Mean Momentum Flux Research Articles

Stratospheric gravity wave momentum flux from radio occultations

AbstractTriples of GPS radio occultation (RO) temperature data are used to derive horizontal and vertical gravity wave (GW) parameters in the stratosphere between 20 km and 40 km from which the vertical flux of horizontal momentum is determined. Compared to previous studies using RO data, better limiting values for the sampling distance (Δd≤250 km) and the time interval (Δt≤15 min) are used. For several latitude bands the mean momentum fluxes (MFs) derived in this study are considerably larger than MF from other satellite missions based on horizontal wavelengths calculated between two adjacent temperature profiles along the satellite track. Error sources for the estimation of MF from RO data and the geometrical setup for the applied method are investigated. Another crucial issue discussed in this paper is the influence of different background separation methods to the final MF. For GW analysis a measured temperature profile is divided into a fluctuation and a background and it is assumed that the fluctuation is caused by GWs only. For the background separation, i.e., the detrending of large‐scale processes from the measured temperature profile, several methods exist. In this study we compare different detrending approaches and for the first time an attempt is made to detrend RO data with ERA‐Interim data from the European Centre for Medium‐Range Weather Forecasts. We demonstrate that the horizontal detrending based on RO data and ERA‐Interim gives more consistent results compared with a vertical detrending.

Inertia gravity wave in the stratosphere and mesosphere observed by Doppler wind and temperature lidar

AbstractWe report on the first observation of persistent inertia gravity wave signatures in the horizontal wind and temperature by Doppler Rayleigh lidar in the middle atmosphere. The observations were performed at the Arctic Lidar Observatory for Middle Atmosphere Research station in northern Norway (69°N,16°E) between 21 and 23 January 2012. The measurements cover the altitude range from 20 km to about 80 km during nighttime and to about 70 km during daytime. We observe amplitudes of 5 to 25 m/s and 1 to 8 K in wind and temperature, respectively. The measured kinetic to potential energy density ratio is about 10, indicating that the majority of variability is due to waves with intrinsic frequencies close to the inertial frequency. The entire wavefield is mainly characterized by the presence of multiple waves; however, quasi‐monochromatic waves could be identified at limited times around 60 km altitude with a mean momentum flux in direction of propagation of 3.8 m2/s2.

A wind-tunnel investigation of wind-turbine wakes in yawed conditions

Wind-tunnel experiments were performed to study the performance of a model wind turbine and its wake characteristics in a boundary layer under different operating conditions, including different yaw angles and tip speed ratios. High-resolution particle image- velocimetry (PIV) was used to measure the three velocity components in a horizontal plane at hub height covering a broad streamwise range from upstream of the turbine to the far- wake region. Additionally, thrust and power coefficients of the turbine were measured under different conditions. These power and thrust measurements, together with the highly-resolved flow measurements, enabled us to systematically study different wake properties.The near-wake region is found to have a highly complex structure influenced by different factors such as tip speed ratio and wake rotation. In particular, for higher tip speed ratios, a noticeable speed-up region is observed in the central part of near wake, which greatly affects the flow distribution in this region. In this regard, the behavior of the near wake for turbines with similar thrust coefficients but different tip speed ratios can vary widely. In contrast, it is shown that the mean streamwise velocity in the far wake of the turbine with zero yaw angle has a self-similar Gaussian distribution, and the strength of wake in this region is consistent with the magnitude of the thrust coefficient.With increasing yaw angle, as expected, the power and thrust coefficients decrease, and the wake deflection increases. The measurements also reveal that, in addition to turbulent momentum flux, lateral mean momentum flux boosts the flow entrainment in only one side of the wake, which results in a faster wake recovery in that side. It is also found that the induced velocity upstream of a yawed turbine has a non-symmetric distribution, and its distribution is in agreement with the available model in the literature. Moreover, the results suggest that in order to accurately predict the load distribution in yawed conditions, both normal and tangential (with respect to the rotor plane) components of the induced velocity upstream of the turbine should be taken into account.

Gravity wave momentum fluxes from MF and meteor radar measurements in the polar MLT region

AbstractAnnual cycles of horizontal winds and gravity wave (GW) momentum fluxes in the mesosphere/lower thermosphere (MLT) are presented for the medium frequency Doppler radar at Saura (SMF radar, located at 69°N, 16°E) for the first time. Four year mean wind and momentum flux fields for 2008 through 2011 clearly show the coupling and interactions between GWs and the mean flow especially in the summer months. GW breaking at mesopause heights results in momentum flux divergence and affects the wind field by forcing a reversal of the wind profile in summer. Height‐time cross sections for the individual years (2008 to 2011) illustrate the year‐to‐year variation of horizontal winds and the vertical fluxes of zonal and meridional momentum. They show similar annual patterns from year to year which are more consistent in the summer months than during winter and have maximum absolute values in 2009. Furthermore, the precise SMF radar measurements give an excellent possibility to evaluate momentum flux estimates from the colocated meteor radar at Andenes. Both radars have different capabilities, and different techniques are applied to derive momentum fluxes. They show comparable results for the 4 year mean annual cycles of horizontal winds and momentum fluxes especially in summer. This holds for both structure and magnitudes in the overlapping heights, where the SMF radar data provide a wider vertical coverage. The best agreement is found for the zonal components of both radars, whereas there are some larger discrepancies in the meridional components, especially in the vertical flux of meridional momentum.

On the Link between Arctic Sea Ice Decline and the Freshwater Content of the Beaufort Gyre: Insights from a Simple Process Model

Abstract Recent satellite and hydrographic observations have shown that the rate of freshwater accumulation in the Beaufort Gyre of the Arctic Ocean has accelerated over the past decade. This acceleration has coincided with the dramatic decline observed in Arctic sea ice cover, which is expected to modify the efficiency of momentum transfer into the upper ocean. Here, a simple process model is used to investigate the dynamical response of the Beaufort Gyre to the changing efficiency of momentum transfer, and its link with the enhanced accumulation of freshwater. A linear relationship is found between the annual mean momentum flux and the amount of freshwater accumulated in the Beaufort Gyre. In the model, both the response time scale and the total quantity of freshwater accumulated are determined by a balance between Ekman pumping and an eddy-induced volume flux toward the boundary, highlighting the importance of eddies in the adjustment of the Arctic Ocean to a change in forcing. When the seasonal cycle in the efficiency of momentum transfer is modified (but the annual mean momentum flux is held constant), it has no effect on the accumulation of freshwater, although it does impact the timing and amplitude of the annual cycle in Beaufort Gyre freshwater content. This suggests that the decline in Arctic sea ice cover may have an impact on the magnitude and seasonality of the freshwater export into the North Atlantic.

Vegetated mixing layer around a finite‐size patch of submerged plants: 1. Theory and field experiments

Dynamics of a vegetated mixing layer around a patch of submerged plants was examined theoretically and experimentally in a lowland river. Theoretical analysis explored analogy with hydrodynamic mixing layers and introduced equations that describe expansion of the layers and alterations in mean flow. Field experiments provided empirical data for validation of the theory and examination of the effects of population density on the mixing layer dynamics. Detailed measurements assessed the structure of turbulent flow, plant morphology, biomechanical properties, bending, and streamlining due to interaction with the flow. The theory was compared with the field experiments and an agreement was concluded. It was found that near the upstream edge of the patch, the vegetated mixing layer expands similar to the canonical mixing layers, although its statistics scale on the velocity differential characteristic for the stabilized part. Downstream mean momentum flux reduces in magnitude and reverses in direction, while turbulence became dominant. This was observed to stabilize the velocity differential and increase the mean velocity of the layer that result in its limited growth described by a logarithmic function. The population density of vegetation was found to control the flow. In dense vegetation, the analogy with mixing layers resembled best, while in sparse vegetation, the flow behaved similar to the boundary layers. This theory can be used for quantitative assessment of seasonal effects in natural vegetated rivers because the population density of vegetation varies during the vegetation season. The companion paper focuses on the structural properties of turbulence in the vegetated shear layers.

0137 後縁に三次元性をもつ平板翼を用いた壁面噴流の操縦(OS1 噴流,後流および剥離流れ現象の探求と技術革新,オーガナイズドセッション)

A flat plate wing was applied to flow management of stronger wall jet with constant velocity ratio of U_m/U_e=4. The normal straight flat plate and three-dimensional wing with notched trailing edge were employed and located at center of the outer layer in the jet with constant angle of attach of -4 degree. The two wings increase the maximum velocity and velocity in the outer layer. Turbulent intensity profile and Reynolds shear stress profile was decreased by flat plate wings. Streamwise mean momentum flux integrated in cross streamwise plane is increased by flat plate wings. The increasing rate of momentum flux is enhanced by the three-dimensional wing.

Three-dimensional properties of Andes mountain waves observed by satellite: A case study

The southern Andes region has been clearly identified in previous satellite and balloon observations and in global models as a hot spot of small-scale gravity wave activity, with monthly mean momentum fluxes exceeding 10 times background values in fall, winter, and spring seasons. This makes this region a focus of interest for global circulation and climate studies. We analyze a case study on 8 May 2006, combining observations from the Atmospheric Infrared Sounder instrument on the Aqua satellite and the High Resolution Dynamics Limb Sounder instrument of the Aura satellite to form a three-dimensional picture of the wave field. The observations show a widespread wave pattern over the southern Andes extending eastward over the south Atlantic. Simulations with the Weather Research Forecasting model clearly identify the waves as orographic in origin, but the observed wave pattern is far from the simple two-dimensional wave field forced by steady flow over a mountain ridge. The morphology of the pattern is consistent with three-dimensional linear theoretical calculations of downstream propagation and latitudinal focusing of mountain waves into the stratospheric jet. The observations confirm the importance of this process in the stratosphere, and we find the process also occurring in the global analysis and forecasts from the European Centre for Medium-Range Weather Forecasting. Our analysis evaluates some strengths and weaknesses of current orographic wave drag parameterizations in global models and the relevance of parameterization assumptions in global models with high resolution. Copyright 2011 by the American Geophysical Union.

Interaction of waves with non-colinear currents

The present paper reports on a study of the interaction of a current-free monochromatic surface wave field with a wave-free uniform current field in a three-dimensional flow frame. The wave and the current fields are not necessarily collinear with each other. The formulation of the wave–current field is done under the assumption of irrotational and inviscid flow. We have developed the three dimensional expressions describing the characteristics of the combined flow in terms of mass, momentum and energy transport conservation. These equations are found efficient to describe the sought-for combined wave–current field. The parameters describing the wave–current field after the interaction are the surface disturbance amplitude and length, mean water depth, mean current-like parameter and direction of the combined flow, which would be calculated from a set of equations that satisfy conservation of mean mass, momentum and energy flux and a dispersion relation on the free surface before and after the interaction. The results are shown in terms of relative changes in wave heights and lengths, current-like parameters and final directions obtained for the combined wave–current field with respect to current-free wave and wave-free current pre-interaction parameters.

Gravity wave momentum fluxes in the MLT—Part I: Seasonal variation at Collm (51.3°N, 13.0°E)

By using a method presented by Hocking (2005), wind variances and gravity wave momentum fluxes in the mesosphere and lower thermosphere have been derived from all-sky interferometric meteor radar wind measurements at Collm, Germany (51.3°N, 13.0°E), during 2004 through 2009. We present vertical profiles of long-term mean monthly mean variances and momentum fluxes. Gravity wave variances show a semi-annual oscillation around 90 km altitude, and generally increase with height. An exception are the summer months where the maximum is measured around 88 km and amplitudes are decreasing above that height. A comparison of momentum fluxes and mean background winds emphasizes the coupling between gravity waves and the background circulation. 3-monthly means of wind and wind variance for individual seasons show stronger interannual variability in winter than in summer, and an indication for mesosphere wind filtering of gravity waves being responsible for this variability.

Regime Transitions of Steady and Time-Dependent Hadley Circulations: Comparison of Axisymmetric and Eddy-Permitting Simulations

Abstract Steady-state and time-dependent Hadley circulations are investigated with an idealized dry GCM, in which thermal forcing is represented as relaxation of temperatures toward a radiative-equilibrium state. The latitude ϕ0 of maximum radiative-equilibrium temperature is progressively displaced off the equator or varied in time to study how the Hadley circulation responds to seasonally varying forcing; axisymmetric simulations are compared with eddy-permitting simulations. In axisymmetric steady-state simulations, the Hadley circulations for all ϕ0 approach the nearly inviscid, angular-momentum-conserving limit, despite the presence of finite vertical diffusion of momentum and dry static energy. In contrast, in corresponding eddy-permitting simulations, the Hadley circulations undergo a regime transition as ϕ0 is increased, from an equinox regime (small ϕ0) in which eddy momentum fluxes strongly influence both Hadley cells to a solstice regime (large ϕ0) in which the cross-equatorial winter Hadley cell more closely approaches the angular-momentum-conserving limit. In axisymmetric time-dependent simulations, the Hadley cells undergo transitions between a linear equinox regime and a nonlinear, nearly angular-momentum-conserving solstice regime. Unlike in the eddy-permitting simulations, time tendencies of the zonal wind play a role in the dynamics of the transitions in the axisymmetric simulation. Nonetheless, the axisymmetric transitions are similar to those in the eddy-permitting simulations in that the role of the nonlinear mean momentum flux divergence in the zonal momentum budget shifts from marginal in the equinox regime to dominant in the solstice regime. As in the eddy-permitting simulations, a mean-flow feedback—involving the upper-level zonal winds, the lower-level temperature gradient, and the poleward boundary of the cross-equatorial Hadley cell—makes it possible for the circulation fields to change at the transition more rapidly than can be explained by the steady-state response to the thermal forcing. However, the regime transitions in the axisymmetric simulations are less sharp than those in the eddy-permitting simulations because eddy–mean flow feedbacks in the eddy-permitting simulations additionally sharpen the transitions.

Estimation of Gravity Wave Momentum Flux and Phase Speeds from Quasi-Lagrangian Stratospheric Balloon Flights. Part II: Results from the Vorcore Campaign in Antarctica

The stratospheric gravity wave field in the Southern Hemisphere is investigated by analyzing observations collected by 27 long-duration balloons that flew between September 2005 and February 2006 over Antarctica and the Southern Ocean. The analysis is based on the methods introduced by Boccara et al. in a companion paper. Special attention is given to deriving information useful to gravity wave drag parameterizations employed in atmospheric general circulation models. The balloon dataset is used to map the geographic variability of gravity wave momentum fluxes in the lower stratosphere. This flux distribution is found to be very heterogeneous with the largest time-averaged value (28 mPa) observed above the Antarctic Peninsula. This value exceeds by a factor of ∼10 the overall mean momentum flux measured during the balloon campaign. Zonal momentum fluxes were predominantly westward, whereas meridional momentum fluxes were equally northward and southward. A local enhancement of southward flux is nevertheless observed above Adélie Land and is attributed to waves generated by katabatic winds, for which the signature is otherwise rather small in the balloon observations. When zonal averages are performed, oceanic momentum fluxes are found to be of similar magnitude to continental values (2.5–3 mPa), stressing the importance of nonorographic gravity waves over oceans. Last, gravity wave intermittency is investigated. Mountain waves appear to be significantly more sporadic than waves observed above the ocean.

AN IMPROVED INTEGRAL MODEL FOR A ROUND TURBULENT BUOYANT JET BASED ON THE TURBULENT MEASUREMENTS

The development of an improved integral model for a round turbulent buoyant jet discharged into a stagnant fluid is reported here, based on the published laboratory measurements of the turbulent quantities in recent years. In order to involve turbulent terms into the integral model, instead of using the control volume method, the differential method that necessitates tedious mathematical procedures is used to derive the equations. The approach is: The threedimensional governing equations of fluid mechanics are firstly transformed to the partial differential forms in a natural coordinate system. Secondly, the solution variables in the instantaneous equations are decomposed into the mean and fluctuating components, and the Reynolds averaging operations are then performed; Finally, the Reynoldsaveraged governing equations are integrated over the jet cross-section, based on the axisymmetrical assumption and the best-fit curves obtained from the experimental data. In the jet axis direction, the total amount of the turbulent momentum flux and the contribution of streamwise pressure gradient is about 10% of the local mean momentum flux. And, the turbulent mass flux of a passive scalar (salinity, temperature, etc.) may be about 15% of the mean flux. Therefore, incorporating the turbulent quantities into the integral model may improve the accuracy of the prediction.

Turbulent flow over transverse ribs in open channel with converging side walls

This paper reports an experimental study of favorable pressure gradient turbulent flow over transverse square ribs in an open channel. The first half of the channel test section has straight parallel side walls, and each of the side walls of the second half of the test section converges linearly to produce the favorable pressure gradient. Two different values of half-angle ( α = 1° and 2°) of the converging side walls were used to vary the pressure gradient. For each value of α, transverse square ribs were glued onto the bottom wall both upstream of and within the converging sections. The pitch-to-height ratio of the ribs was varied to produce d-type, intermediate and k-type rib roughness. A particle image velocimetry was used to conduct detailed measurements over the ribs at several planes upstream of and within the converging sections. From these measurements, mean velocities and turbulent quantities were extracted at selected streamwise locations. The distributions of the mean velocities, turbulent intensities, and the mean and turbulent momentum fluxes at the interface between the rib cavities and the overlying boundary layer were used to provide insight into the interaction between the shear layers. The profiles of mean velocity, turbulent intensities, Reynolds shear stress and triple products were obtained to document the characteristics of favorable pressure gradient turbulent flow over square ribs in an open channel.

Analysis of mean momentum flux in subgrid models of turbulence

Using a recently proposed model spectrum for weakly sheared turbulence [M. Ishihara, K. Yoshida, and Y. Kaneda, Phys. Rev. Lett. 88, 154501 (2002)], the mean momentum fluxes caused by subgrid scales are evaluated analytically. Using the result to calibrate the Smagorinsky coefficient, the value obtained is remarkably close to the traditional result obtained from balancing kinetic energy dissipation. For the Gaussian filter, the same conclusion is obtained when the Smagorinsky model is supplemented with the nonlinear model. It is concluded that when in practice the Smagorinsky model does not correctly predict the mean subgrid scale (SGS) stresses in large eddy simulation (LES), it must be due to either nonuniversal effects at large scales, or due to strong shear that violates the model spectrum assumed in the present analysis. Analysis of a two-parameter mixed model shows that the nonlinear term must be weighted more strongly in high shear conditions than in weak shear.

Cloud‐tracked winds for the first Mars Global Surveyor mapping year

We have measured winds using cloud motion in consecutive Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) wide‐angle global map swaths taken during the first mapping year (Ls 135°–360°–111°). We present a total of ∼11,200 wind vectors collected in the north polar region during Ls 135°–195° (late summer/early fall) and Ls 20°–55° (mid spring) and in the south polar region during Ls 337°–10° (late summer/early fall). For cases with good coverage, we also present the derived mean zonal and meridional winds and the associated eddies. The speed of the zonal winds in 60°N–70°N increases at ∼0.6 m/s/°Ls in late northern summer, and that in 60°S–70°S increases at a rate of ∼0.7 m/s/°Ls in late southern summer. The latitudinal distribution of zonal wind within 50°N–75°N from mid northern summer to early northern fall indicates that winds at higher latitudes are generally weaker than those at lower latitudes, but the rate of increase with time is faster at higher latitudes. There is a cyclonic gyre in the 90°W–0°–30°E sector in the north polar region. There are large‐scale waves in the weekly mean meridional wind and in the biweekly mean eddy momentum flux and eddy kinetic energy fields in the north polar region from mid to late summer. The cloud‐tracked winds in the north are generally consistent with winds calculated by general circulation model at the water condensation level derived from MGS Thermal Emission Spectrometer (TES) observations but appear stronger than the gradient winds derived from TES assuming no flow at the surface.

Numerical simulation of micro-fabricated zero mass-flux jet actuators

Abstract Computational fluid dynamics can play an important role in the design of microelectromechanical systems (MEMS)-based actuators, allowing an investigation of the underlying physical behavior of devices before proceeding to expensive manufacturing processes. Here, we present the results of a series of numerical simulations of synthetic jet actuators, so-called because they synthesize jets from the working fluid. The primary characteristic of these actuators is that over a cycle, the net mass flux is zero, yet the mean momentum flux is non-zero. The simulations consider both the internal and external flow-fields of a micro-scaled device, with the membrane motion specified using a simplified structural analysis. Key features of the flow in the external, orifice and cavity regions are presented and discussed. The actuator output is observed to vary linearly with a non-dimensional input parameter, and this may guide the design of flow control systems that use synthetic jet actuators. Finally, our fabrication strategy for a MEMS-based synthetic jet actuator is presented.

Ocean-atmosphere interaction and synoptic weather conditions in association with the two contrasting phases of monsoon during BOBMEX-1999

Surface meteorological parameters acquired during the field phase experiment, BOBMEX-99, for the stationary periods (SP I and II) of the ship ORV Sagar Kanya over Bay of Bengal have been analysed. Active and weak monsoon conditions were observed during the first and the second phase of the experiment respectively over India as well as over the stationary ship location. The phase mean sea surface temperature (SST) is found to be the same in both the phases, however large differences have been observed in the phase mean values of wind speed, mean sea level pressure, latent heat and momentum fluxes at air-sea interface. Synoptic scale monsoon disturbances formed only during the period of strong north-south pressure gradient over the Bay region. Events of prominent fall in SST and in the upper 15 m ocean layer mean temperature and salinity values during typical rainfall events are cited. The impact of monsoon disturbances on ocean-atmosphere interface transfer processes has been investigated.

Second-order integral model for a round turbulent buoyant jet

The development of a second-order integral model for a round turbulent buoyant jet is reported based on new experimental data on turbulent mass and momentum transport. The mean and turbulent characteristics of a round vertical buoyant jet covering the full range from jets to plumes were investigated using a recently developed combined digital particle image velocimetry (DPIV) and planar laser-induced fluorescence (PLIF) system. The system couples the two well-known techniques to enable synchronized planar measurements of flow velocities and concentrations in a study area. The experimental results conserved the mass and momentum fluxes introduced at the source accurately with closure errors of less than 5%. The momentum flux contributed by turbulence and streamwise pressure gradient was determined to be about 10% of the local mean momentum flux in both jets and plumes. The turbulent mass flux, on the other hand, was measured to be about 7.6% and 15% of the mean mass flux for jets and plumes respectively. While the velocity spread rate was shown to be independent of the flow regime, the concentration-to-velocity width ratio λ varied from 1.23 to 1.04 during the transition from jet to plume. Based on the experimental results, a refined second-order integral model for buoyant jets that achieves the conservation of total mass and momentum fluxes is proposed. The model employs the widely used entrainment assumption with the entrainment coefficient taken to be a function of the local Richardson number. Improved prediction is achieved by taking into account the variation of turbulent mass and momentum fluxes. The variation of turbulent mass flux is modelled as a function of the local Richardson number. The turbulent momentum flux, on the other hand, is treated as a fixed percentage of the local mean momentum flux. In addition, unlike most existing integral models that assume a constant concentration-to-velocity width ratio, the present model adopts a more accurate approach with the ratio expressed as a function of the local Richardson number. As a result, smooth transition of all relevant mean and turbulent characteristics from jet to plume is predicted, which is in line with the underlying physical processes.

Upward Momentum Transfer in the Marine Boundary Layer

Abstract This paper focuses on the study of momentum flux between ocean and atmosphere in light winds and is based on the data collected during several field campaigns, the Atlantic Stratocumulus Transition Experiment, the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment, and the San Clemente Ocean Probing Experiment. Weak wind at sea is frequently accompanied by the presence of fast-traveling ocean swell, which dramatically affects momentum transfer. It is found that the mean momentum flux (uw covariance) decreases monotonically with decreasing wind speed, and reaches zero around a wind speed U ≈ 1.5–2 m s−1, which corresponds to wave age cp/U ≈ 10 for wave/swell conditions of the experiments in this study. Further decrease of the wind speed (i.e., increase of the wave age) leads to a sign reversal of the momentum flux, implying negative drag coefficient. Upward momentum transfer is associated with fast-traveling swell running in the same direction as the wind, and this regim...