Function Of Friction Velocity Research Articles

Aeolian Remobilisation of Volcanic Ash: Outcomes of a Workshop in the Argentinian Patagonia

During explosive volcanic eruptions, large quantities of tephra can be dispersed and deposited over wide areas. Following deposition, subsequent aeolian remobilisation of ash can potentially exacerbate primary impacts on timescales of months to millennia. Recent ash remobilisation events (e.g., following eruptions of Cordón Caulle 2011; Chile, and Eyjafjallajökull 2010, Iceland) have highlighted this to be a recurring phenomenon with consequences for human health, economic sectors, and critical infrastructure. Consequently, scientists from observatories and Volcanic Ash Advisory Centers (VAACs), as well as researchers from fields including volcanology, aeolian processes and soil sciences, convened at the San Carlos de Bariloche headquarters of the Argentinian National Institute of Agricultural Technology to discuss the “state of the art” for field studies of remobilised deposits as well as monitoring, modeling and understanding ash remobilisation. In this article, we identify practices for field characterisation of deposits and active processes, including mapping, particle characterisation and sediment traps. Furthermore, since forecast models currently rely on poorly-constrained dust emission schemes, we call for laboratory and field measurements to better parameterise the flux of volcanic ash as a function of friction velocity. While source area location and extent are currently the primary inputs for dispersion models, once emission schemes become more sophisticated and better constrained, other parameters will also become important (e.g., source material volume and properties, effective precipitation, type and distribution of vegetation cover, friction velocity). Thus, aeolian ash remobilisation hazard and associated impact assessment require systematic monitoring, including the development of a regularly-updated spatial database of resuspension source areas.

Evaluation of the WRF‐lake model over a highland freshwater lake in southwest China

AbstractEvaluation of the Weather Research and Forecasting Model version 3.7.1 lake model performance on lake physical processes and lake‐atmosphere interaction at Erhai Lake was conducted with in situ observation data sets over the lake to verify the model results. The lake model with default parameters presented significant negative bias during the day and positive bias during the night on lake surface temperature. Calibrations of water absorption and extinction coefficients, surface roughness length, and eddy diffusion coefficient were carried out to correct this bias. The added absorption and extinction coefficients increased solar radiation in upper water layer, leading to a little higher surface temperature. Compared to constant surface roughness lengths, parameterization of roughness lengths as functions of friction velocity and fetch length significantly improved simulations of lake surface temperature and sensible heat flux. In Erhai Lake, eddy diffusion coefficient of the Henderson‐Sellers parameterization is smaller than that in deep lakes. The eddy diffusion coefficient was reduced by a factor of 0.05 in dry season and 0.02 in wet season, respectively, to restrict mixing strength, generating a reasonable diurnal range with observed surface temperature. The adjusted parameter simulations matched better with in situ observations in diurnal cycle of temperature and surface heat fluxes but still overestimated wind speed. Topographic correction is effective to correct the bias, indicating that topographic correction over complex terrain was necessary.

An improved lake model for climate simulations: Model structure, evaluation, and sensitivity analyses in CESM1

Lakes can influence regional climate, yet most general circulation models have, at best, simple and largely untested representations of lakes. We developed the Lake, Ice, Snow, and Sediment Simulator (LISSS) for inclusion in the land‐surface component (CLM4) of an earth system model (CESM1). The existing CLM4 lake model performed poorly at all sites tested; for temperate lakes, summer surface water temperature predictions were 10–25°C lower than observations. CLM4‐LISSS modifies the existing model by including (1) a treatment of snow; (2) freezing, melting, and ice physics; (3) a sediment thermal submodel; (4) spatially variable prescribed lake depth; (5) improved parameterizations of lake surface properties; (6) increased mixing under ice and in deep lakes; and (7) correction of previous errors. We evaluated the lake model predictions of water temperature and surface fluxes at three small temperate and boreal lakes where extensive observational data was available. We also evaluated the predicted water temperature and/or ice and snow thicknesses for ten other lakes where less comprehensive forcing observations were available. CLM4‐LISSS performed very well compared to observations for shallow to medium‐depth small lakes. For large, deep lakes, the under‐prediction of mixing was improved by increasing the lake eddy diffusivity by a factor of 10, consistent with previous published analyses. Surface temperature and surface flux predictions were improved when the aerodynamic roughness lengths were calculated as a function of friction velocity, rather than using a constant value of 1 mm or greater. We evaluated the sensitivity of surface energy fluxes to modeled lake processes and parameters. Large changes in monthly‐averaged surface fluxes (up to 30 W m−2) were found when excluding snow insulation or phase change physics and when varying the opacity, depth, albedo of melting lake ice, and mixing strength across ranges commonly found in real lakes. Typical variation among model parameterization choices can therefore cause persistent local surface flux changes much larger than expected changes in greenhouse forcing. We conclude that CLM4‐LISSS adequately simulates lake water temperature and surface energy fluxes, with errors comparable in magnitude to those resulting from uncertainty in global lake properties, and is suitable for inclusion in global and regional climate studies.

Relationship between deposition and attachment rates in Jacobi room model

It is shown in this work that parameters of the Jacobi model, which describes behavior of short-lived radon progeny, are not independent. The relationship between deposition rate of attached radon progeny and attachment rate of their unattached fraction was determined in this paper. It was found that deposition rate increases when the attachment rate is smaller; this effect is more pronounced for larger friction velocity. The deposition rate of attached radon progeny is presented here as a function of friction velocity, ventilation and attachment rate. Deposition rate of attached fraction was estimated in the range 0.012–0.46 h −1, when attachment rate varies from 10 h −1 to 100 h −1.

Deposition rates of unattached and attached radon progeny in room with turbulent airflow and ventilation

In this paper deposition rate coefficients for unattached and attached radon progeny were estimated according to a particle deposition model for turbulent indoor airflow described by Zhao and Wu [2006. Modeling particle deposition from fully developed turbulent flow in ventilation duct. Atmos. Environ. 40, 457–466]. The parameter which characterizes turbulent indoor airflow in this model is friction velocity, u*. Indoor ventilation changes indoor airflow and friction velocity and influences deposition rate coefficients. Correlation between deposition and ventilation rate coefficients in the room was determined. It was shown that deposition rate coefficient increases with ventilation rate coefficient and that these parameters of the Jacobi room model cannot be assumed to be independent. The values of deposition rate coefficients were presented as functions of friction velocity and ventilation rate coefficient. If ventilation rate coefficient varies from 0.1 up to 1 h −1, deposition rate coefficients for unattached and attached fractions were estimated to be in the range 3–110 h −1 and 0.015–0.35 h −1, respectively.

Influence of energetic wind and waves on gas transfer in a large wind–wave tunnel facility

Air–water gas exchange experiments were carried out in a large wind wave tunnel in Marseille, France, to investigate gas transfer processes under energetic wind and wave fields, where macroscale breaking waves create bubble plumes (white caps) and turbulence on the water surface. We measured the gas transfer velocities of N2O, DMS, He, SF6, CH3Br, and total air. Their diffusivity and solubility span a large range, allowing us to investigate gas transfer mechanisms under a variety of physical conditions. We observed that the gas transfer velocities varied with friction velocity in a linear manner. Gas transfer in the presence of pure wind waves is generally consistent with the surface renewal model, as the gas transfer velocity has a strong dependence on diffusivity with an exponent of 0.53(±0.02). Contrary to expectations, the bubble plumes generated by breaking waves contributed relatively little in our pure wind wave experiments. Superposition of mechanically generated waves onto the wind waves in the high wind regime attenuated DMS gas transfer (as a function of friction velocity) across the air–water interface by ∼20% compared with gas transfer under pure wind waves, implying suppression of gas transfer directly across the sheared water surface. Greater transfer of less soluble gases may result from bubble‐mediated gas transfer.

Measurements of aerosol particle dry deposition velocity using the relaxed eddy accumulation technique

The continuous measurements of aerosol particle deposition velocity have been performed from January 2004 to January 2005 using a REA technique with dynamic deadband.We measured aerosol particle deposition velocity in the size range of 10–150 nanometer with 5–10 nanometer steps using differential mobility analyser for sizing.We were able to measure two size classes simultaneously. One size class was changed at one month intervals, another we kept constant at 30 nm to investigate the effect of seasonal and meteorological variation on deposition velocity. We found that the 80–100 nanometer size particles had the lowest deposition velocity, about 0.4 cm s−-1. Deposition velocity increased with decreasing or increasing particle diameter from 80–100 nanometer size. We also found that deposition velocity increases as a function of friction velocity.

Turbulence flux measurement above the overstory of a subtropical Pinus plantation over the hilly region in southeastern China

Continuous turbulence flux measurement using the eddy covariance (EC) technique was made from January 1 to December 31 in 2003 at two and three canopy heights of a subtropical Pinus plantation on the red earth hilly region in southeastern China. To be able to make sure that the measured turbulence flux will equal the net ecosystem/atmosphere exchange, the quality of the data has to be assessed. Three criteria were investigated here, including the power spectra and cospectra analyses, flux variance similarity (integral turbulence test) and energy balance closure. The spectral analyses suggested that above-canopy power spectral slopes for all velocity components and scalars such as CO2, H2O and air temperature followed the expected -2/3 power law in the inertial subrange, and their cospectral slopes were close to -4/3 power law in the inertial subrange. The important contribution of large-scale motions to energy and mass transfer above the canopy at higher measurement level was also confirmed by the spectral analyses. The eddy covariance systems have the ability to resolve fluctuations associated with small-scale eddies and did not induce an obvious underestimation of the measured turbulence flux. The Monin-Obukhov similarity functions for the normalized standard deviation of vertical wind speed and air temperature were well-defined functions of atmospheric stability at two heights above the forest canopy, which indicated that turbulence flux measurements made at two heights were within the surface layer. Nocturnal flux underestimation and departures of this normalized standard deviation of vertical wind speed similarity function from that expected from Monin-Obukhov theory were a function of friction velocity. Thus, an optimal criterion of friction velocity was determined to be greater than 0.2-0.3 m s-1 for nocturnal fluxes so that the eddy covariance flux measurement was under high turbulent mixing conditions. Energy balance closure reached about 72%-81% at the studied site, which was comparable to the 10%-30% of energy imbalance reported in the literature. However, the energy balance closure could be only used as a useful reference criterion.

Development of saltation layer of drifting snow

AbstractThe saltation length of aeolian snow particles and a new parameter, the ejection factor, which expresses the degree of erosion due to drifting snow, were obtained as functions of friction velocity by means of wind-tunnel experiments for semi-hard snow cover. The saturated-snowdrift transport rate was also obtained experimentally as a function of friction velocity. Based on these characteristics and the parameter, the development of the saltation layer of drifting snow along the fetch was simulated under various conditions such as snow hardness, wind speed and snowfall intensity. The main results are as follows. The developing distance denoting the distance required for the saltation layer to attain saturation, X sat, is determined by saltation length, ejection factor and saturated-snowdrift transport rate, all of which depend on wind speed. It is also affected by the magnitude of snowdrift transport rate at the starting point and by the intensity of snowfall if it exists. The dependence of Xsat on wind speed is not simple in the case of semi-hard snow cover: Xsat increases with wind speed under weak to moderate wind conditions and then decreases under moderate to strong wind conditions. It is sensitive to snow hardness: it is about one order longer on hard snow cover than on semi-hard snow cover. Snowfall reduces not only the value of Xsat but also its dependence on snow hardness.

A revised parameterization for gaseous dry deposition in air-quality models

Abstract. A parameterization scheme for calculating gaseous dry deposition velocities in air-quality models is revised based on recent study results on non-stomatal uptake of O3 and SO2 over 5 different vegetation types. Non-stomatal resistance, which includes in-canopy aerodynamic, soil and cuticle resistances, for SO2 and O3 is parameterized as a function of friction velocity, relative humidity, leaf area index, and canopy wetness. Non-stomatal resistance for other chemical species is scaled to those of SO2 and O3 based on their chemical and physical characteristics. Stomatal resistance is calculated using a two-big-leaf stomatal resistance sub-model for all gaseous species of interest. The improvements in the present model compared to its earlier version include a newly developed non-stomatal resistance formulation, a realistic treatment of cuticle and ground resistance in winter, and the handling of seasonally-dependent input parameters. Model evaluation shows that the revised parameterization can provide more realistic deposition velocities for both O3 and SO2, especially for wet canopies. Example model output shows that the parameterization provides reasonable estimates of dry deposition velocities for different gaseous species, land types and diurnal and seasonal variations. Maximum deposition velocities from model output are close to reported measurement values for different land types. The current parameterization can be easily adopted into different air-quality models that require inclusion of dry deposition processes.

Numerical Modeling of Wave-Enhanced Turbulence in the Oceanic Upper Layer

A coupled model of air-wave-sea interaction is modified based on a new roughness formulation and the latest data. The model parameters for aerodynamic roughness from below (ARB) and wave-dependent roughness from above (ARA, z 0a ) are assumed equal. The combined roughness is assumed to be a function of friction velocity, gravity, air and seawater densities, and wave age (c w ). The model is used in a study of wave-enhanced turbulence under breaking waves to predict turbulent dissipation (e), ARA, and drag coefficient (C d ). Both waves and shear production are considered as sources of ocean turbulent energy. The atmospheric part of the model is used only to specify a correct condition at the interface. Numerical experiments are performed to study the e-distribution, z 0a and C d , and to compare with data. The major achievement is model verification using all available data. The first full application of this model is in conjunction with an ocean circulation model in a coupled circulation-wave system. Simulations show that the e-distribution is strongly dependent on local wind-forced wave heights. For each wind and wave state there is a particular wave-dependent depth that is verified by data. The comparison shows that the model predicted e agrees well with the observed e of the z −4 law distribution of Gargett (1989). Simulations also show that waves have an important role in causing e to differ from the classical wall-layer theory and z 0a , with a value of 0.30 for the empirical constant a a . The model-predicted e, z 0a , C d and C gd agree well with data.

Turbulent Flux Measurements Above and Below the Overstory of a Boreal Aspen Forest

Turbulent flux measurements both above and beneath the canopy of a boreal aspen forest are described. Velocity skewness showed that, beneath the aspen canopy, turbulence was dominated by intermittent, downward penetrating gusts. Eulerian horizontal length scales calculated from integration of the autocorrelation function or spectral peaks were 9.0 and 1.4 times the mean aspen height of 21.5 m respectively. Above-canopy power spectral slopes for all velocity components followed the -2/3 power law, whereas beneath-canopy slopes were closer to -1 and showed a spectral short cut in the horizontal and vertical components. Cospectral patterns were similar both above and beneath the canopy. The Monin–Obukhov similarity function for the vertical wind velocity variance was a well-defined function of atmospheric stability, both above and beneath the canopy. Nocturnal flux underestimation and departures of this similarity function from that expected from Monin–Obukhov theory were a function of friction velocity. Energy balance closure greater than 80% was achieved at friction velocities greater than 0.30 and 0.10 m s-1, above and below the aspen canopy, respectively. Recalculating the latent heat flux using various averaging periods revealed a minimum of 15 min were required to capture 90% of the 30-min flux. Linear detrending reduced the flux at shorter averaging periods compared to block averaging. Lack of energy balance closure and erratic flux behaviour led to the recalculation of the latent and sensible heat fluxes using the ratio of net radiation to the sum of the energy balance terms.

Space‐time properties of radar sea spikes and their relation to wind and wave conditions

Low‐grazing angle (LGA) radar sea spikes were observed with a high‐resolution, dual‐polarization, X band imaging radar deployed on the floating instrument platform (FLIP) during the Marine Boundary Layer Experiment (MBLEX) held off the California coast during April–May 1995. Spatiotemporal statistics of observed sea spike events are presented, including duration, velocity, and directional distributions. The dependence of these parameters on wind and wave conditions is compared to theoretical predictions and to similar measurements of breaking wave signatures obtained with passive acoustic techniques by Ding and Farmer [1994]. The density of sea spikes (events per unit time per unit area) and the fractional surface coverage are estimated as a function of friction velocity and compared to theoretical predictions. Though we find consistency between the dynamical aspects of sea spike events and of acoustic wave‐breaking signatures, we do not observe a predicted cubic u* dependence of sea spike density. Differences may be a consequence of the specific nature of low‐grazing angle scattering at X band (and higher) frequencies. We observe an approximately quadratic dependence on u* of sea spike fractional surface coverage which is insensitive to the choice of backscattered power threshold over a 9 dB range.

Measurements of snow mass flux and transport rate at different particle diameters in drifting snow

Wind tunnel experiments were carried out to investigate the snow mass flux and the snow transport rate as functions of friction velocity and particle diameter. Friction velocities obtained with an ultra sonic anemometer was from u *=0.15 to 0.39 m/s. The number flux and the particle size of drifting snow were measured with a new snow particle counter at heights of 16 to 61 mm above the snow surface. The horizontal snow mass flux at each particle diameter d, q d, decreased exponentially with height q d∝exp(− λ d z/( u * 2/ g)), where z is the height, g is the acceleration due to gravity and λ d is a dimensionless parameter. The constant λ d was independent of friction velocity, and at larger particle diameters, approached to 0.3. The total snow transport rate increased with the power (3.96) of friction velocity, but the total snow transport rate at each particle diameter showed that the power decreased with increasing particle diameter, and approached to 3.

Statistical dependencies of sea state characteristics on water temperature and wind-wave age

The extensive data set of photogrammetric measurements of ocean surface fractions covered by the whitecaps, S w and foam patches, S f, is compiled. The correlation analysis confirms the reliability of the sea state characteristics dependencies on sea surface temperature, i.e. on sea water viscosity, which were found earlier on the base of a lesser statistical material. The friction velocity is determined for many cases of observation with taken into account not only the atmospheric stability but wind-wave age also. Correlation of S w and S f with friction velocity are found higher than with wind speed. Quantities S w and S f are functions of friction velocity, sea surface temperature and wind-wave age. Correlation equations which determine S w, S f, duration of wave crest breaking, and foam patches life-time dependencies on these factors are given.

A Net Deposition Model

A net deposition model (dependent on friction velocity) that accounts for ideal deposition of aerosols to a surface, rebound of particles impacting the surface, and reentrainment of particles previously deposited on the surface has been developed. The model time step is based on a scaled time interval between coherent boundary layer structures (bursts), with deposition, rebound, and reentrainment determined for each time step. Deposition and rebound are functions of friction velocity, and reentrainment is based on a Monte-Carlo simulation using a surface renewal concept. The model was used to simulate net deposition of six different particle-surface pairs. Results were found to generally describe experimental data well, although some differences were present.

Breaking waves affecting microwave backscatter: 2. Dependence on wind and wave conditions

This paper is the second of a two‐part series on using microwave techniques for detection and characterization of wave breaking. The statistics of sea spikes detected using the method described in Part 1 are investigated as functions of friction velocity u* and of a Reynolds number Re* based on u* and the dominant surface wavelength. For vertical (VV) and horizontal (HH) polarization, the frequency of sea spikes and their contribution to the mean radar cross section show a roughly cubic dependence on u*. The percentage of wave crests producing sea spikes, Pss, is consistent with the Re* exponent of 1.5 reported by other investigators. At high friction velocities (u* between 40 and 50 cm s−1), sea spikes contribute between 10% and 15% to the mean radar cross section for VV polarization and between 15% and 25% for HH polarization. The data suggest that the average radar cross section of an individual sea spike does not depend on u*. Measurements of Doppler frequency and bandwidth are used to investigate the kinematics of the breaking process.

Forecast and studies of the oceanic mixed layer during the mile experiment

Abstract Forecasts of the oceanic mixed layer are performed for the MILE experiment. Two differential mixed-layer models are used and compared. One neglects the triple-correlation terms and the other forecasts them. The comparison of the models shows the effects of the higher-order closure on mixed-layer modelling with real data. The forecasts are initiated from data and driven by the experimentally measured, wind stress, radiation and latent and sensible heat flux. The duration of forecasts is from several days to a month. The resultant temperature and velocity profiles are compared against available data. The velocity profiles show stress related amplitudes that oscillate at the inertial frequency. The dissipation function is computed. The results are compared for low and high wind velocities against those of Caldwell and Dillon (1979). A scatter diagram of the integrated dissipation over the mixed-layer depth is computed as a function of friction velocity. The slope of the scatter diagram is compared a...