Wind Field Structure Research Articles

Hydrodynamic model of wind field in a moving typhoon

An algorithm is developed for solving the hydrodynamic equations describing wind field structure in a typhoon. The specific feature of the algorithm consists in solving an overdetermined set of equations: given radial profiles of pressure and vertical velocity component, horizontal wind components are found. Projection of angular momentum on tangential direction plays a role of “error of closure.” The calculated wind fields in resting and moving typhoons are presented. As an example, the whole lifecycle of Typhoon Ivan (September 2–17, 2004) is described in detail.

Estimating Hurricane Wind Structure in the Absence of Aircraft Reconnaissance

Abstract New objective methods are introduced that use readily available data to estimate various aspects of the two-dimensional surface wind field structure in hurricanes. The methods correlate a variety of wind field metrics to combinations of storm intensity, storm position, storm age, and information derived from geostationary satellite infrared (IR) imagery. The first method estimates the radius of maximum wind (RMW) in special cases when a clear symmetric eye is identified in the IR imagery. The second method estimates RMW, and the additional critical wind radii of 34-, 50-, and 64-kt winds for the general case with no IR scene–type constraint. The third method estimates the entire two-dimensional surface wind field inside a storm-centered disk with a radius of 182 km. For each method, it is shown that the inclusion of infrared satellite data measurably reduces error. All of the methods can be transitioned to an operational setting or can be used as a postanalysis tool.

Simulation of additive transport from the land surface on the basis of experimental data on heat transfer in the atmospheric surface layer

A method is proposed to study the transport of a passive additive in the atmospheric surface layer with the use of the atmospheric transfer function. This method makes it possible to estimate the spatial distribution of the concentration of a passive additive in the atmospheric surface layer from the additive’s surface source without experimentally determining the vertical profile of the transport coefficient or without resorting to various hypotheses for the character of its behavior. The transfer function, which contains the information on the wind-field structure, can be obtained from simple one-point measurements of surface-and air-temperature fluctuations and from subsequent spectral processing of the data. The effects of the wind-velocity profile and turbulence on the spatial distribution of additive concentration are assessed. This method allows one to simplify experiments during development and verification of the models of atmospheric diffusion. This method may also be useful in emergency situations to predict the propagation of hazardous additives.

Vulnerability of spruce (Picea abies) and beech (Fagus sylvatica) forest stands to storms and consequences for silviculture

The form and magnitude of storm damage and stand disclosure patterns were assessed in 332 randomly chosen pure and regular stands of spruce (Picea abies L.) and beech (Fagus sylvatica L.) after storm lothar, within a region of the Swiss Midlands. This data was analysed in relation to maximal wind speed, measured with Doppler radar techniques and other influential factors such as relief, allometric characteristics, silvicultural history, and neighbourhood. In addition, storm damage, assessed from aerial photographs over an extended perimeter (about 70,000 ha) was considered. A storm of the magnitude of lothar (December 26 1999), with an average maximal wind speed of 45 m s−1 (160 km h−1) appears to have a highly chaotic wind field structure, with great spatial and temporal variation of wind gusts. Wind speeds were not a significant predictor for damage in spruce stands and only weakly influential for beech. The consequences of this high randomness were analysed to estimate the return time of such a storm at the stand level. It lies between 86 and 113 years for spruce, 357 and 408 for beech. Only a few independent variables were significant and the overall explanatory strength of the model was unexpectedly low (R 2=0.07 for spruce and 0.30 for beech). Among the more reliable predisposing factors were mixture and aspect combined with gradient. An admixture of 10% or more broadleaved tree species or wind-firm conifers like Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] significantly reduced the vulnerability of spruce stands (by a factor of more than three). On wind-exposed aspects, damage was more than twice the average. Steeper slopes caused a significant reduction in susceptibility (by a factor of six for slopes over 50%, in comparison to gentle slopes <20%). Other factors such as height to diameter ratio of trees or time since last thinning did not appear to be significant predictors.

Mediterranean wave climate variability and its links with NAO and Indian Monsoon

This study examines the variability of the monthly average significant wave height (SWH) field in the Mediterranean Sea, in the period 1958–2001. The analysed data are provided by simulations carried out using the WAM model (WAMDI group, 1988) forced by the wind fields of the ERA-40 (ECMWF Re-Analysis). Comparison with buoy observations, satellite data, and simulations forced by higher resolution wind fields shows that, though results underestimate the actual SWH, they provide a reliable representation of its real space and time variability. Principal component analysis (PCA) shows that the annual cycle is characterised by two main empirical orthogonal functions (EOF) patterns. Most inter-monthly variability is associated with the first EOF, whose positive/negative phase is due to the action of Mistral/Etesian wind regimes. The second EOF is related to the action of southerly winds (Libeccio and Sirocco). The annual cycle presents two main seasons, winter and summer characterised, the first, by the prevalence of eastwards and southeastwards propagating waves all over the basin, and the second, by high southwards propagating waves in the Aegean Sea and Levantin Basin. Spring and fall are transitional seasons, characterised by northwards and northeastwards propagating waves, associated to an intense meridional atmospheric circulation, and by attenuation and amplification, respectively, of the action of Mistral. These wave field variability patterns are associated with consistent sea level pressure (SLP) and surface wind field structures. The intensity of the SWH field shows large inter-annual and inter-decadal variability and a statistically significant decreasing trend of mean winter values. The winter average SWH is anti-correlated with the winter NAO (North Atlantic Oscillation) index, which shows a correspondingly increasing trend. During summer, a minor component of the wave field inter-annual variability (associated to the second EOF) presents a statistically significant correlation with the Indian Monsoon reflecting its influence on the meridional Mediterranean circulation. However, the SLP patterns associated with the SWH inter-annual variability reveal structures different from NAO and Monsoon circulation. In fact, wave field variability is conditioned by regional storminess in combination with the effect of fetch. The latter is likely to be the most important. Therefore, the inter-annual variability of the mean SWH is associated to SLP patterns, which present their most intense features above or close to Mediterranean region, where they are most effective for wave generation.

Sequential observations of the local neutral wind field structure associated with E region plasma layers

The first sequential rocket experiment to study intermediate layers in over 30 years was launched from Wallops Island, Virginia on the night of 29–30 June 2003. Using an onsite digisonde to determine the presence of sporadic E and conditions indicating the possible presence of an intermediate layer, four rockets were launched over a 4‐hour period. Three of the rockets launched with at least an hour separation; they each contained chemical release experiments and plasma impedance probes. All four payloads encountered two plasma layers on both the ascent and descent of the flights. The lower‐altitude layer, located at approximately 100 km, is clearly a sporadic E layer. The higher‐altitude layer, located between 120 and 130 km, displays many characteristics of an intermediate layer, but it exhibits little downward motion over time. The neutral wind profiles resulting from the chemical tracer experiment are presented here along with the vertical drift velocities derived from the wind measurements. These are compared with the electron density profiles. They show a good agreement between the convergent regions in the velocity profiles and the location of the sporadic E layers. However, agreement between the center of the convergent vertical drift regions and the location of the higher‐altitude layer is poor. The inclusion in the drift calculation of electric field data from the instrumented rocket significantly improves the overall agreement between the convergent vertical drift region center and the intermediate layer center. The convergent region is within 4 km of the intermediate layer. Further, the density depletions surrounding the layers coincide with the regions of divergent drift. The relatively large discrepancy observed between the shear in the vertical drift and the location of the intermediate layer implies that other factors such as horizontal motion structure variations may be important. Thus intermediate layer formation theory and subsequent evolution is still not fully understood.

A Preliminary Numerical Study on Asymmetric Wind Field Structure of Tropical Cyclones

This paper investigates the method of using the experiential model of tropical cyclone's wind distribution to estimate the tropical cyclone size (the radius of gale cycle). The “best size” data of the Northwest Pacific of 2001 compiled by Joint Typhoon Warning Center (JTWC) is used to determine the empirical constants of different models, and compute their estimating precisions. The results show that the “V Bogus” model is better than other experiential models. Moreover, based on the “V Bogus” model, we construct the wind asymmetry distribution model (“revised V Bogus”) of tropical cyclone by the asymmetry distribution of fitting size, estimate the value of characteristic parameters in this model in different seasons and geographical regions, which provide new evidence for the study and application of tropical cyclone size and asymmetric structure.

Investigation of the tidal variations in a 3-D dynamics-chemistry-transport model of the middle atmosphere

The Monitoring of Stratospheric Depletion of Ozone Layer (MSDOL) system is a software tool that is used to assimilate continuously satellite ozone data into a 3-D dynamics-chemistry-transport mechanistic model of the middle atmosphere. Because atmospheric tides lead to variations in temperature, which then affect ozone amounts, we suspect that the ability of the model to converge toward the true 3-D ozone field should therefore depend on how well atmospheric tides are simulated in the model. In this paper, the MSDOL model run without ozone data assimilation was used to explore the representation of the diurnal and semidiurnal tidal components between 10 and 80 km . Comparison with the predictions from the Global-Scale Wave Model (GSWM) showed the model to be in good qualitative agreement with GSWM results, despite quantitative differences. These differences might be explained by the differences in the representation of the background atmosphere, tidal forcing and tidal dissipation in MSDOL and GSWM. A series of sensitivity simulations were then conducted by changing some input parameters. The sensitivity of the tidal amplitude to changes of the tidal forcing and the effect of variations in the structure of the background wind and temperature fields were described; in particular, it was shown that significant fluctuations of the diurnal amplitude (∼20%) could be obtained due to variable tidal forcing. The results here provide some hints on the way the model should be used in the future. Indeed, they suggest the need for updating the model with revised tidal forcing, and for assimilating some part of the real background atmosphere in order to provide real-time estimates of the tides and improve the accuracy of the ozone data assimilation result.

Mesoscale wind and wave field modeling in the area of the Odra Lagoon

The Odra lagoon is a small inner lagoon of the western Baltic Sea which is almost completely landlocked. The wind field in this area is influenced by the formation of atmospheric boundary layers, which generate highly inhomogeneous wind fields, which can be detected by means of in situ measurements and remote sensing techniques. Different numerical wind field estimation techniques were tested within the lagoon and validated against in situ measurements. The output of a coarse-grid standard model of the DWD (Deutscher Wetterdienst) for Europe, as well as a “nested-grid” model run of a mesoscale atmospheric model, was used to estimate the local wind field structure. Two data interpolation techniques employing a set of preprocessed wind fields, generated by the mesoscale atmospheric model, and in situ measurements at pile stations within the area gave best results regarding the reconstruction of the wind field variability in space and time. Subsequent model runs with a shallow-water wave model gave better hi...

Atmospheric Wind Retrievals from Satellite Soundings over the Middle- and High-Latitude Oceans

A technique that uses satellite-based surface wind and temperature soundings for deriving three-dimensional atmospheric wind fields is developed for climate studies over the middle- and high-latitude oceans. In this technique, the thermal wind derived from the satellite soundings is added to the surface wind to obtain a first-guess, nonmass-conserved atmospheric wind profile. Then a Lagrange multiplier in a variational formalism is used to force the first-guess wind to conserve mass. Two mass conservation schemes are proposed. One is to use the meridional mass transport conservation equation as a constraint to derive the meridional wind first, and then the vertically integrated mass conservation equation is used to infer the zonal wind. The zonal and meridional winds are obtained separately in this approach. The second scheme is to use the vertically integrated mass conservation equation as a constraint to retrieve the zonal and meridional winds simultaneously from the first-guess field. Temperature soundings from the Television and Infrared Observational Satellite (TIROS) Operational Vertical Sounder (TOVS) Pathfinder Path A dataset and a Special Sensor Microwave Imager (SSM/I) satellite-based surface wind field are used to derive the wind fields. The two mass conservation schemes yield two different wind fields. They are compared with the European Centre for Medium-Range Weather Forecasts (ECMWF) and National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalyses and radiosonde observations over the Southern Ocean. The general circulation structure of both wind fields is similar to the reanalysis winds. However, the annual-mean bias of the first method is small in both the zonal and meridional winds compared to radiosonde observations, while the zonal wind bias of the second method is as large as −4 m s−1. The main reason for the difference is that the second method requires that the Lagrange multiplier be zero on the latitudinal boundaries. This forces the retrieved zonal wind to approach the first-guess zonal wind. In contrast, the first method does not require latitudinal boundary conditions, allowing a larger correction to the first-guess zonal wind.

Evidence for electrodynamic linkages between spread-F, ion rain, the intermediate layer, and sporadic E: results from observations and simulations

High resolution ISR observations of the evening and nighttime E- and F-regions at Arecibo during both “normal” low-activity periods and during an active spread-F event reveal apparent electrodynamic links between the low-lying layers, ion-rain, and the spread-F instability process. In particular, during a 22 November 1994 event, streaks of ionization—ion rain [Mathews et al., Geophys. Res. Lett. 24 (1997) 1411]—appear to descend from the highly structured F-region base onto the 100– 110 km Tidal Ion Layer (TIL) system. The evening intermediate tidal ion layer, if present, is almost totally disrupted while the spread-F event is characterized by deep, almost vertical folds in electron concentration. The amplitudes of these structures are 100% with fold-structures extending from the nominal F-region base to well above the nominal peak of the F-region. Apparent periods range from less than 10 min for small-scale structures to about 40 min for the largest-scales. We suggest that the E-region structures are a manifestation of polarization electric fields that “map” into the E-region and are well in excess of 1 mV/ m in contrast to low-activity period small-scale fields of order 1 mV/ m . These E-fields are hypothesized to be generated in the “field” of linear (low-activity) and/or non-linear (high-activity spread-F periods) Perkins-like instabilities that are individually of relatively small horizontal-scales but occur over large horizontal distances in the bottom-side of the nighttime F-region. We present numerical simulations of 3-dimensional ion trajectories in tidal wind and electric field structures similar to those implied by these observations. These simulations extend the results of Machuga and Mathews (J. Atmos. Solar-Terres. Phys., 2001, 63(14), 1519–1528, this issue.) (Paper I) and demonstrate the formation of parallel sheets of ionization that form in the tilted plane oriented transverse to the bulk flow of the F-region that sweeps ∼1 mV/ m amplitude E-fields quasi-periodically through a fixed location in the E-region. A fixed pointing radar sees these ionization sheets as ion-rain-like features.

Dynamical effects of ice sublimation in a frontal wave

The dynamical role of ice sublimation in weather systems is briefly reviewed. Observations are presented from the Fronts and Atlantic Storm-Track Experiment (FASTEX) Intensive Observation Period 16 that show variations of static stability, humidity and mesoscale circulation corresponding to those associated theoretically with sublimation of ice precipitation. It is thus suggested that the observations display the mechanism proposed by Clough and Franks in which forward mesoscale flows are associated with moist adiabatic descent supported by the sublimation cooling. This mechanism was suggested as an important stage in the evolution of many mesoscale rain bands. A set of three model simulations of the event has been made with versions of The Mel. Office's Unified Model. Of these a mesoscale model integration with 11 km resolution and 45 levels clearly displays the symptoms, and is diagnosed to demonstrate its consistency with the Clough-Franks mechanism. An integration omitting the cooling due to sublimation differs significantly from the full model experiment in the structure of low-level wind fields, frontal troughs and mesoscale precipitation distribution. Il is also demonstrated that the static-stability transition, mesoscale circulation and mid-tropospheric potential-vorticity perturbations are substantially weakened in this integration, thus confirming that the Clough-Franks mechanism is also operating in the numerical weather prediction (NWP) model. We deduce from these studies that ice precipitation and its sublimation has a major role in determining mesoscale circulation and structure in mid-latitude weather systems, affecting stratification and the formation of features such as fronts and rain bands. These are substantially affected by the fall and evaporation of ice crystals, which are both important to temperature and moisture transports and the behaviour of NWP models on time-scales of hours to days. In our integrations dynamical feedback due to sublimation cooling coincided with extreme negative potential-vorticity values and potentially conditional symmetric instability, hence the anticyclonic motion occurring in the cloud head or deep cloud of the moist warm sector as in this case. In moist warm sectors a substantial role for sublimation may be anticipated more generally, particularly for air trajectories receiving most ice precipitation. We suggest that the described phenomenon be referred to as sublimation enhanced descent or SED. It is concluded that in view of tins demonstrated sensitivity substantial attention should be given to refining microphysical parametrizations in NWP models, and that radar and sounding observations from the FASTEX experiment provide a suitable basis for validating these schemes.

Spectral Characteristics and Multi-Scale Structure of the Boundary-Layer Wind Field During Cold Front Passages Over East China

In this paper, data obtained from a 164 m and a195 m meteorological tower in the northern suburb ofNanjing have been used to estimate and analyzetime-space distributions for velocity spectra andscales of multi-scaling turbulence during thepassages of two cold fronts. Results show that anon-precipitating weak cold front and aprecipitating cold front were clearly revealed bytheir wind field structures. The frontal passageinfluenced all meteorological variables over aperiod of 18–24 hours for the former, and a longerperiod of 44–56 hours for the latter. During these periods there occurred gust surges and eddymotions of various meso- and micro-scales with periodsof 3–4 hours and 1–20 minutes respectively. In the inertialsubrange Kolmogorov's ‘-2/3’ power law for thevelocity spectrum is partly distorted and theturbulence in the atmospheric boundary layer is not isotropic.

Comparative Evaluation of Eddy Exchange Coefficients for Strong and Weak Wind Stable Boundary Layer Modeling

Abstract Five local K-closure formulations and a TKE closure were incorporated in a one-dimensional version of the Pielke’s model, and a comparative evaluation of the closure schemes was made for strong and weak wind stable boundary layer (SBL). The Cabauw (Netherlands) and EPRI-Kincaid site (United States) observations were used for this purpose. The results indicate that for the strong wind case study, the profiles of turbulent diffusivities in terms of shape, depth of significant mixing, and the height above the surface where diffusion reaches a maximum are more or less the same for the various closure schemes. Only the magnitudes of mixing produced by various closure schemes are different. This difference produced by various closure formulations causes minor but noticeable changes in the mean wind field and thermodynamic structure of the model SBL. However, although the profiles of turbulent diffusivities become weak, variable, and poorly defined under weak wind conditions, the mean profiles become in...

Thessaloniki '91 field measurement campaign—I. Wind field and atmospheric boundary layer structure over greater Thessaloniki area, under light background flow

In this study, the major characteristics of the sea and land breeze circulation cells over the Greater Thessaloniki Area (GTA) are presented. During the Thessaloniki '91 Field Measurement Campaign (15 September–13 October) the local flows were examined under different conditions of background wind using a network of ten surface measuring stations, two tethered balloons and a high-resolution monostatic Sodar. The measurements showed that the sea breeze developing over the area maintained two circulation cells. The first was generated over the Thessaloniki Bay during the morning hours and caused sea breeze spreading with the wind direction perpendicular to the long convoluted shoreline at moderate wind speed. The second circulation cell was established over the entire Thermaikos Gulf at noon and was maintained with a southerly wind direction. During the night, a land breeze developed and the wind pattern was influenced by various drainage flows resulting from the surrounding complex topography. The measurements also confirmed the influence of the heat island effect and the creation of a shallow convection layer, during the night which gave rise to layering aloft. Finally, tethersonde ascents revealed the depth of the sea breeze, which was found to range from 300 m to more than 500 m, depending on the prevailing synoptic flow.

A wintertime polar low over the eastern Weddell Sea (Antarctica): A study with AVHRR, TOVS, SSM/I and conventional data

The baroclinic development of an intense meso-scale cyclone (MC) over the Weddell Sea near the Antarctic coast close to the German Georg-von-Neumayer Station (GvN) during the period 26–28 April 1989 was studied by means of satellite and conventional data. ECMWF analyses, radiosonde data and surface observations were used together with AVHRR data, TOVS and SSM/I retrievals for the description of the synoptic and subsynoptic environment associated with the development of the MC. The MC had a diameter of about 500 km, a lifetime of about 40 hours and reached the intensity of a polar low. Wind speeds up to 19 m/s (with gusts up to 24 m/s) were recorded at GvN as the MC approached on 27 April and remained quasi-stationary for about 24 hours. Its development took place in baroclinic conditions of strong low-level cold air advection close to the sea ice front. The genesis of the MC seemed to be triggered by a 500 hPa short-wave trough and a resemblance to a baroclinic development at a boundary layer front was noticed. Low-level thickness fields from TOVS data reflected the baroclinic structure of the MC, but gradients were relatively weak. Wind speed retrievals from SSM/I data did not allow a full analysis of the wind field structure for this case, as they were limited to ice-free ocean. They showed the incipient MC lying in an area with increased wind speed and high surface fluxes of sensible heat. In the mature stage, a maximum in the SSM/I wind speed field was found in the northwestern part of the MC. A meso-scale analysis with a limited area assimilation system for the mature stage showed only weak support of the low-level MC by upper level cyclonic vorticity advection.

Investigation of a fine layered wind field structure in the stratosphere by the artificial luminous cloud method

The wind field fine structure at an altitude of 15–32 km was studied from the results of balloon experiments with different systems of vertical smoke traces. The resolution of the method allows us to detect the layers with a vertical resolution of 25m. Very high spatial stability of the fine wind structure in the stratosphere was clearly demonstrated.

Estimating Pathways of Migrating Insects Carried in Atmospheric Winds1

A numerical trajectory analysis scheme is used to estimate the pathways of insects in the lowest 2 km of the atmosphere during three hypothetical pest migrations. The investigation incorporated winds-aloft data from a series of special meteorological field experiments conducted over the eastern United States during the spring of 1979. Hypothetical pest-catch sites were chosen from which trajectories were constructed upwind (back-trajectories) at seven levels between the surface and 800 mb (ca. 2,000 m). Comparisions were made between 12-h data (the normal operational interval) interpolated to 3 hand 3-h observed data to examine temporal variabilities in the near-surface wind fields. In addition, back-trajectories were constructed with data using only the regular upper air sounding network versus the regular network supplemented with additional sites to explore differences due to increases in the horizontal resolution of the wind. Finally, an intercomparsion study was conducted with several trajectory analysis methods of pest migration studies reported in the literature and the objective procedure described here. Results suggest that increased resolution of the vertical structure of the horizontal wind fields substantially decreases the uncertainties in the estimated paths of migrants. Increases in the temporal and horizontal frequencies of the wind produced differences in back-trajectory endpoints that were 4–5 times smaller than those from the improved vertical resolution. Backtracking methods of previous researchers appeared to provide reasonable approximations of potential source regions for migrants, but only when both surface and upper air data were incorporated into the analysis and with lower-tropospheric airflow conditions that were reasonably homogeneous.

Lightning location relative to storm structure in a supercell storm and a multicell storm

Relationships between lightning location and storm structure are examined for one radar volume scan in each of two mature, severe storms. One of these storms had characteristics of a supercell storm, and the other was a multicell storm. Data were analyzed from dual‐Doppler radar and dual‐VHF lightning‐mapping systems. The distributions of VHF impulse sources were compared with radar reflectivity, vertical air velocity, and their respective gradients. In the supercell storm, lightning tended to occur along streamlines above and downshear of the updraft and reflectivity cores; VHF impulse sources were most concentrated in reflectivities between 30 and 40 dBZ and were distributed uniformly with respect to updraft speed. In the multicell storm, on the other hand, lightning tended to coincide with the vertical reflectivity and updraft core and with the diverging streamlines near the top of the storm. The results suggest to us that the location of lightning in these severe storms was most directly associated with the wind field structure relative to updraft and reflectivity cores. Since the magnitude and vertical shear of the environmental wind are fundamental in determining the reflectivity and wind field structure of a storm, we suggest that these environmental parameters are also fundamental in determining lightning location.

Experiments on scalar dispersion within a model plant canopy part I: The turbulence structure

This is the first of a series of three papers describing experiments on the dispersion of trace heat from elevated line and plane sources within a model plant canopy in a wind tunnel. Here we consider the wind field and turbulence structure. The model canopy consisted of bluff elements 60 mm high and 10 mm wide in a diamond array with frontal area index 0.23; streamwise and vertical velocity components were measured with a special three-hot-wire anemometer designed for optimum performance in flows of high turbulence intensity. We found that: (i) The momentum flux due to spatial correlations between time-averaged streamwise and vertical velocity components (the dispersive flux) was negligible, at heights near and above the top of the canopy. (ii) In the turbulent energy budget, turbulent transport was a major loss (of about one-third of local production) near the top of the canopy, and was the principal gain mechanism lower down. Wake production was greater than shear production throughout the canopy. Pressure transport just above the canopy, inferred by difference, appeared to be a gain in approximate balance with the turbulent transport loss. (iii) In the shear stress budget, wake production was negligible. The role of turbulent transport was equivalent to that in the turbulent energy budget, though smaller. (iv) Velocity spectra above and within the canopy showed the dominance of large eddies occupying much of the boundary layer and moving downstream with a height-independent convection velocity. Within the canopy, much of the vertical but relatively little of the streamwise variance occurred at frequencies characteristic of wake turbulence. (v) Quadrant analysis of the shear stress showed only a slight excess of sweeps over ejections near the top of the canopy, in contrast with previous studies. This is a result of improved measurement techniques; it suggests some reappraisal of inferences previously drawn from quadrant analysis.