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A new mixed scaling parameterZ=z/(Lh)^1/2is proposed for similarity in the stable atmospheric surface layer, wherezis the height,Lis the Obukhov length, andhis the boundary layer depth. In comparison with the parameterζ=z/Lfrom Monin–Obukhov similarity theory (MOST), the new parameterZleads to improved mean profile similarity for wind speed and air temperature in large-eddy simulations. It also yields the same linear similarity relation for CASES-99 field measurements, including in the strongly stable (but still turbulent) regime where large deviations from MOST are observed. Results further suggest that similarity for turbulent energy dissipation rate depends on bothZandζ. The proposed mixed scaling ofZand relevance ofhcan be explained by physical arguments related to the limit ofz-less stratification that is reached asymptotically above the surface layer. The presented evidence and fitted similarity relations are promising, but the results and arguments are limited to a small sample of idealized stationary stable boundary layers. Corroboration is needed from independent datasets and analyses, including for complex and transient conditions not tested here.

 

The wind‐blown flux of sand generates dunes, wind erosion, and mineral dust aerosols. Existing models predict sand flux using the wind friction velocity that characterizes near‐surface turbulent momentum fluxes. However, these models struggle to accurately predict sand fluxes. Here we analyze root causes of these model discrepancies using high‐frequency field measurements of winds and sand fluxes. We find that friction velocity is only predictive of sand fluxes on long timescales, when it correlates with horizontal wind speed. On shorter timescales, and for non‐ideal surface conditions, friction velocity is much less predictive, likely because the near‐surface wind momentum budget is dominated by other, less predictable terms. We furthermore find that variability in 30‐min averaged sand fluxes at a given friction velocity is not driven by changes in turbulence but by changes in surface conditions, raising a challenge for models. These findings can improve sand flux models and clarify their limitations.

 

A persistent spatial organization of eddies is identified in the lowest portion of the stably stratified planetary boundary layer. The analysis uses flow realizations from published large-eddy simulations (Sullivan et al. in J Atmos Sci 73(4):1815–1840, 2016) ranging in stability from near-neutral to almost z-less stratification. The coherent turbulent structure is well approximated as a series of uniform momentum zones (UMZs) and uniform temperature zones (UTZs) separated by thin layers of intense gradients that are significantly greater than the mean. This pattern yields stairstep-like instantaneous flow profiles whose shape is distinct from the mean profiles that emerge from long-term averaging. However, the scaling of the stairstep organization is closely related to the resulting mean profiles. The differences in velocity and temperature across the thin gradient layers remain proportional to the surface momentum and heat flux conditions regardless of stratification. The vertical thickness of UMZs and UTZs is proportional to height above the surface for near-neutral and weak stratification, but becomes thinner and less dependent on height as the stability increases. Deviations from the logarithmic mean profiles for velocity and temperature observed under neutral conditions are therefore predominately due to the reduction in eddy size with increasing stratification, which is empirically captured by existing Monin–Obukhov similarity relations for momentum and heat. The zone properties are additionally used to explain trends in the turbulent Prandtl number, thus providing a connection between the eddy organization, mean profiles, and turbulent diffusivity in stably stratified conditions.

 

The inertial subrange of turbulent scales is commonly reflected by a power law signature in ensemble statistics such as the energy spectrum and structure functions – both in theory and from observations. Despite promising findings on the topic of fractal geometries in turbulence, there is no accepted image for the physical flow features corresponding to this statistical signature in the inertial subrange. The present study uses boundary layer turbulence measurements to evaluate the self-similar geometric properties of velocity isosurfaces and investigate their influence on statistics for the velocity signal. The fractal dimension of streamwise velocity isosurfaces, indicating statistical self-similarity in the size of ‘wrinkles’ along each isosurface, is shown to be constant only within the inertial subrange of scales. For the transition between the inertial subrange and production range, it is inferred that the largest wrinkles become increasingly confined by the overall size of large-scale coherent velocity regions such as uniform momentum zones. The self-similarity of isosurfaces yields power-law trends in subsequent one-dimensional statistics. For instance, the theoretical 2/3 power-law exponent for the structure function can be recovered by considering the collective behaviour of numerous isosurface level sets. The results suggest that the physical presence of inertial subrange eddies is manifested in the self-similar wrinkles of isosurfaces. 

Modification to the law of the wall represented by a dimensionless correction function ϕRSL(z/h) is derived using atmospheric turbulence measurements collected at two sites in the Amazon in near-neutral stratification, where z is the distance from the forest floor and h is the mean canopy height. The sites are the Amazon Tall Tower Observatory for z/h∈[1,2.3] and the Green Ocean Amazon (GoAmazon) site for z/h∈[1,1.4]. A link between the vertical velocity spectrum Eww(k) (k is the longitudinal wavenumber) and ϕRSL is then established using a co-spectral budget (CSB) model interpreted by the moving-equilibrium hypothesis. The key finding is that ϕRSL is determined by the ratio of two turbulent viscosities and is given as νt,BL/νt,RSL, where νt,RSL=(1/A)∫0∞τ(k)Eww(k)dk, νt,BL=kv(z−d)u*, τ(k) is a scale-dependent decorrelation time scale between velocity components, A=CR/(1−CI)=4.5 is predicted from the Rotta constant CR=1.8, and the isotropization of production constant CI=3/5 given by rapid distortion theory, kv is the von Kármán constant, u* is the friction velocity at the canopy top, and d is the zero-plane displacement. Because the transfer of energy across scales is conserved in Eww(k) and is determined by the turbulent kinetic energy dissipation rate (ε), the CSB model also predicts that ϕRSL scales with LBL/Ld, where LBL is the length scale of attached eddies to z=d, and Ld=u*3/ε is a macro-scale dissipation length.

 

using a combination of field experiments and numerical simulations. Specifically, Large Eddy Simulations (LES) were used to resolve emissions of isoprene and monoterpenes, turbulent transport, and air chemistry. The coupled chemistry-transport LES included the effects of isoprene and monoterpenes reactivity due to reactions with hydroxyl radical and ozone. The LES results are used to compute vertically resolved budgets of isoprene and monoterpenes in the rainforest canopy in response to emissions, turbulent transport, surface deposition, and air chemistry. Results indicated that emission and dispersion dominated the isoprene budget as the gases were transported out of the canopy space. In a region limited by nitrogen oxides (with prevailing nitric oxide levels of < 0.5 parts per billion), the in-canopy chemical destruction removed approximately 10% of locally emitted monoterpenes. Hydroxyl radical production rates from the ozonolysis of monoterpenes amounted to ≈ 2 × 106 radicals cm􀀀 3 s􀀀 1 and had similar magnitude to the light-dependent hydroxyl radical formation. One key conclusion was that the Amazonia rainforest abundantly emitted monoterpenes whose in-canopy ozonolysis yielded hydroxyl radicals in amounts similar to the magnitude of light-dependent formation. Reactions of monoterpenes and isoprene with hydroxyl radical and ozone were necessary for the maintenance of the Amazon rainforest canopy as a photochemically active environment suitable to generate oxidants and secondary organic aerosols. 

Desert dust accounts for a substantial fraction of the total atmospheric aerosol loading. It produces important impacts on the Earth system due to its nutrient content and interactions with radiation and clouds. However, current climate models greatly underestimate its airborne lifetime and transport. For instance, super coarse Saharan dust particles (with diameters greater than 10 µm) have repeatedly been detected in the Americas, but models fail to reproduce their transatlantic transport. In this study, we investigated the extent to which vertical turbulent mixing in the Saharan Air Layer (SAL) is capable of delaying particle deposition. We developed a theory based on the solution to a one‐dimensional dust mass balance and validated our results using large‐eddy simulation (LES) of a turbulent shear layer. We found that eddy motion can increase the lifetime of suspended particles by up to a factor of 2 when compared with laminar flows. Moreover, we found that the increase in a lifetime can be reliably estimated solely as a function of the particle Peclet number (the ratio of the mixing timescale to the settling timescale). By considering both the effects of turbulent mixing and dust asphericity, we explained to a large extent the presence of super coarse Saharan dust in the Caribbean observed during the Saharan Aerosol Long‐Range Transport and Aerosol‐Cloud‐Interaction Experiment (SALTRACE) field campaign. The theory for the lifetime of coarse particles in turbulent flows developed in this study is also expected to be applicable in other similar geophysical problems, such as phytoplankton sinking in the ocean mixed layer.