than one hour tends to be independent of wind speed for the present datasets. The spectra of the horizontal kinetic energy and the ratio of the crosswind velocity variance to the along-wind variance vary substantially between networks. This study was unable to isolate the cause of such differences. As a result, the basic behaviour of the submeso motions in the stable boundary layer cannot be generalized into a universal theory, at least not from existing data.

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Atmospheric Disturbances that Generate Intermittent Turbulence in Nocturnal Boundary Layers

Cited by 196 publications

References 35 publications

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

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

Low Level Jets in the Pantanal Wetland Nocturnal Boundary Layer – Case Studies

Characteristics of Submeso Winds in the Stable Boundary Layer

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