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Turbulent Fluxes (turbulent + flux)

Distribution by Scientific Domains

Earth and Environmental Science	73%

Selected Abstracts

Air,sea exchanges in the equatorial area from the EQUALANT99 dataset: Bulk parametrizations of turbulent fluxes corrected for airflow distortion

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 610 2005
A. Brut
Abstract Turbulent fluxes of momentum, sensible heat and water vapour were calculated using both the eddy covariance (EC) and the inertial dissipation (ID) methods applied to data collected on board the research vessel La Thalassa during 40 days of the EQUALANT99 oceanographic campaign. The aim of this experiment was to establish accurate parametrizations of air,sea fluxes for the equatorial Atlantic area from a large dataset. However, the accuracy of turbulent fluxes measured aboard ships is strongly affected by the distortion of airflow patterns generated by obstacles such as the ship and mast. For the EQUALANT99 experiment, the effects of airflow distortion were estimated using physical simulations in a water channel. To reproduce the conditions of the campaign, a neutral boundary layer was simulated in the water channel and a detailed model of the ship La Thalassa was built. Correction coefficients for the mean wind speed were evaluated from these physical simulations. They show a dependence on both the azimuth angle of the flow (i.e. the horizontal direction of the flow with respect to the ship's longitudinal axis) and the angle of incidence of the wind. The correction for airflow distortion was applied to the measured wind speed and also included in the flux computation using the ID method. Compared with earlier studies which applied a single correction per flux sample, it appears that our results for the corrected transfer coefficients present greater dependence on neutral wind speed than the previous parametrizations; the method also shows encouraging results, with a decrease in the scatter of the transfer coefficients parametrization. However, the distortion could not be corrected for in the fluxes calculated using the EC method, because this technique integrates a wide range of turbulence scales for which the airflow distortion cannot be simulated in a water channel. Fluxes computed using the ID and EC methods are presented and compared in order to determine which method, in the configuration of the EQUALANT99 experiment, provides the best resulting transfer coefficients. According to the results, fluxes of momentum and latent heat computed by ID were better for deriving the drag and humidity coefficients. The EC method seemed better adapted to calculate sensible-heat fluxes than the ID method, although a high scatter remained on the Stanton neutral number. Copyright © 2005 Royal Meteorological Society [source]

Variance scaling in shallow-cumulus-topped mixed layers

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 628 2007
R. A. J. Neggers
Abstract Scaling of thermodynamic variance in shallow-cumulus-topped mixed layers is studied using large-eddy simulation (LES). First, the performance of the top-down scaling (the turbulent flux at mixed-layer top divided by w*) is evaluated for transient shallow-cumulus convection over land. The results indicate that this scaling fails to capture all the variance in the top half of the mixed layer when shallow cumulus clouds are present. A variance-budget analysis is then performed, to derive a new scaling for the variance at mixed-layer top, which differs from the standard top-down scaling by a factor of one Richardson number. The essential new features of the proposed scaling are that the local vertical gradient is retained and that a balance is assumed between gradient production of variance and removal by transport and dissipation, using an adjustment time-scale given by w*/h. Evaluation against LES for a range of different cases, including a dry convective boundary layer as well as steady-state marine and transient continental shallow cumulus, reveals a data-collapse of the newly-scaled variance, for all hours and all cases in the top half of the mixed layer. The corresponding vertical structure is shown to resemble a power-law function. The results suggest that the structure of variance in the dry convective boundary layer is similar to that in the sub-cloud mixed layer. In transient situations, the scaling reproduces the time-development of variance at sub-cloud mixed-layer top. The new cloud-base variance scale is then further interpreted in the context of statistical cloud schemes, which depend on the variance as the second moment of the associated probability density function. The results suggest that the area fraction of the moist convective thermals uniquely depends on the ratio of cloud-base transition-layer depth to sub-cloud mixed-layer depth. This puts ,valve'- or ventilation-type closures for the cloud-base mass flux in the context of the variance budget for the sub-cloud layer. Copyright © 2007 Royal Meteorological Society [source]

Effects of shear sheltering in a stable atmospheric boundary layer with strong shear

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 596 2004
Ann-Sofi Smedman
Abstract Data from two marine field experiments in the Baltic Sea with stable stratification have been analysed. The purpose was to test the concept of the ,detached' or ,top-down' eddies and the ,shear-sheltering' mechanism in the presence of a low-level wind speed maximum in the atmosphere. Data used include turbulence and profile measurements on two 30 m towers and concurrent wind profiles throughout the boundary layer obtained from pilot-balloon soundings. Measurements show that large eddies are being suppressed when there is a low-level wind speed maximum present somewhere in the layer 40,300 m above the water surface and when the stratification is slightly stable. The effect is seen both in normalized standard deviations of the velocity components and in corresponding component spectra. In previous work it was argued that the relatively large eddies, which dominate the low wave number spectra in measurements in the surface layer, are detached or top-down eddies generated higher up in the boundary layer, that interact with the surface layer. The low-level wind maximum introduces a distinct layer with strong vorticity which, according to the shear-sheltering hypothesis, prevents these eddies from penetrating downwards. In the limit of the wind maximum occurring at a very low height (less than about 100 m), usual turbulence statistics characteristic of the ,canonical' boundary layer are found. Combining all the statistics, it is demonstrated that the wavelength of maximum spectral energy is locally related to a turbulence length-scale, which shows that for values of the Richardson number of unity or less the effect of the local wind gradient is greater than that of static stability. The reduction of length-scale with the strength of a low-level wind maximum, explains the observed reduction (by a factor of two) of the turbulent flux of sensible heat at the surface. This result indicates that the shear-sheltering mechanism is likely to play an important role in the turbulent exchange process at the surface in sea areas where low-level wind maxima are a frequently occurring phenomenon, such as the Baltic and other large water bodies surrounded by landmasses. Copyright © 2004 Royal Meteorological Society [source]

Carbon dioxide exchange of a Russian boreal forest after disturbance by wind throw

GLOBAL CHANGE BIOLOGY, Issue 3 2002
Alexander Knohl
Abstract The exchange of carbon dioxide (CO2) between the atmosphere and a forest after disturbance by wind throw in the western Russian taiga was investigated between July and October 1998 using the eddy covariance technique. The research area was a regenerating forest (400 m × 1000 m), in which all trees of the preceding generation were uplifted during a storm in 1996. All deadwood had remained on site after the storm and had not been extracted for commercial purposes. Because of the heterogeneity of the terrain, several micrometeorological quality tests were applied. In addition to the eddy covariance measurements, carbon pools of decaying wood in a chronosequence of three different wind throw areas were analysed and the decay rate of coarse woody debris was derived. During daytime, the average CO2 uptake flux was ,3 µmol m,2s,1, whereas during night-time characterised by a well-mixed atmosphere the rates of release were typically about 6 µmol m,2s,1. Suppression of turbulent fluxes was only observed under conditions with very low friction velocity (u* , 0.08 ms,1). On average, 164 mmol CO2 m,2d,1 was released from the wind throw to the atmosphere, giving a total of 14.9 mol CO2 m,2 (180 g CO2 m,2) released during the 3-month study period. The chronosequence of dead woody debris on three different wind throw areas suggested exponential decay with a decay coefficient of ,0.04 yr,1. From the magnitude of the carbon pools and the decay rate, it is estimated that the decomposition of coarse woody debris accounted for about a third of the total ecosystem respiration at the measurement site. Hence, coarse woody debris had a long-term influence on the net ecosystem exchange of this wind throw area. From the analysis performed in this work, a conclusion is drawn that it is necessary to include into flux networks the ecosystems that are subject to natural disturbances and that have been widely omitted into considerations of the global carbon budget. The half-life time of about 17 years for deadwood in the wind throw suggests a fairly long storage of carbon in the ecosystem, and indicates a very different long-term carbon budget for naturally disturbed vs. commercially managed forests. [source]

Hydrometeorological behaviour of pine and larch forests in eastern Siberia

HYDROLOGICAL PROCESSES, Issue 1 2004
Shuko Hamada
Abstract Seasonal changes in the water and energy exchanges over a pine forest in eastern Siberia were investigated and compared with published data from a nearby larch forest. Continuous observations (April to August 2000) were made of the eddy-correlation sensible heat flux and latent heat flux above the canopy. The energy balance was almost closed, although the sum of the turbulent fluxes sometimes exceeded the available energy flux (Rn , G) when the latent heat flux was large; this was related to the wind direction. We examined the seasonal variation in energy balance components at this site. The seasonal variation and magnitude of the sensible heat flux (H) was similar to that of the latent heat flux (,E), with maximum values occurring in mid-June. Consequently, the Bowen ratio was around 1·0 on many days during the study period. On some clear days just after rainfall, ,E was very large and the sum of H and ,E exceeded Rn , G. The evapotranspiration rate above the dry canopy from May to August was 2·2 mm day,1. The contributions of understory evapotranspiration (Eu) and overstory transpiration (Eo) to the evapotranspiration of the entire ecosystem (Et) were both from 25 to 50% throughout the period analysed. These results suggest that Eu plays a very important role in the water cycle at this site. From snowmelt through the tree growth season (23 April to 19 August 2000), the total incoming water, comprised of the sum of precipitation and the water equivalent of the snow at the beginning of the melt season, was 228 mm. Total evapotranspiration from the forest, including interception loss and evaporation from the soil when the canopy was wet, was 208,254 mm. The difference between the incoming and outgoing amounts in the water balance was from +20 to ,26 mm. The water and energy exchanges of the pine and larch forest differed in that ,E and H increased slowly in the pine forest, whereas ,E increased rapidly in the larch forest and H decreased sharply after the melting season. Consequently, the shape of the Bowen ratio curves at the two sites differed over the period analysed, as a result of the differences in the species in each forest and in soil thawing. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Impact of an extreme melt event on the runoff and hydrology of a high Arctic glacier

HYDROLOGICAL PROCESSES, Issue 6 2003
Sarah Boon
Abstract On 28,30 July 2000, an extreme melt event was observed at John Evans Glacier (JEG), Ellesmere Island (79° 40,N, 74° 00,W). Hourly melt rates during this event fell in the upper 4% of the distribution of melt rates observed at the site during the period 1996,2000. Synoptic conditions during the event resulted in strong east-to-west flow over the northern sector of the Greenland Ice Sheet, with descending flow on the northwest side reaching Ellesmere Island. On JEG, wind speeds during the event averaged 8·1 m s,1 at 1183 m a.s.l., with hourly mean wind speeds peaking at 11·6 m s,1. Air temperatures reached 8°C, and rates of surface lowering measured by an ultrasonic depth gauge averaged 56 mm day,1. Calculations with an energy balance model suggest that increased turbulent fluxes contributed to melt enhancement at all elevations on the glacier, while snow albedo feedback resulted in increased melting due to net radiation at higher elevations. The event was responsible for 30% of total summer melt at 1183 m a.s.l. and 15% at 850 m a.s.l. Conditions similar to those during the event occurred on only 0·1% of days in the period 1948,2000, but 61% of events occurred in the summer months and there was an apparent clustering of events in the 1950s and 1980s. Such events have the potential to impact significantly on runoff, mass balance and drainage system development at high Arctic glaciers, and changes in their incidence could play a role in determining how high Arctic glaciers respond to climate change and variability. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Effect of snowpack removal on energy balance, melt and runoff in a small supraglacial catchment

HYDROLOGICAL PROCESSES, Issue 14 2002
Ian C. Willis
Abstract Modelling melt and runoff from snow- and ice-covered catchments is important for water resource and hazard management and for the scientific study of glacier hydrology, dynamics and hydrochemistry. In this paper, a distributed, physically based model is used to determine the effects of the up-glacier retreat of the snowline on spatial and temporal patterns of melt and water routing across a small (0·11 km2) supraglacial catchment on Haut Glacier d'Arolla, Switzerland. The melt model uses energy-balance theory and accounts for the effects of slope angle, slope aspect and shading on the net radiation fluxes, and the effects of atmospheric stability on the turbulent fluxes. The water routing model uses simplified snow and open-channel hydrology theory and accounts for the delaying effects of vertical and horizontal water flow through snow and across ice. The performance of the melt model is tested against hourly measurements of ablation in the catchment. Calculated and measured ablation rates show a high correlation (r2 = 0·74) but some minor systematic discrepancies in the short term (hours). These probably result from the freezing of surface water at night, the melting of the frozen layer in the morning, and subsurface melting during the afternoon. The performance of the coupled melt/routing model is tested against hourly discharge variations measured in the supraglacial stream at the catchment outlet. Calculated and measured runoff variations show a high correlation (r2 = 0·62). Five periods of anomalously high measured discharge that were not predicted by the model were associated with moulin overflow events. The radiation and turbulent fluxes contribute c. 86% and c. 14% of the total melt energy respectively. These proportions do not change significantly as the surface turns from snow to ice, because increases in the outgoing shortwave radiation flux (owing to lower albedo) happen to be accompanied by decreases in the incoming shortwave radiation flux (owing to lower solar incidence angles) and increases in the turbulent fluxes (owing to higher air temperatures and vapour pressures). Model sensitivity experiments reveal that the net effect of snow pack removal is to increase daily mean discharges by c. 50%, increase daily maximum discharges by >300%, decrease daily minimum discharges by c. 100%, increase daily discharge amplitudes by >1000%, and decrease the lag between peak melt rates and peak discharges from c. 3 h to c. 50 min. These changes have important implications for the development of subglacial drainage systems. Copyright © 2002 John Wiley & Sons, Ltd. [source]

Two decades of urban climate research: a review of turbulence, exchanges of energy and water, and the urban heat island

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 1 2003
A. John Arnfield
Abstract Progress in urban climatology over the two decades since the first publication of the International Journal of Climatology is reviewed. It is emphasized that urban climatology during this period has benefited from conceptual advances made in microclimatology and boundary-layer climatology in general. The role of scale, heterogeneity, dynamic source areas for turbulent fluxes and the complexity introduced by the roughness sublayer over the tall, rigid roughness elements of cities is described. The diversity of urban heat islands, depending on the medium sensed and the sensing technique, is explained. The review focuses on two areas within urban climatology. First, it assesses advances in the study of selected urban climatic processes relating to urban atmospheric turbulence (including surface roughness) and exchange processes for energy and water, at scales of consideration ranging from individual facets of the urban environment, through streets and city blocks to neighbourhoods. Second, it explores the literature on the urban temperature field. The state of knowledge about urban heat islands around 1980 is described and work since then is assessed in terms of similarities to and contrasts with that situation. Finally, the main advances are summarized and recommendations for urban climate work in the future are made. Copyright © 2003 Royal Meteorological Society. [source]

Advanced tilt correction from flow distortion effects on turbulent CO2 fluxes in complex environments using large eddy simulation

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 643 2009
F. Griessbaum
Abstract Measurement of the turbulent fluxes of gases, momentum and heat can be biased by obstacles such as buildings or instrument platforms distorting the flow of air to the flux instruments. Standard methods have long been used to account for non-horizontal mean flows. Here we demonstrate a novel approach to correct for the effects of flow distortion which combines numerical flow modelling with eddy covariance measurements of the fluxes. This approach applies a flow distortion correction to the data prior to the application of the standard planar-fit and double-rotation methods. This new direction-dependent flow correction allows the determination of the correct orthogonal wind vector components and hence the vertical turbulent fluxes. We applied the method to a 10 Hz dataset of 3D wind components, temperature, and the concentrations of carbon dioxide and water vapour, as measured on top of a military tower above the city of Münster in northwest Germany during spring and summer 2007. Significant differences appeared between the fluxes that were calculated with the standard rotation methods alone and those that underwent flow distortion correction prior to the application of the rotation methods. The highest deviations of 27% were obtained for the momentum flux. Pronounced differences of 15% and 8% were found for the diurnal net fluxes of carbon dioxide and water vapour, respectively. The flow distortion correction for the carbon dioxide fluxes yielded the same magnitude as the WPL (Webb,Pearman,Leuning) correction for density fluctuations. Copyright © 2009 Royal Meteorological Society [source]

Scaling turbulent atmospheric stratification.

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 631 2008
I: Turbulence, waves
Abstract In this first of a three-part series, we argue that the dynamics of turbulence in a stratified atmosphere should depend on the buoyancy over a wide range of vertical scales and on energy flux over a wide range of horizontal scales; it should be scaling, but anisotropic, not isotropic. We compare the leading statistical theories of atmospheric stratification which are conveniently distinguished by the elliptical dimension Ds which quantifies their degree of spatial stratification. This includes the mainstream isotropic 2-D (large scales), isotropic 3-D (small scales) theory but also the more recent linear gravity wave theories (Ds = 7/3) and the classical fractionally integrated flux (FIF) 23/9-D unified scaling model. In the latter, the horizontal wind has a k,5/3 spectrum as a function of horizontal wavenumber determined by the energy flux and a k,11/5 energy spectrum as a function of vertical wavenumber determined by the buoyancy force variance flux. In this model, the physically important notion of scale is determined by the turbulent dynamics, it is not given a priori (i.e. the by usual Euclidean distance). The 23/9-D FIF model is the most physically and empirically satisfying, being based on turbulent (spectral) fluxes. The FIF model as originally proposed by Schertzer and Lovejoy is actually a vast family of scaling models broadly compatible with turbulent phenomenology and with the classical turbulent laws of Kolmogorov, Corrsin and Obukov. However, until now it has mostly been developed on the basis of structures localized in space,time. In this paper, we show how to construct extreme FIF models with wave-like structures which are localized in space but unlocalized in space,time, as well as a continuous family of intermediate models which are akin to Lumley,Shur models in which some part of the localized turbulent energy ,leaks' into unlocalized waves. The key point is that the FIF requires two propagators (space,time Green's functions) which can be somewhat different. The first determines the space,time structure of the cascade of fluxes; this must be localized in space,time in order to satisfy the usual turbulence phenomenology. In contrast, the second propagator relates the turbulent fluxes to the observables; although the spatial part of the propagator is localized as before, in space,time it can be unlocalized. (It is still localized in space, now in wave packets.) We display numerical simulations which demonstrate the requisite (anisotropic, multifractal) statistical properties as well as wave-like phenomenologies. In parts II and III we will examine the empirical evidence for the spatial and temporal parts, respectively, of the model using state-of-the-art lidar data of aerosol backscatter ratios (which we use as a surrogate for passive scalar concentration). Copyright © 2008 Royal Meteorological Society [source]

Air,sea exchanges in the equatorial area from the EQUALANT99 dataset: Bulk parametrizations of turbulent fluxes corrected for airflow distortion

Three-dimensional simulation of the ASTEX Lagrangian 1 field experiment with a regional numerical weather prediction model

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 597 2004
Robert Sigg
Abstract The Atlantic Stratocumulus Transition Experiment (ASTEX) first Lagrangian experiment (Lagrangian 1) is here simulated with a modified version of the regional forecast model HIRLAM (High Resolution Limited Area Model). The main modification is that moist turbulent fluxes are accounted for in the model. Trajectory calculations show good agreement with earlier estimations. The initially rather shallow stratocumulus topped marine boundary layer is deepening along the trajectory, and in the end cumulus clouds are formed that penetrate the boundary-layer top. The model predicts this change in cloudiness, but the boundary layer is too shallow in the model. A simulation with modified initial conditions shows improved results, but is still too slow in increasing the boundary-layer depth. Additional factors that influence the boundary-layer growth are: the increase in sea surface temperatures, lower modelled wind speeds, low entrainment rates due to coarse vertical resolution, and synoptic-scale subsidence. An anticyclone at the surface moved slightly northward during the simulation. The anticyclone was accompanied at 500 hPa by a deepening cyclone and, therefore, one would expect synoptic subsidence in the area of the Lagrangian 1. The modelled negative vertical wind component at the boundary-layer top oscillates, and this is examined using spectral analysis. The results show that the vertical velocity is influenced by cumulus clouds on time-scales up to 15 h with a peak at 9 h. The horizontal and vertical wavelengths of the vertical velocity disturbances are estimated from model output to be 400,500 km and 6,10 km, respectively. Using the estimated vertical wavelength and linear theory for hydrostatic inertia,gravity waves, a horizontal wavelength of 350,550 km was calculated for a frequency of 9 h. The model results thus indicate that these types of waves are responsible for the undulating vertical velocity. Finally, an estimation of the synoptic-scale vertical velocity is calculated by filtering out all scales smaller than 15 h from the vertical velocity signal. This results in subsidence both at the beginning and the end of the Lagrangian with vertical velocities between ,0.1 and ,0.4 cm s,1. Copyright © 2004 Royal Meteorological Society [source]

On the accuracy of retrieved wind information from Doppler lidar observations

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 587 2003
F. Davies
Abstract A single pulsed Doppler lidar was successfully deployed to measure air flow and turbulence over the Malvern hills, Worcester, UK. The DERA Malvern lidar used was a pulsed Doppler lidar. The lidar pulse repetition rate was 120 Hz and had a pulse duration of . The system was set up to have 41 range gates with range resolution of 112 m. This gave a theoretical maximum range of approximately 4.6 km. The lidar site was 2 km east of the Malvern hill ridge which runs in a north,south direction and is approximately 6 km long. The maximum height of the ridge is 430 m. Two elevation scans (Range,Height Indicators) were carried out parallel and perpendicular to the mean surface flow. Since the surface wind was primarily westerly the scans were carried out perpendicular and parallel to the ridge of the Malvern hills. The data were analysed and horizontal winds, vertical winds and turbulent fluxes were calculated for profiles throughout the boundary layer. As an aid to evaluating the errors associated with the derivation of velocity and turbulence profiles, data from a simple idealized profile was also analysed using the same method. The error analysis shows that wind velocity profiles can be derived to an accuracy of 0.24 m s,1 in the horizontal and 0.3 m s,1 in the vertical up to a height of 2500 m. The potential for lidars to make turbulence measurements, over a wide area, through the whole depth of the planetary boundary layer and over durations from seconds to hours is discussed. Copyright © 2003 Royal Meteorological Society [source]