Session L1: Geophysical: Atmospheric III

Experimental study of turbulence induced wall temperature fluctuations

Turbulent heat transport is critical in engineering applications and atmospheric flows. The relative strength of background shear and buoyancy near the wall influences coherent structures responsible for much of the heat transport. Previous studies show that shear dominated flow causes streaky-like structures; whereas buoyancy dominated flow causes cell-like structures. In this work, we investigated the influence of flow structures on the wall temperature and heat flux in a convective atmospheric boundary layer. Turbulence data at different heights and high frequency wall temperature were obtained during the Boundary Layer Late Afternoon and Sunset Turbulence field campaign at Lannemezan, France from 7 June -- 8 July, 2011. Conditional averaging confirms that the warm wall causes warm ejection events, and cold sweep events cause cooling of the wall. The wall temperature structures move along the wind and their advection speed is close to the wind speed of the upper logarithmic layer and mixed layer, have a size of about 0.2 times the boundary layer depth, become streakier with stability and its standard deviation follows a -1/3 power law with stability parameter, Obukhov length.   

Surface-Pressure Fluctuations due to Coherent Microscale Processes in the Atmospheric Boundary Layer

Recent work has found evidence for microscale coherent turbulence in the atmospheric boundary layer due to the breakdown of Kelvin-Helmholtz billows. It is hypothesized that these structures, which advect with the mean flow, may be identified by transient fluctuations they cause in the boundary wall pressure. Surface-pressure fluctuations beneath the turbulent atmospheric boundary layer were measured at Reese Technology Center, located in the Llano Estacado mesa region of West Texas. Two multi-element arrays of piezoelectric bimorph infrasound sensors, selected for their sensitivity to frequencies as low as 0.1 Hz, were used to measure the pressure fluctuations. The boundary layer was simultaneously profiled up to 200 meters in elevation by an on-site meteorological instrument tower. All measurements were taken continuously for 44 hours. To study coherent propagating events, methods of beamforming and model reduction were applied to the surface-pressure fluctuations. Through dynamic mode decomposition, wavelike eigenfunctions of the pressure dynamics are identified that propagate with either advective or acoustic speeds.   

On the characterization of coherent structures within a neutrally-stratified atmospheric boundary layer

Up to this point, a clear characterization of wind turbulence and extreme gust events through experimentation has frustrated countless researchers. The statistical analysis of fluctuating components has been exhausted while the conditional analysis of extreme events, though insightful, often results in constricted conclusions that cannot be bridged from study to study. Thus the current study shifts towards an understanding of the fundamental turbulent flow structures within a neutrally-stratified atmospheric boundary layer. Two approaches to characterize coherent wind structures are presented. The first approach identifies hairpin-vortex heads by correlating three-dimensional, fluctuating data from two high-speed anemometers situated at 40m and 50m heights on a wind mast. The model assumes that a hairpin-vortex head can be approximated as a transverse vortex with a Vatistas viscous core of assumed radius when the hairpin-vortex head impinges onto the two anemometers. The second approach employs large-scale particle tracking velocimetry to follow seeded bubbles next to the wind mast. The results obtained with both approaches are then compared, and the advantages and shortcomings of each method are discussed.   

Nested large-eddy simulations of nighttime shear-instability waves and transient warming in a steep valley

This numerical study investigates the nighttime flow dynamics in a steep valley. The Owens Valley in California is highly complex, and represents a challenging terrain for large-eddy simulations (LES). To ensure a faithful representation of the nighttime atmospheric boundary layer (ABL), realistic external boundary conditions are provided through grid nesting. The model obtains initial and lateral boundary conditions from reanalysis data, and bottom boundary conditions from a land-surface model. We demonstrate the ability to extend a mesoscale model to LES resolutions through a systematic grid-nesting framework, achieving accurate simulations of the stable ABL over complex terrain. Nighttime cold-air flow was channeled through a gap on the valley sidewall. The resulting katabatic current induced a cross-valley flow. Directional shear against the down-valley flow in the lower layers of the valley led to breaking Kelvin-Helmholtz waves at the interface, which is captured only on the LES grid. Later that night, the flow transitioned from down-slope to down-valley near the western sidewall, leading to a transient warming episode. Simulation results are verified against field observations and reveal good spatial and temporal precision.   

Delay in convection in nocturnal boundary layer due to aerosol-induced cooling

Heat transfer processes in the nocturnal boundary layer (NBL) influence the surface energy budget, and play an important role in many micro-meteorological processes including the formation of inversion layers, radiation fog, and in the control of air-quality near the ground. Under calm clear-sky conditions, radiation dominates over other transport processes, and as a result, the air layers just above ground cool the fastest after sunset. This leads to an anomalous post-sunset temperature profile characterized by a minimum a few decimeters above ground (Lifted temperature minimum). We have designed a laboratory experimental setup to simulate LTM, involving an enclosed layer of ambient air, and wherein the boundary condition for radiation is decoupled from those for conduction and convection. The results from experiments involving both ambient and filtered air indicate that the high cooling rates observed are due to the presence of aerosols. Calculated Rayleigh number of LTM-type profiles is of the order 10$^{5}$-10$^{7}$ in the field and of order 10$^{3}$-10$^{5}$ in the laboratory. In the LTM region, there is convective motion when the Rayleigh number is greater than 10$^{4}$ rather than the critical Rayleigh number (Ra$_{c}$ = 1709). The diameter of convection rolls is a function of height of minimum of LTM-type profiles. The results obtained should help in the parameterization of transport process in the nocturnal boundary layer, and highlight the need to accounting the effects of aerosols and ground emissivity in climate models.   

Direct Numerical Simulation of the Convective Boundary Layer

The inversion of the dry shear-free Convective Boundary Layer is investigated by means of Direct Numerical Simulation (DNS). This work is motivated by the importance of entrainment and related mechanisms at the inversion of the atmospheric boundary layer, combined with the uncertainty of Large-Eddy Simulations (LES) there. Despite moderate Reynolds numbers attainable, results show that the achieved scale separation is enough to capture the expected $1/2$ power law evolution in time, and the expected structure---an inversion-capped outer layer, whose statistics are comparable to LES results and atmospheric data when normalized with the convective scales, and an inner layer near the surface comparable to that of the heated plate case. In agreement with some previous investigations, the entrainment ratio $A$ is a factor of two less than the nominal $0.2$ even though the corresponding entrainment velocity is within 5\% of the Zero-order Model prediction with $A=0.2$. To understand this apparent discrepancy, we use DNS data to directly determine the behavior of the terms of an exact equation for the entrainment ratio.   

DNS of stratified spatially-developing turbulent thermal boundary layers

Direct numerical simulations (DNS) of spatially-developing turbulent thermal boundary layers under stratification are performed. It is well known that the transport phenomena of the flow is significantly affected by buoyancy, particularly in urban environments where stable and unstable atmospheric boundary layers are encountered. In the present investigation, the Dynamic Multi-scale approach by Araya et al. (JFM, 670, 2011) for turbulent inflow generation is extended to thermally stratified boundary layers. Furthermore, the proposed Dynamic Multi-scale approach is based on the original rescaling-recycling method by Lund et al (1998). The two major improvements are: (i) the utilization of two different scaling laws in the inner and outer parts of the boundary layer to better absorb external conditions such as inlet Reynolds numbers, streamwise pressure gradients, buoyancy effects, etc., (ii) the implementation of a Dynamic approach to compute scaling parameters from the flow solution without the need of empirical correlations as in Lund et al (1998). Numerical results are shown for ZPG flows at high momentum thickness Reynolds numbers ($\sim$ 3,000) and a comparison with experimental data is also carried out.   

Wind Tunnel Simulation of the Atmospheric Boundary Layer

To simulate the interaction of large Vertical Axis Wind Turbines (VAWT) with the Atmospheric Boundary Layer (ABL) in the laboratory, we implement a variant of Counihan's technique in which a combination of a castellated barrier, elliptical vortex generators, and floor roughness elements is used to create an artificial ABL profile in a standard closed loop wind tunnel. We report hotwire measurements in a plane normal to the flow direction at various downstream positions and free stream velocities to examine the development and formation of the artificial ABL. It was found possible to generate a boundary layer at $Re_{\theta} \sim 10^{6}$, with a mean velocity that followed the 1/7 power law of a neutral ABL over rural terrain and longitudinal turbulence intensities and power spectra that compare well with the data obtained by Hultmark in 2010 for high Reynolds number flat plate turbulent boundary layers.   

Global Intermittency in Stably Stratified Turbulent Ekman Layers

Current weather models rely on similarity theories to represent fluxes in the lower atmosphere that assume turbulence is relatively homogeneous in space and time. In addition, many SGS models for LES assume turbulence is homogeneous to apply planar-averaging when computing model coefficients.However, under very stable conditions turbulence can be intermittent, resulting in inhomogeneity in space. Measurements in the atmosphere under very stable conditions have also shown oscillatory behavior of turbulent quantities (Coulter 90;Van de Weil et al 01). A number of possible mechanisms causing this have been described previously (Hunt 85;Nappo 91;Mahrt 99). In this study we investigate the dynamics of intermittency and oscillatory behavior using DNS, in order to improve our understanding and turbulence closures. For very stable cases, TKE production is found to drop down to zero resulting in laminarization of the flow field. This causes a drop in wall stress, which in turn allows the flow to accelerate, thus increasing shear culminating with increased production of TKE. The period of variations in TKE from high to low and back is on the order of a few hours, as found in the atmosphere. Dependencies of the period and other properties of the variations on Richardson number are explored.   

The critical layer for gravity waves in sheared rotating stratified flows

We re-examined the propagation of gravity waves through a critical layer surrounded by two inertial levels in the case of a constant vertically sheared flow. This problem involves a transition from balanced (where the quasi-geostrophic approximation applies) to sheared gravity waves. The three-dimensional disturbance is described analytically using both an exact solution and a WKB approximation valid for large Richardson numbers. In contradiction with past studies which show that there is finite reflection and did not analyse the transmission (Yamanaka and Tanaka, 1984), we find that reflection is extremely too small to be significant. The reasons that previous authors made incorrect evaluations are related to the fact that (i) the equations yielding to these results are extremely involved and (ii) the values of reflection and transmission coefficients are exponentially small or null, e.g. quite difficult to cross check numerically. Interestingly, these values are exactly like in the much simpler non-rotating case analysed by Booker and Bretherton (1966). Some practical implications for the problem of the emission of gravity waves by potential vorticity anomalies, analysed recently in Lott et al. (2013), are also discussed.