Weak Wind Conditions Research Articles

Pedestrian Level Turbulent Wind Flow around an Elevated Building

Wind flow turbulence plays a critical role in the pedestrian level wind environment, particularly for the airflow around buildings with specific architectural features. The elevated design, a feature was proved to improve the pedestrian level weak wind conditions in high-density cities in our earlier study. This present study aims to further investigate the pedestrian level wind flow turbulence in an elevated building surrounding. The predictions were conducted by comparing the CFD simulation and a wind tunnel experiment. Specifically, the Detached Eddy Simulation (DES), the Steady-state and the Unsteady-state RANS (SRANS and URANS), were assessed. The effects of mesh resolutions and inflow fluctuating algorithms on the performance of the DES model were thoroughly evaluated. DES can better reproduce the mean flow fields than the other two models. The unsteady flow fluctuations around the elevated building are analyzed with energy spectral density. Meanwhile, the periodical wind flow fields around the elevated building are accurately obtained.

Effect of wave-current interactions on sediment resuspension in large shallow Lake Taihu, China.

The disturbance of the water-sediment interface by wind-driven currents and waves plays a critical role in sediment resuspension and internal nutrient release in large, shallow lakes. This study analyzed the effects of the interactions between wind-induced currents an1d waves on the driving mechanism of sediment resuspension in Lake Taihu, the third largest freshwater lake in China, using acoustic and optic techniques to collect long-term, high-frequency, synchronous in situ measurements of wind, currents, waves, and suspended solid concentrations (SSCs). The results suggested that water turbidity started to increase at wind speeds of approximately 4m/s and significantly increased when wind speeds exceeded 6m/s. In most cases, wind-induced waves were the main energy source for changes in turbidity. Wave-generated shear stress contributed more than 95% to sediment resuspension and that only in weak wind conditions (<4m/s) did the lake bottom shear stresses generated by currents and waves contributed equally. The relationship between SSC and bottom shear stress generated by wave was established by fitting the observed results. The processes of sediment dynamics were divided into four stages (A through D) according to three shear-stress thresholds. In stage A, SSC remained stable (about 45mg/L) and τw was less than 0.02N/m2. In stage B, the sediment bed was starting to be activated (SSC 45∼60mg/L) and τw was in the range of 0.02∼0.07N/m2. In stage C, a medium amount of sediment was suspended (SSC 60∼150mg/L) and τw ranged from 0.07 to 0.3N/m2. In stage D, large amount of sediment was suspended (SSC 150∼300mg/L) and τw was larger than 0.3N/m2. The findings of this paper reveal the driving mechanism of sediment resuspension, which may further help to evaluate internal nutrient release in large shallow Lake Taihu.

Early Warning Signals for Regime Transition in the Stable Boundary Layer: A Model Study

The evening transition is investigated in an idealized model for the nocturnal boundary layer. From earlier studies it is known that the nocturnal boundary layer may manifest itself in two distinct regimes, depending on the ambient synoptic conditions: strong-wind or overcast conditions typically lead to weakly stable, turbulent nights; clear-sky and weak-wind conditions, on the other hand, lead to very stable, weakly turbulent conditions. Previously, the dynamical behaviour near the transition between these regimes was investigated in an idealized setting, relying on Monin–Obukhov (MO) similarity to describe turbulent transport. Here, we investigate a similar set-up, using direct numerical simulation; in contrast to MO-based models, this type of simulation does not need to rely on turbulence closure assumptions. We show that previous predictions are verified, but now independent of turbulence parametrizations. Also, it appears that a regime shift to the very stable state is signaled in advance by specific changes in the dynamics of the turbulent boundary layer. Here, we show how these changes may be used to infer a quantitative estimate of the transition point from the weakly stable boundary layer to the very stable boundary layer. In addition, it is shown that the idealized, nocturnal boundary-layer system shares important similarities with generic non-linear dynamical systems that exhibit critical transitions. Therefore, the presence of other, generic early warning signals is tested as well. Indeed, indications are found that such signals are present in stably stratified turbulent flows.

Modal identifiability of a cable-stayed bridge using proper orthogonal decomposition

The recent research on proper orthogonal decomposition (POD) has revealed the linkage between proper orthogonal modes and linear normal modes. This paper presents an investigation into the modal identifiability of an instrumented cable-stayed bridge using an adapted POD technique with a band-pass filtering scheme. The band-pass POD method is applied to the datasets available for this benchmark study, aiming to identify the vibration modes of the bridge and find out the so-called deficient modes which are unidentifiable under normal excitation conditions. It turns out that the second mode of the bridge cannot be stably identified under weak wind conditions and is therefore regarded as a deficient mode. To judge if the deficient mode is due to its low contribution to the structural response under weak wind conditions, modal coordinates are derived for different modes by the band-pass POD technique and an energy participation factor is defined to evaluate the energy participation of each vibration mode under different wind excitation conditions. From the non-blind datasets, it is found that the vibration modes can be reliably identified only when the energy participation factor exceeds a certain threshold value. With the identified threshold value, modal identifiability in use of the blind datasets from the same structure is examined.

Mode identifiability of a multi-span cable-stayed bridge utilizing stabilization diagram and singular values

This study investigates the mode identifiability of a multi-span cable-stayed bridge in terms of a benchmark study using stabilization diagrams of a system model identified using stochastic subspace identification (SSI). Cumulative contribution ratios (CCRs) estimated from singular values of system models under different wind conditions were also considered. Observations revealed that wind speed might influence the mode identifiability of a specific mode of a cable-stayed bridge. Moreover the cumulative contribution ratio showed that the time histories monitored during strong winds, such as those of a typhoon, can be modeled with less system order than under weak winds. The blind data Acc 1 and Acc 2 were categorized as data obtained under a typhoon. Blind data Acc 3 and Acc 4 were categorized as data obtained under wind conditions of critical wind speeds around 7.5 m/s. Finally, blind data Acc 5 and Acc 6 were categorized as data measured under weak wind conditions.

A comparison of the motions of surface drifters with offshore wind properties in the Gulf of Finland, the Baltic Sea

We analyse the interconnections of wind forcing and trajectories of in-situ surface drifters deployed in the Gulf of Finland in different seasons of the years 2011 and 2013. The water masses of this brackish water body are usually strongly stratified, resulting in a layered system. The drifters were designed to follow the uppermost layer of the sea with a thickness of about 2 m. The drifter speed was 2.9–6.3% of the wind speed in 2011 when the drifters were more strongly impacted by the wind, and 1.9–2.9% in 2013 after the drifters had been modified to reduce the wind impact. The trajectories of drifters varied, in many instances the drift was almost aligned with the direction towards which the wind was blowing. In some cases the motion of drifters was systematically to the left of the wind vector apparently owing to boundary effects in the gulf. While on most occasions the drift was mostly along the gulf, on several occurrences rapid across-gulf transport was observed on time scales of 6–12 days. These observations suggest that the motions in the upper layer of the Gulf of Finland (and in similar environments) are intermittently driven by two drivers: the wind impact (including the accompanying Ekman drift) under moderate and strong winds and by underlying synoptic- and basin-scale circulation patterns in weak wind conditions.

Double crests of peak height in the equatorial ionospheric F2 layer observed by COSMIC

AbstractFor the first time, we report daytime double crests of peak height (hmF2) in the F2 layer based on the Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC) observations during 2007–2014. Evident double crests of hmF2 occurred at around ±10° geomagnetic latitude with a trough over the magnetic equator at low solar activity and at March equinox. This phenomenon is referred to as an equatorial height anomaly (EHA) of the ionospheric F2 layer. The double crests became less obvious at September equinox and disappeared at solstices. At solstices only one crest was observed in the summer hemisphere, which is probably associated with transequatorial neutral winds. In addition, the double EHA crests generally take place during 10:00–14:00 local times. Our results indicate that the EHA favors the conditions of strong vertical plasma drifts and weak transequatorial neutral winds during low solar activity. The EHA feature is reproduced by the Thermosphere Ionosphere Electrodynamics Global Circulation Model at March equinox and low solar activity.

Numerical Analysis on Biogenic Emission Sources Contributing to Urban Ozone Concentration in Osaka, Japan

This study conducted analyses on biogenic volatile organic compounds (BVOC) emission sources contributing to urban ozone (O₃) concentration in Osaka Prefecture, Japan in summer 2010 by using the Weather Research and Forecasting model (WRF) version 3.5.1 and the Community Multiscale Air Quality model (CMAQ) version 5.0.1. This prefecture is characterized by highly urbanized area with small forest area. The contributions of source regions surrounding Osaka were estimated by comparing the baseline case and zero-out cases for BVOC emissions from each source region. The zero-out emission runs showed that the BVOC emissions substantially contributed to urban O₃ concentration in Osaka (10.3 ppb: 15.9% of mean daily maximum 1-h O₃ concentration) with day-by-day variations of contributing source regions, which were qualitatively explained by backward trajectory analyses. Although O₃ concentrations were especially high on 23 July and 2 August 2010, the contribution of BVOC on 23 July (35.4 ppb: 25.6% of daily maximum O₃) was much larger than that on 2 August (20.9 ppb: 14.2% of daily maximum O₃). To investigate this difference, additional zero-out cases for anthropogenic VOC (AVOC) emissions from Osaka and for VOC emissions on the target days were performed. On 23 July, the urban O₃ concentration in Osaka was dominantly increased by the transport from the northwestern region outside Osaka with large contribution of O₃ that was produced through BVOC reactions by the day before and was retained over the nocturnal boundary layer. On 2 August, the concentration was dominantly increased by the local photochemical production inside Osaka under weak wind condition with the particularly large contribution of AVOC emitted from Osaka on the day.

Gustiness and coherent structure under weak wind period in atmospheric boundary layer

Statistical analysis of turbulent and gusty characteristics in the atmospheric boundary layer under weak wind period has been carried out. The data used in the analysis were from the multilevel ultrasonic anemometer-thermometers at 47m, 120m, and 280m levels on Beijing 325m meteorological tower. The time series of 3D atmospheric velocity were analyzed by using conventional Fourier spectral analysis and decompose into three parts: basic mean flow (period > 10 min), gusty disturbances (1 min < period < 10 min) and turbulence fluctuations (period < 1 min). The results show that under weak mean wind condition: 1) the gusty disturbances are the most strong fluctuations, contribute about 60% kinetic energy of eddy kinetic energy and 80% downward flux of momentum, although both the eddy kinetic energy and momentum transport are small in comparison with those in strong mean wind condition; 2) the gusty wind disturbances are anisotropic; 3) the gusty wind disturbances have obviously coherent structure, and their horizontal and vertical component are negatively correlated and make downward transport of momentum more effectively; 4) the friction velocities related to turbulence and gusty wind are approximately constant with height in the surface layer.

Emissions of nitrogen oxides from US urban areas: estimation from Ozone Monitoring Instrument retrievals for 2005–2014

Abstract. Satellite remote sensing of tropospheric nitrogen dioxide (NO2) can provide valuable information for estimating surface nitrogen oxides (NOx) emissions. Using an exponentially modified Gaussian (EMG) method and taking into account the effect of wind on observed NO2 distributions, we estimate 3-year moving-average emissions of summertime NOx from 35 US (United States) urban areas directly from NO2 retrievals of the Ozone Monitoring Instrument (OMI) during 2005–2014. Following conclusions of previous studies that the EMG method provides robust and accurate emission estimates under strong-wind conditions, we derive top-down NOx emissions from each urban area by applying the EMG method to OMI data with wind speeds greater than 3–5 m s−1. Meanwhile, we find that OMI NO2 observations under weak-wind conditions (i.e., &lt; 3 m s−1) are qualitatively better correlated to the surface NOx source strength in comparison to all-wind OMI maps; therefore, we use them to calculate the satellite-observed NO2 burdens of urban areas and compare with NOx emission estimates. The EMG results show that OMI-derived NOx emissions are highly correlated (R &gt; 0.93) with weak-wind OMI NO2 burdens as well as with bottom-up NOx emission estimates over 35 urban areas, implying a linear response of the OMI observations to surface emissions under weak-wind conditions. The simultaneous EMG-obtained effective NO2 lifetimes (~ 3.5 ± 1.3 h), however, are biased low in comparison to the summertime NO2 chemical lifetimes. In general, isolated urban areas with NOx emission intensities greater than ~ 2 Mg h−1 produce statistically significant weak-wind signals in 3-year average OMI data. From 2005 to 2014, we estimate that total OMI-derived NOx emissions over all selected US urban areas decreased by 49 %, consistent with reductions of 43, 47, 49, and 44 % in the total bottom-up NOx emissions, the sum of weak-wind OMI NO2 columns, the total weak-wind OMI NO2 burdens, and the averaged NO2 concentrations, respectively, reflecting the success of NOx control programs for both mobile sources and power plants. The decrease rates of these NOx-related quantities are found to be faster (i.e., −6.8 to −9.3 % yr−1) before 2010 and slower (i.e., −3.4 to −4.9 % yr−1) after 2010. For individual urban areas, we calculate the R values of pair-wise trends among the OMI-derived and bottom-up NOx emissions, the weak-wind OMI NO2 burdens, and ground-based NO2 measurements, and high correlations are found for all urban areas (median R= 0.8), particularly large ones (R up to 0.97). The results of the current work indicate that using the EMG method and considering the wind effect, the OMI data allow for the estimation of NOx emissions from urban areas and the direct constraint of emission trends with reasonable accuracy.

Field Observations of Surf Zone–Inner Shelf Exchange on a Rip-Channeled Beach

AbstractCross-shore exchange between the surf zone and the inner shelf is investigated using Lagrangian and Eulerian field measurements of rip current flows on a rip-channeled beach in Sand City, California. Surface drifters released on the inner shelf during weak wind conditions moved seaward due to rip current pulses and then returned shoreward in an arcing pattern, reentering the surf zone over shoals. The cross-shore velocities of the seaward- and shoreward-moving drifters were approximately equal in magnitude and decreased as a function of distance offshore. The drifters carried seaward by the rip current had maximum cross-shore velocities as they exited the surf zone and then decelerated as they moved offshore. The drifters moving shoreward accelerated as they approached the surfzone boundary with maximum cross-shore velocities as they reentered the surf zone over shoals. It was found that Stokes drift was not solely responsible for the onshore transport across the surfzone boundary. The cross-shore diffusivity on the inner shelf was greatest during observations of locally contained cross-shore exchange. These field observations provide evidence that the cross-shore exchange between the surf zone and inner shelf on a rip-channeled beach is due to wave-driven rip current circulations and results in surface material being contained within the nearshore region.

Assessment of wind energy potential for selecting wind turbines: An application to Thailand

Wind energy assessment plays a critical role on determining installation of wind turbines in many countries worldwide. Wind analysis and economic analysis are proposed to systematically assess the potential of electrical energy production for commercial wind turbines. Statistical information of wind data and power curves of wind turbines are applied for determination of factors in decision making such as annual energy production and capacity factor. Accordingly, financial indices and reductions of both fossil fuel consumption and carbon dioxide generation are examined for installing potential wind turbines. The uncertainties of financial indices are determined under variances of parametric inputs within confidence intervals. Case studies are investigated in the central region of Thailand, which has a high demand for electricity. Annual mean wind speed is between 3m/s and 5m/s along directions of tropical monsoons. With the proposed methodology, a wind turbine with low cut-in wind speed, such as the Vestas™ V60 850kW, can be recommended as one of the most feasible wind turbines for electricity generation in green economy under weak and moderate wind conditions in the considered sites. The annual energy production has the most impact on the uncertainty in the net present value, among other financial parameters.

Turbulent transfer coefficient and roughness length in a high-altitude lake, Tibetan Plateau

A persistent unstable atmospheric boundary layer was observed over Lake Ngoring, caused by higher temperature on the water surface compared with the overlying air. Against this background, the eddy covariance flux data collected from Lake Ngoring were used to analyse the variation of transfer coefficients and roughness lengths for momentum, heat and moisture. Results are discussed and compared with parameterization schemes in a lake model. The drag coefficient and momentum roughness length rapidly decreased with increasing wind velocity, reached a minimum value in the moderate wind velocity and then increased slowly as wind velocity increased further. Under weak wind conditions, the surface tension or small scale capillary wave becomes more important and increases the surface roughness. The scalar roughness length ratio was much larger than unity under weak wind conditions, and it decreased to values near unity as wind velocity exceeded 4.0 m s−1. The lake model could not reproduce well the variation of drag coefficient, or momentum roughness length, versus wind velocity in Lake Ngoring, but it did simulate well the sensible heat and latent heat fluxes, as a result of complementary opposite errors.

New model fit functions of the plasmapause location determined using THEMIS observations during the ascending phase of Solar Cycle 24

AbstractIt is well known that the plasmapause is influenced by the solar wind and magnetospheric conditions. Empirical models of its location have been previously developed such as those by O'Brien and Moldwin (2003) and Larsen et al. (2006). In this study, we identified the locations of the plasmapause using the plasma density data obtained from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites. We used the data for the period (2008–2012) corresponding to the ascending phase of Solar Cycle 24. Our database includes data from over a year of unusually weak solar wind conditions, correspondingly covering the plasmapause locations in a wider L range than those in previous studies. It also contains many coronal hole stream intervals during which the plasmasphere is eroded and recovers over a timescale of several days. The plasmapause was rigorously determined by requiring a density gradient by a factor of 15 within a radial distance of 0.5 L. We first determined the statistical correlation of the plasmapause locations with several solar wind parameters as well as geomagnetic indices. We found that the plasmapause locations are well correlated with the solar wind speed and the interplanetary magnetic field Bz, therefore the y component of the convective electric field, and some energy coupling functions such as the well‐known Akasofu's epsilon parameter. The plasmapause locations are also highly correlated with the geomagnetic indices, Dst, AE, and Kp, as recognized previously. Finally, we suggest new model fit functions for the plasmapause locations in terms of the solar wind parameters and geomagnetic indices. When applied to a new data interval outside the model training interval, our model fit functions work better than existing ones. The new model fit functions developed here extend the range of conditions from those used in previous works.

Dependence of Turbulence-Related Quantities on the Mechanical Forcing for Wind Tunnel Stratified Flow

Wind tunnel experiments of thermally stratified flows are analyzed with the objective of determining the influence of the thermal stratification on turbulence-related quantities. The experimental apparatus consisted of an aluminum plate inserted at the bottom surface of the wind tunnel test section. In order to create a thermally stratified layer, the plate was cooled down to 5oC and the air entering the tunnel was kept at approximately 20oC. As the wind speed increases, such stable layer was soon destroyed. Data shows that quantities such as temperature, variances, dissipation rates and turbulent fluxes are substantially reduced when surface cooling is present and that such a reduction is enhanced for weak wind conditions, as these are cases that preserve the stratification. On the other hand, third and fourth order statistical moments, as well as the mean wind speed are much less affected by the stratification.

Characterization and sources of black carbon in PM(2.5) at a site close to a roadway in Gwangju, Korea, during winter.

Continuous measurements of black carbon (BC) concentrations in PM2.5 were conducted using a single-wavelength aethalometer (@880 nm, Magee Sci., AE16) at a site close to a roadway (∼70 m from roadside) in Gwangju, Korea, during winter (December-February) to investigate the characteristics and sources of BC particles. The BC concentrations ranked in the order of January > December > February, probably due to lower boundary layer height, ambient temperature, and wind speed during January. Diurnal patterns in BC and carbon monoxide (CO) levels exhibited peak concentrations during the morning and evening hours coinciding with rush-hour traffic, with a strong correlation (R(2)) ranging from 0.52 (December) to 0.87 (January). It was found that wind speed was an important factor controlling BC concentrations at the site. Very high BC concentrations, up to ∼18.0 μg m(-3), were observed at wind speeds < 1.5 m s(-1). The BC concentrations acquired under weak wind conditions are highly correlated with CO with ΔBC/ΔCO (the slope of BC and CO correlation) of 0.0063 (R(2) = 0.55, p < 0.01) and 0.0065 (R(2) = 0.59, p < 0.01) μg m(-3) ppbv(-1) during day and night, respectively, suggesting no significant difference in the fraction of diesel vehicles between the daytime and nighttime periods. Two BC episodes, "A" and "B", were classified based on BC, PM2.5, and secondary SO4(2-) concentrations, and discussed to investigate the difference in the evolution of the BC observed. Episode "A" was associated with high BC and low PM2.5 and SO4(2-) concentrations, while episode "B" was associated with high concentrations of BC, PM2.5, and SO4(2-). Based on the temporal profiles of BC, NO, and NOx concentrations, CO/NOx ratio, and potential source contribution function map for BC, the BC observed during episode "A" was mostly attributed to locally produced emissions (e.g., traffic). However, the BC during episode "B" was influenced by long-range transport of air masses from China, as well as the local emissions.

NEW CALIBRATION METHOD APPLICABLE TO SIGNIFICANT WAVE HEIGHTS OBTAINED BY X-BAND RADAR

This paper presents a new calibration method of wave heights obtained using marine X-band radar wave measurement system. Realtime information about sea state, such as wave height, period, and direction, is crucial for coastal protection as well as offshore operational management. In the last 30 years, routine sea state measurements have been carried out using different measuring devices such as mooring buoys, pressure gages, and acoustic type gages. Although such devices provide reliable measurements, they are subject to frequent damages and losses. Therefore, considerable interest has been shown lately in the use of remote sensing techniques to measure waves and surface currents. In recent years, it has been demonstrated that radar images of the ocean surface provide reliable information about the spatial behavior of wave fields. Many systems which utilize a commercial marine X-band radar for providing time series of radar backscatter images from the sea surface have been developed. (Borge et al., 2004, Plant and Zunk, 1997 ) Wave measurement system using marine X-band radar has many advantages over conventional wave gages, which are more robust in severe sea state, easy to maintain, economic maintenance cost, etc. However, one critical shortcoming of X-band radar system using in operational wave measurements is inaccuracy in the estimation of wave heights during weak local wind conditions. This is a critical weakness when people try to use measured wave data in the statistical estimation of design waves. It is not an easy task to overcome this critical shortcoming. It is because this problem resides in the X-band radar system’s inherent sea surface scanning mechanism, which is called Bragg Resonance mechanism. It is known that under various conditions signatures of the sea surface are visible in the near ranges (usually less than 5.4 km) of marine radar images. These signatures are known as sea clutter because they are undesirable for navigational purposes and generally suppressed by filter algorithm. Sea clutter is mainly due to the backscatter through Bragg resonance with ocean waves of wavelengths similar to those of the transmitted electromagnetic waves. The longer waves become visible in the radar images because they modulate the sea clutter signals. For the case of X-band radar (frequency of 9.4 GHz), sea surface with ripple waves (wavelength of about 2 cm) provide Bragg resonance, henceforth results in accurate estimation of wave heights. On the other hand, swell waves without ripples when no local wind available, are tend to be significantly underestimated.

Analysis of the causes of heavy aerosol pollution in Beijing, China: A case study with the WRF-Chem model

The causes and variability of a heavy haze episode in the Beijing region was analyzed. During the episode, the PM2.5 concentration reached a peak value of 450μg/kg on January 18, 2013 and rapidly decreased to 100μg/kg on January 19, 2013, characterizing a large variability in a very short period. This strong variability provides a good opportunity to study the causes of the haze formation. The in situ measurements (including surface meteorological data and vertical structures of the winds, temperature, humidity, and planetary boundary layer (PBL)) together with a chemical/dynamical regional model (WRF-Chem) were used for the analysis. In order to understand the rapid variability of the PM2.5 concentration in the episode, the correlation between the measured meteorological data (including wind speed, PBL height, relative humidity, etc.) and the measured particle concentration (PM2.5 concentration) was studied. In addition, two sensitive model experiments were performed to study the effect of individual contribution from local emissions and regional surrounding emissions to the heavy haze formation. The results suggest that there were two major meteorological factors in controlling the variability of the PM2.5 concentration, namely, surface wind speed and PBL height. During high wind periods, the horizontal transport of aerosol particles played an important role, and the heavy haze was formed when the wind speeds were very weak (less than 1m/s). Under weak wind conditions, the horizontal transport of aerosol particles was also weak, and the vertical mixing of aerosol particles played an important role. As a result, the PBL height was a major factor in controlling the variability of the PM2.5 concentration. Under the shallow PBL height, aerosol particles were strongly confined near the surface, producing a high surface PM2.5 concentration. The sensitivity model study suggests that the local emissions (emissions from the Beijing region only) were the major cause for the heavy haze events. With only local emissions, the calculated peak value of the PM2.5 concentration was 350μg/kg, which accounted for 78% of the measured peak value (450μg/kg). In contrast, without the local emissions, the calculated peak value of the PM2.5 concentration was only 100μg/kg, which accounted for 22% of the measured peak value.

Observations of the scale-dependent turbulence and evaluation of the flux–gradient relationship for sensible heat for a closed Douglas-fir canopy in very weak wind conditions

Abstract. Observations of the scale-dependent turbulent fluxes, variances, and the bulk transfer parameterization for sensible heat above, within, and beneath a tall closed Douglas-fir canopy in very weak winds are examined. The daytime sub-canopy vertical velocity spectra exhibit a double-peak structure with peaks at timescales of 0.8 s and 51.2 s. A double-peak structure is also observed in the daytime sub-canopy heat flux co-spectra. The daytime momentum flux co-spectra in the upper bole space and in the sub-canopy are characterized by a relatively large cross-wind component, likely due to the extremely light and variable winds, such that the definition of a mean wind direction, and subsequent partitioning of the momentum flux into along- and cross-wind components, has little physical meaning. Positive values of both momentum flux components in the sub-canopy contribute to upward transfer of momentum, consistent with the observed sub-canopy secondary wind speed maximum. For the smallest resolved scales in the canopy at nighttime, we find increasing vertical velocity variance with decreasing timescale, consistent with very small eddies possibly generated by wake shedding from the canopy elements that transport momentum, but not heat. Unusually large values of the velocity aspect ratio within the canopy were observed, consistent with enhanced suppression of the horizontal wind components compared to the vertical by the very dense canopy. The flux–gradient approach for sensible heat flux is found to be valid for the sub-canopy and above-canopy layers when considered separately in spite of the very small fluxes on the order of a few W m−2 in the sub-canopy. However, single-source approaches that ignore the canopy fail because they make the heat flux appear to be counter-gradient when in fact it is aligned with the local temperature gradient in both the sub-canopy and above-canopy layers. While sub-canopy Stanton numbers agreed well with values typically reported in the literature, our estimates for the above-canopy Stanton number were much larger, which likely leads to underestimated modeled sensible heat fluxes above dark warm closed canopies.

Turbulence regimes and the validity of similarity theory in the stable boundary layer over complex terrain of the Loess Plateau, China

AbstractTo gain an insight into the characteristics of turbulence in a stable boundary layer over the complex terrain of the Loess Plateau, data from the Semi‐Arid Climate and Environment Observatory of Lanzhou University are analyzed. We propose a method to identify and efficiently isolate nonstationary motions from turbulence series, and then we examine the characteristics of nonstationary motions (nonstationary motions refer to gusty events on a greater scale than local shear‐generated turbulence). The occurrence frequency of nonstationary motions is found to depend on the mean flow, being more frequent in weak wind conditions and vanishing when the wind speed, U, is greater than 3.0 m s−1. When U exceeds the threshold value of 1.0 m s−1 for the gradient Richardson number Ri ≤ 0.3 and 1.5 m s−1 for Ri &gt; 0.3, local shear‐generated turbulence on timescales of less than 4 min depends systematically on U with an average rate of 0.05 U. However, for the weak wind condition, neither the mean wind speed nor the stability is an important factor for local turbulence. Then turbulence is categorized into three regimes based on the behaviors of nonstationary motions and local turbulence. Regime 1 considers stationary turbulence with a wind speed greater than 3.0 m s−1, and the Monin‐Obukhov similarity theory (MOST) can be used to calculate the turbulence momentum flux. Regime 2 examines intermittent turbulence where the MOST is competent to evaluate the local turbulence momentum flux but not nonstationary motions. Regime 3 involves wind speed that is less than the threshold value, where nonstationary motions are dominant, local turbulence is independent of the mean flow, and where the MOST may well be invalid.