Vertical Velocity Variance Research Articles

UMA NOVA EXPRESSÃO PARA A VARIÂNCIA DA VELOCIDADE TURBULENTA CONVECTIVA DERIVADA DE UM MODELO LES

The aim of this study is to derive a new expression for the vertical turbulent velocity variance using LES data in a convective boundary layer.

A study of propagation of nonlinear acoustic-gravity waves in the middle and upper atmosphere by means of numerical modeling

numerical model of the vertical propagation and decay of nonlinear acoustic-gravity waves (AGW) from the Earth surface to the upper atmosphere is described. Monochromatic vertical velocity variations at the Earth surface are used as the AGW source in the model. The numerical method for solving three-dimensional hydrodynamic equations is based on finite-difference representation of the fundamental laws of conservation, which makes it possible to calculate not only smooth, but also physically correct generalized solutions of the hydrodynamic equations. The equations are solved in a range of altitudes from the ground up to 500 km. The background temperature, density, molecular viscosity and thermal conductivity coefficient are specified according to standard atmosphere models. The dependence of the characteristics of the waves on the amplitude of the wave source at the lower boundary is examined. The amplitudes of the AGW increase with the altitude, and the waves can break down due to nonlinear effects in the middle and upper atmosphere, depending on the amplitude of the source.

Constraints on crustal attenuation and three-dimensional spatial distribution of stress drop in Switzerland

We employ two different methods to estimate attenuation-corrected source spectra and calculate Brune stress drops of small to medium earthquakes in Switzerland and the surrounding region. We test trade-offs between source and travel path contributions by comparing results between a data-driven and a model-dependent method of distinguishing between the two. Systematic differences between results from both methods can best be reconciled by assuming intrinsic whole-path attenuation to be frequency dependent. A two-parameter grid search based on a power-law Q( f ) function reveals trade-offs that prevent us from quantifying a more exact frequency dependence. However, a comparison of the two-source parameter inversion methods with synthetic tests provide evidence for a non-negligible frequency dependence of Q between 1 and 30 Hz. Consideration of these implications on the attenuation model, as well as consideration of lateral and vertical variations of velocity and quality factor reduces the scatter of the obtained stress drop estimates. Synthetic tests confirm that both methods are able to robustly resolve lateral variations of Brune stress drop with quantifiable uncertainty estimates. Resulting lateral variations show reduced stress drop along the Alpine deformation front. This pattern points to tectonic causes and may be due to variations in differential stress.

An Optimal Inverse Method Using Doppler Lidar Measurements to Estimate the Surface Sensible Heat Flux

Inverse methods are widely used in various fields of atmospheric science. However, such methods are not commonly used within the boundary-layer community, where robust observations of surface fluxes are a particular concern. We present a new technique for deriving surface sensible heat fluxes from boundary-layer turbulence observations using an inverse method. Doppler lidar observations of vertical velocity variance are combined with two well-known mixed-layer scaling forward models for a convective boundary layer (CBL). The inverse method is validated using large-eddy simulations of a CBL with increasing wind speed. The majority of the estimated heat fluxes agree within error with the proscribed heat flux, across all wind speeds tested. The method is then applied to Doppler lidar data from the Chilbolton Observatory, UK. Heat fluxes are compared with those from a mast-mounted sonic anemometer. Errors in estimated heat fluxes are on average 18 %, an improvement on previous techniques. However, a significant negative bias is observed (on average $$-63\,\%$$ ) that is more pronounced in the morning. Results are improved for the fully-developed CBL later in the day, which suggests that the bias is largely related to the choice of forward model, which is kept deliberately simple for this study. Overall, the inverse method provided reasonable flux estimates for the simple case of a CBL. Results shown here demonstrate that this method has promise in utilizing ground-based remote sensing to derive surface fluxes. Extension of the method is relatively straight-forward, and could include more complex forward models, or other measurements.

Roles of breaking waves and Langmuir circulation in the surface boundary layer of a coastal ocean

[1] Breaking waves and Langmuir circulation are two important turbulent processes in the wind-driven upper ocean. To investigate their roles in generating turbulence in the surface boundary layer of a coastal ocean, a large eddy simulation model is used to simulate the turbulence measurements collected at the Martha's Vineyard Coastal Observatory's Air-Sea Interaction Tower, during the Coupled Boundary Layers and Air-Sea Transfer (CBLAST) experiment in 2003. The model provides reasonable predictions for the vertical profiles of vertical velocity variance, turbulent kinetic energy (TKE), energy dissipation rates, and heat flux. It shows breaking waves dominating turbulence generation near the ocean surface and turbulent large eddies characteristic of Langmuir circulation deeper in the water column. Diagnostic analysis of TKE budget in the model shows a dominant balance between turbulent transport and dissipation near the surface and a dominant balance between shear production and dissipation at deeper depths. Although the Stokes production is a significant term in the TKE budget balance near the surface, it is smaller than shear production. The turbulent transport is large in the near-surface zone and is still significant in the region affected by Langmuir circulation. These results are in agreement with a conclusion inferred from a recent analysis of the near-surface turbulence measurements at the CBLAST site.

Aspects of Convective Boundary Layer Turbulence Measured by a Dual-Doppler Lidar System

Abstract Special designed dual-Doppler setups can be used to retrieve simultaneous measurements of two wind components with high temporal resolution in several heights throughout the atmospheric boundary layer. During a field campaign in summer 2011, different scan strategies were performed to demonstrate the opportunities of obtaining variance profiles of the vertical and horizontal wind components in complex terrain. A simplified error analysis reveals the effects of the error propagation of the uncorrelated noise of the single lidar systems. A comparison shows that the course of the derived horizontal wind component is in accordance to in situ measurements. The dual-Doppler vertical wind velocity reflects the up- and downdrafts in a convective boundary layer and is even able to reflect a light rain event. The normalized profiles of the vertical velocity variances reproduce the well-known decrease from about one-third of the boundary layer height to its top. The horizontal velocity variance did not reveal a systematic behavior on the considered days.

Shear velocity structure and mineralogy of the transition zone beneath the East Pacific Rise

Models of seismic velocity as a function of depth through the upper mantle provide some of the strongest constraints on the mineralogy and composition of the mantle. Although receiver function studies have provided new information on the depths of upper mantle discontinuities they do not provide as much information on seismic gradients between discontinuities and absolute velocities within the upper mantle. The waveforms and travel times of upper mantle turning waves provide the strongest constraints on vertical variations in upper mantle velocity although in the past they suffered from the lack of dense profiles of data sampling a single part of the upper mantle that would minimize effects of 3D variations in velocity. Here we model three dense profiles of triplicated upper mantle broadband S and SS waves recorded by USArray, Canadian and NARS-Baja stations located in western North America. Earthquakes along the East Pacific Rise were recorded along profiles within 5° back azimuth windows and with stations at a maximum of 0.5° separation. The distance range covered is from 35° to 60° and thus the waves sample the mantle from the shallow upper mantle to depths near 1000 km. The data were inverted using a conjugate gradient algorithm that utilizes the reflectivity synthetic technique. The results show a much smaller gradient within the transition zone than the PREM model with larger jumps in velocity across the 410 km and 660 km depth discontinuities. These results are consistent with velocities predicted for a pyrolite composition mantle transition zone. Compositional models with lower olivine content, such as piclogite, are not consistent with our seismic model.

Monte Carlo-based subgrid parameterization of vertical velocity and stratiform cloud microphysics in ECHAM5.5-HAM2

Abstract. A new method for parameterizing the subgrid variations of vertical velocity and cloud droplet number concentration (CDNC) is presented for general circulation models (GCMs). These parameterizations build on top of existing parameterizations that create stochastic subgrid cloud columns inside the GCM grid cells, which can be employed by the Monte Carlo independent column approximation approach for radiative transfer. The new model version adds a description for vertical velocity in individual subgrid columns, which can be used to compute cloud activation and the subgrid distribution of the number of cloud droplets explicitly. Autoconversion is also treated explicitly in the subcolumn space. This provides a consistent way of simulating the cloud radiative effects with two-moment cloud microphysical properties defined at subgrid scale. The primary impact of the new parameterizations is to decrease the CDNC over polluted continents, while over the oceans the impact is smaller. Moreover, the lower CDNC induces a stronger autoconversion of cloud water to rain. The strongest reduction in CDNC and cloud water content over the continental areas promotes weaker shortwave cloud radiative effects (SW CREs) even after retuning the model. However, compared to the reference simulation, a slightly stronger SW CRE is seen e.g. over mid-latitude oceans, where CDNC remains similar to the reference simulation, and the in-cloud liquid water content is slightly increased after retuning the model.

Interaction between coherent structures and surface temperature and its effect on ground heat flux in an unstably stratified boundary layer

Surface layer plumes, thermals, downdrafts and roll vortices are the most prominent coherent structures in an unstably stratified boundary layer. They contribute most of the temperature and vertical velocity variance, and their time scales increase with height. The effects of these multi-scale structures (surface layer plumes scale with surface layer depth, thermals scale with boundary layer height and the resulting roll vortices scale with convective time scale) on the surface temperature and ground heat flux were studied using turbulence measurements throughout the atmospheric boundary layer and the surface temperature measurements from an infrared camera. Plumes and thermals imprint on the surface temperature as warm structures and downdrafts imprint as cold structures. The air temperature trace shows a ramp-like pattern, with small ramps overlaid on a large ramp very close to the surface; on the other hand, surface temperature gradually increases and decreases. Turbulent heat flux and ground heat flux show similar patterns, with the former lagging the latter. The maximum values of turbulent heat flux and ground heat flux are 4 and 1.2 times the respective mean values during the ejection event. Surface temperature fluctuations follow a similar power-law exponent relationship with stability as suggested by surface layer similarity theory.

Numerical simulation of tropical cumulus congestus during TOGA COARE

Recent observational studies of tropical deep convection typically include some mention of cumulus congestus, a third mode of tropical convection, in addition to shallow trade cumulus and deep convection. This study analyzes congestus behavior in a multiday cloud‐resolving model simulation based on the Tropical Ocean‐Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE) field campaign. Simulation results exhibit a pronounced congestus cloud mode, present during both suppressed and active phases of the intraseasonal oscillation (ISO), with a unique signature consistent with cloudy‐air detrainment near the 0°C isotherm. Congestus clouds in the simulation contribute 34% of the total precipitation during a 10 day transition period from suppressed to active phases, a number which corresponds well with previous estimates of the congestus contribution to precipitation. Domain‐mean profiles and statistics from conditionally sampled buoyant cloud cores are compared with similar quantities from a recent model intercomparison of RICO trade cumulus. In many respects, cumulus congestus act like overgrown trade cumulus clouds. Both cloud types demonstrate multiple cloud fraction maxima associated with cloud base and detrainment layers. Profiles of buoyancy flux and vertical velocity variance suggest that the buoyancy production of turbulence behaves similarly in both cloud types. The greater precipitation production in the simulated congestus clouds nearly balances the surface latent heat flux, and thus the congestus contribution to moistening the atmosphere is limited. The computational configuration is a compromise between providing both sufficient resolution to represent shallow cumulus and sufficient domain size to handle broader, deep convective clouds.

The GASS/EUCLIPSE model intercomparison of the stratocumulus transition as observed during ASTEX: LES results

Large‐eddy simulations of a Lagrangian transition from a vertically well‐mixed stratocumulus‐topped boundary layer to a situation in which shallow cumuli penetrate an overlying layer of thin and broken stratocumulus are compared with aircraft observations collected during the Atlantic Stratocumulus Transition Experiment. Despite the complexity of the case and the long simulation period of 40 h, the six participating state‐of‐the‐art models skillfully and consistently represent the observed gradual deepening of the boundary layer, a negative buoyancy flux at the top of the subcloud layer and the development of a double‐peaked vertical velocity variance profile. The moisture flux from the subcloud to the stratocumulus cloud layer by cumulus convection exhibits a distinct diurnal cycle. During the night the moisture flux at the stratocumulus cloud base exceeds the surface evaporation flux, causing a net drying of the subcloud layer, and vice versa during daytime. The spread in the liquid water path (LWP) among the models is rather large during the first 12 h. From additional sensitivity experiments it is demonstrated that this spread is mainly attributable to differences in the parameterized precipitation rate. The LWP differences are limited through a feedback mechanism in which enhanced drizzle fluxes result in lower entrainment rates and subsequently a reduced drying at cloud top. The spread is furthermore reduced during the day as cloud layers with a greater LWP absorb more solar radiation and hence evaporate more.

Derivation of Turbulent Kinetic Energy from a First-Order Nonlocal Planetary Boundary Layer Parameterization

Abstract Turbulent kinetic energy (TKE) is derived from a first-order planetary boundary layer (PBL) parameterization for convective boundary layers: the nonlocal K-profile Yonsei University (YSU) PBL. A parameterization for the TKE equation is developed to calculate TKE based on meteorological profiles given by the YSU PBL model. For this purpose buoyancy- and shear-generation terms are formulated consistently with the YSU scheme—that is, the combination of local, nonlocal, and explicit entrainment fluxes. The vertical transport term is also formulated in a similar fashion. A length scale consistent with the K profile is suggested for parameterization of dissipation. Single-column model (SCM) simulations are conducted for a period in the second Global Energy and Water Cycle Experiment (GEWEX) Atmospheric Boundary Layer Study (GABLS2) intercomparison case. Results from the SCM simulations are compared with large-eddy simulation (LES) results. The daytime evolution of the vertical structure of TKE matches well with mixed-layer development. The TKE profile is shaped like a typical vertical velocity (w) variance, and its maximum is comparable to that from the LES. By varying the dissipation length from −23% to +13% the TKE maximum is changed from about −15% to +7%. After normalization, the change does not exceed the variability among previous studies. The location of TKE maximum is too low without the effects of the nonlocal TKE transport.

Similarity of the velocity profile in geometrically distorted flow model

A method is presented to measure the depth-averaged horizontal velocity in distorted physical models, i.e., the depth-averaged horizontal velocity can be equivalent to a point velocity measured at approximately 0.6h below the water surface (h is the water depth). Further a scale for the Chézy coefficient in distorted models is deduced, and the velocity distributions in the vertical in prototype and distorted models are analyzed. By comparing data of the vertical variation of the horizontal velocity measured in a physical distorted model (with distortion ratio of 4.0) with prototype data, inaccuracies via the one-point method, the three-point method and the five-point method are analyzed. It is concluded that it is more accurate to measure the depth-averaged velocity via the three-point method than via the one-point method in the distorted model. What's more, the point velocity at 0.6h below the water surface approximates the depth-averaged velocity in the distorted physical model and the one-point method is proposed to be used in practice.

Full tensor CSEM and MT for subsalt structural imaging in the Red Sea: Implications for seismic and electromagnetic integration

The mapping of complex salt, subsalt sediments, and basement structures in the Red Sea was addressed through the integration of coincident high-resolution wide-azimuth (WAZ) 3D seismic data, full tensor gravity gradiometry (FTG), and MT/CSEM techniques. The data acquisition was designed to enable advanced multi-geophysical integration schemes to image the subsalt geologic structures. Multi-offset, multicomponent electromagnetic measurements were recorded for both MT and CSEM with overlapping frequency bands. The analysis of the data and the preliminary 3D inversion results indicate that the two methods provide coherent information for the evaporitic overburden and the subsalt sediment sections. In addition, the MT data also show sensitivity to the deep basement structures. The obtained resistivity distributions strongly correlate with imaged seismic events from the 3D migrated seismic volumes and with the lateral- and vertical-velocity variations in the salt and presalt sections. These data suggest that the acquired electromagnetic data set can support the interval velocity-depth model building in areas of low seismic sensitivity such as the intrasalt and subsalt sections.

The Generation of Turbulence below Midlevel Cloud Bases: The Effect of Cooling due to Sublimation of Snow

AbstractIn the author’s experience as a forecaster, commercial aircraft sometimes report turbulence beneath midlevel clouds that extend above upper frontal zones. Turbulence caused by Kelvin–Helmholtz instability occurs in upper frontal zones with strong vertical shear of horizontal winds. However, the turbulence seems to occur not only in the cloud bases (where upper frontal zones are) but also below the cloud bases where the vertical shear is not strong. Because those clouds are usually accompanied by precipitation that does not reach the ground, cooling by evaporation or sublimation seems to contribute to the generation of turbulence. In this paper, the mechanisms generating turbulence below midlevel cloud bases are examined by using observations and high-resolution three-dimensional numerical simulations with idealized initial conditions. The numerical simulations showed that the following sequence of events led to turbulence. Falling snow sublimated below cloud bases and cooled the air, which created absolute instability. This generated Rayleigh–Bénard convection cells. The vertical motion caused turbulence. The horizontal scale of the convection was about 800–1000 m, and the variations of vertical wind velocity were up to about 7 m s−1. The cloud base was accompanied by a virga-like distribution of snow. Sensitivity experiments showed the appropriate conditions to cause the turbulence: 1) the cloud-base temperature was between about 0° and −15°C, 2) the relative humidity in subcloud layers was sufficiently low, and 3) the stability in subcloud layers was weak. The results of the numerical simulations agreed well with the observations.

Scaling Laws in Canopy Flows: A Wind-Tunnel Analysis

An analysis of velocity statistics and spectra measured above a wind-tunnel forest model is reported. Several measurement stations downstream of the forest edge have been investigated and it is observed that, while the mean velocity profile adjusts quickly to the new canopy boundary condition, the turbulence lags behind and shows a continuous penetration towards the free stream along the canopy model. The statistical profiles illustrate this growth and do not collapse when plotted as a function of the vertical coordinate. However, when the statistics are plotted as function of the local mean velocity (normalized with a characteristic velocity scale), they do collapse, independently of the streamwise position and freestream velocity. A new scaling for the spectra of all three velocity components is proposed based on the velocity variance and integral time scale. This normalization improves the collapse of the spectra compared to existing scalings adopted in atmospheric measurements, and allows the determination of a universal function that provides the velocity spectrum. Furthermore, a comparison of the proposed scaling laws for two different canopy densities is shown, demonstrating that the vertical velocity variance is the most sensible statistical quantity to the characteristics of the canopy roughness.

Statistics of Turbulence in the Stable Boundary Layer Affected by Gravity Waves

In order to investigate effects of interactions between turbulence and gravity waves in the stable boundary layer on similarity theory relationships, we re-examined a dataset, collected during three April nights in 1978 and in 1980 on the 300-m tower of the Boulder Atmospheric Observatory (BAO). The BAO site, located in Erie, Colorado, USA, 30 km east of the foothills of the Rocky Mountains, has been known for the frequent detection of wave activities. The considered profiles of turbulent fluxes and variances were normalized by two local, gradient-based scaling systems, and subsequently compared with similarity functions of the Richardson number, obtained based on data with no influence of gravity currents and topographical factors. The first scaling system was based on local values of the vertical velocity variance $$\sigma _\mathrm{w}$$ and the Brunt–Vaisala frequency $$ N,$$ while the second one was based on the temperature variance $$\sigma _{\theta }$$ and $$N.$$ Analysis showed some departures from the similarity functions (obtained for data with virtually no influence of mesoscale motions); nonetheless the overall dependency of dimensionless moments on the Richardson number was maintained.

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

Situated in South America midwest region, Pantanal is a unique biome, alternating dry and flooded periods. An important seasonal variability characteristic from Pantanal's energy balance is the occurrence of situations in which sensible heat flux 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 (<img src=image/10.5923.j.ajee.20130301.06_001.gif></img>) occurrence and associated turbulent structure. For this, scale action of different Low-Level Jets (<img src=image/10.5923.j.ajee.20130301.06_002.gif></img>) types on Pantanal's Nocturnal Boundary Layer was investigated through case studies. Six events, distributed the following way, were used: Two events without <img src=image/10.5923.j.ajee.20130301.06_003.gif></img>, two events <img src=image/10.5923.j.ajee.20130301.06_004.gif></img> weak shear and two events <img src=image/10.5923.j.ajee.20130301.06_005.gif></img> strong shear. From the two events without <img src=image/10.5923.j.ajee.20130301.06_006.gif></img>, one of them is observed during the dry season and the other, during the flooded one. The same procedure was applied to other events (<img src=image/10.5923.j.ajee.20130301.06_007.gif></img> weak shear and <img src=image/10.5923.j.ajee.20130301.06_008.gif></img> 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. Remarkable differences were observed among turbulence in NBL characteristics during the “dry” and “flooded” periods. It was observed that <img src=image/10.5923.j.ajee.20130301.06_009.gif></img> weak shear acts like a forcing that generates action of top-down mechanisms in the Pantanal's Nocturnal Boundary-layer. On the other hand, <img src=image/10.5923.j.ajee.20130301.06_010.gif></img>strong shear causes eddies blocking situations with length scales greater than the <img src=image/10.5923.j.ajee.20130301.06_011.gif></img> 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 numerical simulations of the flow structure above the region.

The role of hydrodynamics in structuring in situ ammonium uptake within a submerged macrophyte community

Lay AbstractAquatic macrophytes (such as macroalgae and sea grasses) are able to take up nutrients directly from the surrounding water in which they inhabit. How fast nutrients are transported into their structures (such as leaves or fronds) can be influenced by water flow. However, macrophytes tend to form canopies made up of many individual structures, and the presence of these canopies has large effects on local water flow. This implies that in shallow regions the architecture of different canopies may well be a key factor in setting rates of nutrient transport to the sea floor. Predicting these effects is complicated because in many natural systems macrophyte communities often form patchy distributions consisting of bare and vegetated areas, as well as patches of different species with different canopy types. Here we use a new application of stable isotope labeling to directly measure the in situ uptake of ammonium at the transition between short, dense Caulerpa prolifera (a macroalga) and tall, sparse Cymodocea nodosa (a sea grass) canopies. We found that as water flowed over the dense macroalgae into the sparse sea grass canopy, it accelerated, resulting in horizontal and vertical variations in water velocity and mixing that appeared to explain variations in the ammonium uptake of the different types of organisms living on and within the canopies. These results highlight how the interplay between the physical structure of organisms and flow may determine the nutrient transport of individual species within macrophyte communities, creating nutrient transport microhabitats that may influence biodiversity and ecosystem functioning.

A method to diagnose boundary‐layer type using Doppler lidar

AbstractA new technique for objective classification of boundary layers is applied to ground‐based vertically pointing Doppler lidar and sonic anemometer data. The observed boundary layer has been classified into nine different types based on those in the Met Office ‘Lock’ scheme, using vertical velocity variance and skewness, along with attenuated backscatter coefficient and surface sensible heat flux. This new probabilistic method has been applied to three years of data from Chilbolton Observatory in southern England and a climatology of boundary‐layer type has been created. A clear diurnal cycle is present in all seasons. The most common boundary‐layer type is stable with no cloud (30.0% of the dataset). The most common unstable type is well mixed with no cloud (15.4%). Decoupled stratocumulus is the third most common boundary‐layer type (10.3%) and cumulus under stratocumulus occurs 1.0% of the time. The occurrence of stable boundary‐layer types is much higher in the winter than the summer and boundary‐layer types capped with cumulus cloud are more prevalent in the warm seasons. The most common diurnal evolution of boundary‐layer types, occurring on 52 days of our three‐year dataset, is that of no cloud with the stability changing from stable to unstable during daylight hours. These results are based on 16393 hours, 62.4% of the three‐year dataset, of diagnosed boundary‐layer type. This new method is ideally suited to long‐term evaluation of boundary‐layer type parametrisations in weather forecast and climate models.