Turbulence vertical structure of the boundary layer during the afternoon transition

Darbieu, Clara; Lohou, Fabienne; Lothon, Marie; Arellano, Jordi Vilà-Guerau de; Couvreux, Fleur; Durand, Pierre; Pino, David; Patton, Edward G.; Nilsson, Erik; Blay-Carreras, E.; Gioli, Beniamino

doi:10.5194/acp-15-10071-2015

Cited by 56 publications

(61 citation statements)

References 49 publications

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“…Moreover, Darbieu et al (2015) also found steeper slopes in spectra derived from aircraft measurements. Lothon et al (2009) and Darbieu et al (2015) assume that a steeper slope could also be caused by asymmetric convective structures, i.e., by anisotropy of the w field.…”

Section: Characteristics Of Vertical Velocity Datamentioning

confidence: 95%

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Observed spatiotemporal variability of boundary-layer turbulence over flat, heterogeneous terrain

et al. 2016

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Abstract. In the spring of 2013, extensive measurements with multiple Doppler lidar systems were performed. The instruments were arranged in a triangle with edge lengths of about 3 km in a moderately flat, agriculturally used terrain in northwestern Germany. For 6 mostly cloud-free convective days, vertical velocity variance profiles were calculated. Weighted-averaged surface fluxes proved to be more appropriate than data from individual sites for scaling the variance profiles; but even then, the scatter of profiles was mostly larger than the statistical error. The scatter could not be explained by mean wind speed or stability, whereas time periods with significantly increased variance contained broader thermals. Periods with an elevated maximum of the variance profiles could also be related to broad thermals. Moreover, statistically significant spatial differences of variance were found. They were not influenced by the existing surface heterogeneity. Instead, thermals were preserved between two sites when the travel time was shorter than the large-eddy turnover time. At the same time, no thermals passed for more than 2 h at a third site that was located perpendicular to the mean wind direction in relation to the first two sites. Organized structures of turbulence with subsidence prevailing in the surroundings of thermals can thus partly explain significant spatial variance differences existing for several hours. Therefore, the representativeness of individual variance profiles derived from measurements at a single site cannot be assumed.

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Section: Characteristics Of Vertical Velocity Datamentioning

confidence: 95%

“…Lothon et al (2009) and Darbieu et al (2015) assume that a steeper slope could also be caused by asymmetric convective structures, i.e., by anisotropy of the w field.…”

Section: Characteristics Of Vertical Velocity Datamentioning

confidence: 99%

Observed spatiotemporal variability of boundary-layer turbulence over flat, heterogeneous terrain

et al. 2016

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“…This could be related to the quasi-stationary behaviour discussed in Darbieu et al (2015) and Nilsson et al (2015a), where no changes in turbulence structure or characteristics are evident after normalization by the decreasing surface sensible heat fluxes. The difficulties increase in the very late afternoon.…”

Section: Afternoon Transitionmentioning

confidence: 99%

“…The layer between the pink and the red symbols was named the pre-residual layer by Nilsson et al (2015b). It is characterized by very low turbulence and results from the adjustment of turbulence to the decreasing surface fluxes (Darbieu et al, 2015). Figure 11 presents the variations of the time when the virtual temperature flux (which is a combination of the surface sensible heat flux and the latent heat flux) becomes negative, t_Hv0, through the IOPs and the various points.…”

Section: Afternoon Transitionmentioning

confidence: 99%

Boundary-layer turbulent processes and mesoscale variability represented by numerical weather prediction models during the BLLAST campaign

et al. 2016

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Abstract. This study evaluates the ability of three operational models, with resolution varying from 2.5 to 16 km, to predict the boundary-layer turbulent processes and mesoscale variability observed during the Boundary Layer Late-Afternoon and Sunset Turbulence (BLLAST) field campaign. We analyse the representation of the vertical profiles of temperature and humidity and the time evolution of nearsurface atmospheric variables and the radiative and turbulent fluxes over a total of 12 intensive observing periods (IOPs), each lasting 24 h. Special attention is paid to the evolution of the turbulent kinetic energy (TKE), which was sampled by a combination of independent instruments. For the first time, this variable, a central one in the turbulence scheme used in AROME and ARPEGE, is evaluated with observations.In general, the 24 h forecasts succeed in reproducing the variability from one day to another in terms of cloud cover, temperature and boundary-layer depth. However, they exhibit some systematic biases, in particular a cold bias within the daytime boundary layer for all models. An overestimation of the sensible heat flux is noted for two points in ARPEGE and is found to be partly related to an inaccurate simplification of surface characteristics. AROME shows a moist bias within the daytime boundary layer, which is consistent with overestimated latent heat fluxes. ECMWF presents a dry bias at 2 m above the surface and also overestimates the sensible heat flux. The high-resolution model AROME resolves the vertical structures better, in particular the strong daytime inversion and the thin evening stable boundary layer. This model is also able to capture some specific observed features, such as the orographically driven subsidence and a welldefined maximum that arises during the evening of the water vapour mixing ratio in the upper part of the residual layer due to fine-scale advection. The model reproduces the order of magnitude of spatial variability observed at mesoscale (a few tens of kilometres). AROME provides a good simulation of the diurnal variability of the turbulent kinetic energy, while ARPEGE shows the right order of magnitude.

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“…Recent simulations (Darbieu et al, 2015) have also been used to study TKE and other turbulence characteristics such as anisotropy, evolution of spectra and integral length scales during the afternoon transition. This was also studied by Pino et al (2006) using LES by prescribing an instantaneous change to zero-buoyancy flux, similar to Nieuwstadt and Brost (1986) but with the additional effect of shear production.…”

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confidence: 99%

Turbulence kinetic energy budget during the afternoon transition – Part 1: Observed surface TKE budget and boundary layer description for 10 intensive observation period days

et al. 2016

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Abstract. The decay of turbulence kinetic energy (TKE) and its budget in the afternoon period from midday until zerobuoyancy flux at the surface is studied in a two-part paper by means of measurements from the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign for 10 intensive observation period days. Here, in Part 1, near-surface measurements from a small tower are used to estimate a TKE budget. The overall boundary layer characteristics and mesoscale situation at the site are also described based upon taller tower measurements, radiosoundings and remote sensing instrumentation. Analysis of the TKE budget during the afternoon transition reveals a variety of different surface layer dynamics in terms of TKE and TKE decay. This is largely attributed to variations in the 8 m wind speed, which is responsible for different amounts of nearsurface shear production on different afternoons and variations within some of the afternoon periods. The partitioning of near-surface production into local dissipation and transport in neutral and unstably stratified conditions was investigated. Although variations exist both between and within afternoons, as a rule of thumb, our results suggest that about 50 % of the near-surface production of TKE is compensated for by local dissipation near the surface, leaving about 50 % available for transport. This result indicates that it is important to also consider TKE transport as a factor influencing the near-surface TKE decay rate, which in many earlier studies has mainly been linked with the production terms of TKE by buoyancy and wind shear. We also conclude that the TKE tendency is smaller than the other budget terms, indicating a quasi-stationary evolution of TKE in the afternoon transition. Even though the TKE tendency was observed to be small, a strong correlation to mean buoyancy production of −0.69 was found for the afternoon period. For comparison with previous results, the TKE budget terms are normalized with friction velocity and measurement height and discussed in the framework of Monin-Obukhov similarity theory. Empirically fitted expressions are presented. Alternatively, we also suggest a non-local parametrization of dissipation using a TKE-length scale model which takes into account the boundary layer depth in addition to distance above the ground. The non-local formulation is shown to give a better description of dissipation compared to a local parametrization.

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Turbulence vertical structure of the boundary layer during the afternoon transition

Cited by 56 publications

References 49 publications

Observed spatiotemporal variability of boundary-layer turbulence over flat, heterogeneous terrain

Observed spatiotemporal variability of boundary-layer turbulence over flat, heterogeneous terrain

Boundary-layer turbulent processes and mesoscale variability represented by numerical weather prediction models during the BLLAST campaign

Turbulence kinetic energy budget during the afternoon transition – Part 1: Observed surface TKE budget and boundary layer description for 10 intensive observation period days

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