Doppler Sodar Research Articles

Some aspects of the local atmospheric circulation in the Castelporziano Estate derived from sodar wind measurements

To characterize the local low-level circulation in the Tyrrhenian Sea coastal area near Rome, the wind velocity field observed by Doppler sodar operating in the Estate of Castelporziano, has been studied. The prevailing diurnal behavior of the wind speed and wind direction as a function of the season was highlighted for the winter and the summer 2007. Two different patterns of the local circulation can be alternately observed in the night to early morning and after the sunset to late evening. The data analysis has shown that the statistical distribution of wind direction during the night, in winter, presents great occurrence at low levels in the northeastern sector and around 100°. Moreover, it has been observed that the nocturnal components of the wind direction at low-level changes with the height, and these changes can be associated with the effects induced by the large-scale flow on the local circulation at higher levels.

Characterization of wind noise by the boundary layer meteorology

The fluctuations in pressure generated by turbulent motions of the atmospheric boundary layer are a principal noise source in outdoor acoustic measurements. The mechanics of wind noise involve not only stagnation pressure fluctuations at the sensor, but also shearing and self-interaction of turbulence throughout the flow, particularly at low frequencies. The contributions of these mechanisms can be described by the boundary-layer meteorology. An experiment was conducted at the National Wind Institute's 200-meter meteorological tower, located outside Lubbock, Texas in the Llano Estacado region. For two days, a 44-element 400-meter diameter array of unscreened NCPA-UMX infrasound sensors recorded wind noise continuously, while the tower and a Doppler SODAR measured vertical profiles of the boundary layer. Analysis of the fluctuating pressure with the meteorological data shows that the statistical structure of wind noise depends on both mean velocity distribution and buoyant stability. The root-mean-square pressure exhibits distinct scalings for stable and unstable stratification. Normalization of the pressure power spectral density depends on the outer scales. In stable conditions, the kurtosis of the wind noise increases with Reynolds number. Measures of noise intermittency are explored with respect to the meteorology.

A comparative study of typhoon wind profiles derived from field measurements, meso-scale numerical simulations, and wind tunnel physical modeling

To investigate the applicability and limitations of both physical (wind tunnel test) and numerical (weather prediction) simulations of the atmospheric boundary layer, field measurements from a Doppler SODAR and a wind-profiler were combined to serve as validation criteria when comparing the results from a numerical simulation conducted by the Weather Research and Forecast model (WRF) and from wind tunnel testing. The comparisons focused on the simulation of the typhoon boundary layer, and revealed that a major drawback of wind tunnel testing is the use of an unrealistic approaching wind profile. As a result, the wind tunnel test results should only be considered valid when the measured wind profile is influenced predominantly by the underlying terrain. Meanwhile, the relatively coarse resolution of the underlying terrain model used in the numerical weather prediction system may lead to an inaccurate mean wind speed profile at lower altitudes, especially when the winds are coming from the land fetch.

Estimation of turbulence intensities under strong wind conditions via turbulent kinetic energy dissipation rates

The turbulence intensity, defined as the standard deviation of the longitudinal wind speeds divided by the mean wind velocity, is one of the important turbulent characteristics in atmospheric boundary layers. In-situ measurements of turbulence intensities above heights of 10m are, however, limited by the availability of suitable instrumentation. The present study discusses a method to estimate turbulence intensity from the turbulent kinetic Energy Dissipation Rate (EDR) through the use of a turbulent length scale model. Provided that the EDRs can be extracted directly from raw measurements taken by remote sensing techniques, the proposed method is applicable for estimating the turbulent velocity fluctuations where only remotely sensed EDRs are available. The validity of the proposed method was evaluated in two case studies. When the measurements taken by a Doppler sodar under various weather conditions were adopted to calculate both the longitudinal turbulent velocity fluctuations and to extract the EDRs, the comparisons between the observed and estimated turbulent velocity fluctuations quantitatively show the validity of the proposed method. It has been found that the proposed method is valid when (a) there are no topographic obstacles upstream, (b) the wind speed in a typhoon boundary layer is high enough to make the selected turbulent length scale model applicable and (c) the EDR values are large enough to ignore any noise contained in the EDR data.

Ground-Based Remote Sensing of the ABL Structure in Moscow and Its Use to Estimate Pollutant Surface Emission Rates

AbstractCurrently used methods to estimate surface pollutant emissions require a set of specific air-sampling surveys. Data from a network of ground-based sodars and a network of air-quality stations in Moscow, Russia, are used to estimate the emission rates of carbon monoxide (CO) and nitric oxide (NO). The sodar network, consisting of three “LATAN-3” Doppler sodars and three “MTP-5” microwave temperature profilers, is used to measure the vertical profiles of vertical and horizontal wind velocity, wind direction, and temperature, which are used to determine the average mixing-layer height. The network of ground-based air-quality stations, consisting of 17 automated stations distributed uniformly across Moscow, continuously measured the CO and NO concentrations. This study focuses on an anticyclonic episode of high surface pressure over Moscow during 30 July–1 August 2012. After sunrise, the solar-induced convection effectively moderated the pollutant levels in the lowest 100–200 m. After sunset, convective mixing stopped and the wind weakened, which allowed CO and NO to reach hazardous levels. With an assumption of an average mixing-layer height of 150 m, the resulting estimate of surface emission of CO is ~6 μg m−2 s−1, whereas that for NO is ~0.6 μg m−2 s−1.

Simulating acoustic scattering from atmospheric temperature fluctuations using a k-space method

This paper describes a numerical method for simulating far-field scattering from small regions of inhomogeneous temperature fluctuations. Such scattering is of interest since it is the mechanism by which acoustic wind velocity profiling devices (Doppler SODAR) receive backscatter. The method may therefore be used to better understand the scattering mechanisms in operation and may eventually provide a numerical test-bed for developing improved SODAR signals and post-processing algorithms. The method combines an analytical incident sound model with a k-space model of the scattered sound close to the inhomogeneous region and a near-to-far-field transform to obtain far-field scattering patterns. Results from two test case atmospheres are presented: one with periodic temperature fluctuations with height and one with stochastic temperature fluctuations given by the Kolmogorov spectrum. Good agreement is seen with theoretically predicted far-field scattering and the implications for multi-frequency SODAR design are discussed.

Evaluation of WRF-Predicted Near-Hub-Height Winds and Ramp Events over a Pacific Northwest Site with Complex Terrain

AbstractOne challenge with wind-power forecasts is the accurate prediction of rapid changes in wind speed (ramps). To evaluate the Weather Research and Forecasting (WRF) model's ability to predict such events, model simulations, conducted over an area of complex terrain in May 2011, are used. The sensitivity of the model's performance to the choice among three planetary boundary layer (PBL) schemes [Mellor–Yamada–Janjić (MYJ), University of Washington (UW), and Yonsei University (YSU)] is investigated. The simulated near-hub-height winds (62 m), vertical wind speed profiles, and ramps are evaluated against measurements obtained from tower-mounted anemometers, a Doppler sodar, and a radar wind profiler deployed during the Columbia Basin Wind Energy Study (CBWES). The predicted winds at near–hub height have nonnegligible biases in monthly mean under stable conditions. Under stable conditions, the simulation with the UW scheme better predicts upward ramps and the MYJ scheme is the most successful in simulating downward ramps. Under unstable conditions, simulations using the YSU and UW schemes show good performance in predicting upward ramps and downward ramps, with the YSU scheme being slightly better at predicting ramps with durations longer than 1 h. The largest differences in mean wind speed profiles among simulations using the three PBL schemes occur during upward ramps under stable conditions, which were frequently associated with low-level jets. The UW scheme has the best overall performance in ramp prediction over the CBWES site when evaluated using prediction accuracy and capture-rate statistics, but no single PBL parameterization is clearly superior to the others when all atmospheric conditions are considered.

Evidence of Very Shallow Summertime Katabatic Flows in Dronning Maud Land, Antarctica

AbstractThe three-axis “Latan-3” Doppler sodar was operated near the Finnish Antarctic station Aboa in Dronning Maud Land (73.04°S, 13.40°W) in the austral summer of 2010/11. The measuring site is located at a practically flat, slightly sloped (about 1%) surface of the glacier. The sodar was operated in multiple-frequency parallel mode with 20–800-m sounding range, 20-m vertical resolution, and 10-s temporal resolution. To reveal the wind and temperature profiles below the sounding range as well as turbulent fluxes at 2 and 10 m, the data from a 10-m meteorological mast were used. During the measurements, the atmospheric boundary layer was within the sounding range of the sodar most of the time. Despite a large variety of observed sodar echo patterns and wind speed profiles, several cases of clear steady katabatic flows were observed. Practically all of them were easterly, whereas the uphill direction is southern. The thickness of the katabatic flow varied from a few tens to several hundreds of meters; the wind speed maximum could be as low as 5 m. Thin katabatic flows had lower wind speed and much stronger temperature gradients (up to 1 K m−1) but had smaller surface heat flux than did the thicker ones.

Diurnally forced convergence lines in the Australian Tropics

AbstractObservations and analyses of low‐level convergence lines over the Gulf of Carpentaria region of northeastern Australia made during the second and third Gulf Lines EXperiments (GLEX II and GLEX III) are reported.During GLEX II, the initiation of the North Australian Cloud Line (NACL) was documented using a small autonomous aircraft and two Doppler sodars. NACLs are long convergence lines that form on the western side of northern Cape York Peninsula and are typically marked by a line of contiguous cumulus or cumulus congestus cloud. The observations show that the degree of asymmetry between west‐coast and east‐coast sea breezes (as characterized by the wind field) depends on the strength of the background easterlies, and that NACLs develop only when the background easterlies are sufficiently large (≥ 5 m s−1).GLEX III focused on morning glories, which are (mostly) southwestward‐moving bore‐like convergence lines that originate over the southern part of the gulf region. The field experiment involved Doppler sodar measurements as well as high‐temporal‐resolution data from operational automatic weather stations. The sodar measurements showed that undular bore‐like morning glory cloud lines develop only when the background easterlies are sufficiently weak (≤ 10 m s−1) and that strong bore‐like morning glories develop when the background winds exceed about 10 m s−1. If the background easterlies are too strong, no morning glories develop. Numerical simulations show that the structure of convergence line produced depends on the strength of the collision between the sea breezes from each side of the Cape York Peninsula, which in turn depends on the strength of the background easterlies. When the easterlies are weak (≤ 10 m s−1), the sea breezes have similar depths and strengths, and their collision is relatively violent, whereas when the background easterlies are strong (> 10 m s−1) the sea breezes have very different depths and strengths and their collision is comparatively benign.

A Climatological Study of the Nocturnal Boundary Layer over a Complex-Terrain Station

AbstractTwo years of Doppler sodar measurements are used to study the time–height structure of the nocturnal boundary layer (NBL), its seasonal variation, and the characteristics of different types of NBL. A total of 220 clear-sky nights during which the inversion layer is clearly visible on a sodar echogram are examined. The NBL depth estimated with sodar data using a wind maxima criterion matches reasonably well with radiosonde-based NBL depth estimates. The NBL exhibits clear seasonal variation with greater depths during the monsoon season. Shallow NBLs are generally observed in winter. The evolution of NBL height shows two distinctly different patterns (called type 1 and type 2), particularly in the second half of the night. Type 1 NBL depth is nearly constant and the wind speed in this type is generally weak and steady throughout the night, while type 2 is characterized by moderate to strong winds with considerable variations in NBL height. The local circulation generated by the complex topography is clearly seen in type 1 throughout the night, whereas it is seen only in the first half of the night in type 2. Type 1 NBLs seem to be more prevalent over Gadanki, India, with nearly 61% of total nights showing type 1 characteristics. Furthermore, type 1 NBL shows large seasonal variability with the majority of type 1 cases in winter. The type 2 cases are mostly observed in monsoon (~60%) followed by summer (39%). The surface meteorological parameters during type 1 and type 2 cases are examined. Differences between type 1 and type 2 NBL patterns are discussed in relation to the surface forcing.

Wavy Vertical Motions in the ABL Observed by Sodar

Some characteristics of wavelike motions in the atmospheric boundary layer observed by sodar are considered. In an experiment carried out in February 1993 in Milan, Italy, Doppler sodar measurements were accompanied by in situ measurements of temperature and wind velocity vertical profiles using a tethered balloon up to 600 m. The oscillations of elevated wavy layers containing intense thermal turbulence, usually associated with temperature-inversion zones, were studied by using correlation and spectral analysis methods. The statistics of the occurrence of wavelike and temperature-inversion events are presented. The height distributions of Brunt–Vaisala frequency and wind shear and their correlation within elevated inversion layers were determined, with a strong correlation observed between the drift rate of the wavy layers and the vertical velocity measured by Doppler sodar inside these layers. Spectral analysis showed similarities regarding their frequency characteristics. The phase speed and propagation direction of waves were estimated from the time delay of the signals at three antennae to provide estimates of wavelength. Moreover, wavelengths were estimated from the intrinsic frequency obtained from sodar measurements of the Doppler vertical velocity and oscillations of wavy turbulent layers. The two wavelength estimates are in good agreement.

Pre-monsoon/monsoon thunderstorm characteristics over Pune—An investigation using Doppler Sodar observations

Doppler sodar observations of three dimensional (3D) wind fields and thermal structure of convective boundary layer (CBL) on a few thunderstorm days of 2009 during pre-monsoon (May and June; June due to delayed arrival of monsoon over Pune) and monsoon (July and August) are analyzed. They reveal the typical signatures of wind fields for the late afternoon thunderstorm (TS) such as deceleration of winds with or without change in direction leading to convergence a few minutes (∼15–30 min) prior to the onset of TS. Pre-monsoon TS are characterized by broad updrafts and narrow downdrafts in CBL in contrast to the narrow updrafts and broad downdrafts of a normal day (i.e. No-TS day). Mean vertical velocity averaged over CBL period shows net updraft on TS days and net downdraft on No-TS day for the pre-monsoon cases. Similarly calm winds are observed in the CBL on TS-days that support enhanced free convection. During the monsoon period updrafts are observed on both TS and No-TS days with higher values on TS days in comparison, due to the dominance of large-scale monsoon flow over local convection. Relatively higher turbulence kinetic energy (TKE) in CBL is observed on all TS days. Analysis shows that TKE maximum for the day is attained about 1.5–2.0 h prior to the onset of afternoon TS. Mixed-layer depth, determined from TKE profile, is higher than lifting condensation level (LCL) on TS days in May and June indicating saturation of air parcels in updrafts.

Low-Level Jets in the Moscow Region in Summer and Winter Observed with a Sodar Network

We evaluate the statistical properties of low-level jets (LLJs) observed by means of a network of Doppler sodars in the Moscow region, Russia. Continuous long-term measurements of the echo-signal intensity and wind-velocity profiles were carried out in July 2005 and in 2008–2010 synchronously in the centre of Moscow and at a rural site. The summertime nocturnal LLJs have a very clear diurnal cycle and exhibit features predicted by the Blackadar mechanism. In contrast, the long-lasting wintertime jets do not have any clear diurnal variability. The urban environment strongly influences LLJs in both seasons: above the city LLJs are higher, weaker and observed more rarely than at the rural site. In very cold periods (air temperature below −8°C) no LLJs were observed over the city, instead convection emerged in the urban boundary layer. The results are based on observations made in July 2005, January and December 2009, and January 2011.

Use of a High-Resolution Sodar to Study Surface-layer Turbulence at Night

Measurements in the atmospheric surface layer are generally made with point sensors located in the first few tens of metres. In most cases, however, these measurements are not representative of the whole surface layer. Standard Doppler sodars allow a continuous display of the turbulent thermal structure and wind profiles in the boundary layer up to 1000 m, with a few points, if any, in the surface layer. To overcome these limitations a new sodar configuration is proposed that allows for a higher resolution in the surface layer. Because of its capabilities (echo recording starting at 2 m, echo intensity vertical resolution of approximately 2 m, temporal resolution of 1 s) this sodar is called the surface-layer mini-sodar (SLM-sodar). Features and capabilities of the SLM-sodar are described and compared with the sodar. The comparison of the thermal vertical structure given by the SLM-sodar and the sodar provides evidence that, in most cases, the surface layer presents a level of complexity comparable to that of the entire boundary layer. Considering its high vertical resolution, the SLM-sodar is a promising system for the study of the nocturnal surface layer. The nocturnal SLM-sodar measurements have shown that, depending on wind speed, the structure of the surface layer may change substantially within a short time period. At night, when the wind speed is greater than 3 m s−1, mechanical mixing destroys the wavy structure present in the nocturnal layer. Sonic anemometer measurements have shown that, in such cases, also the sensible heat flux varies with height, reaching a peak in correspondence with the wind speed peak. Under these conditions the assumption of horizontal homogeneity of the surface layer and the choice of the averaging time need to be carefully treated.

音響ドップラー・ソーダの風向・風速補正の試み

音響ドップラー・ソーダの風向・風速補正の試み

局所的風況予測モデル(LAWEPS)と小型ドップラーソーダによる年間風況予測と検証

局所的風況予測モデル(LAWEPS)と小型ドップラーソーダによる年間風況予測と検証

Local Circulation Diurnal Patterns and Their Relationship with Large-Scale Flows in a Coastal Area of the Tyrrhenian Sea

In order to characterise the local low-level circulation in the Tyrrhenian Sea coastal area near Rome, the wind field observed by conventional anemometers, Doppler sodar, and rawinsonde has been analysed. The prevailing diurnal behaviour of wind speed and direction as a function of season was highlighted, and the existence of two different patterns of the local circulation, mainly due to land and sea breezes and to the drainage flow from the mouth of the Tiber valley, revealed. The comparison between the low-level circulation and synoptic flow allowed us to determine the influence of the large-scale flow on nocturnal currents that are observed at the Pratica di Mare site and the way that wind direction evolves during the day. Numerical simulations are consistent with experimental data and depict the main features of the low-level wind field in the area.

Experimental study of turbulence and vertical temperature profile in the urban boundary layer

We observed the turbulence and vertical profile of temperature in and above the roughness sublayer (RS) using a tower, a tethered balloon and a Doppler sodar in Himeji City. A 3-D ultrasonic anemometer-thermometer was sited at a height of 54 m on the tower. The present results showed that in the RS, the ratio of friction velocity to wind speed, u*/u, observed under unstable conditions of − 4.2 < z′/L < −0.5 were close to those under neutral conditions, where z´ = z – d; the displacement height d = 2 m; L is the Monin-Obkhov stability length. In addition, σw/u* were lower than the universal function presented by Panofsky et al. (1977). The cross correlation coefficients for u′w′ and T′w′ under neutral and unstable conditions were also studied.

Meteorological wind energy potential in the Alps using ERA40 and wind measurement sites in the Tyrolean Alps

AbstractThe peculiarities of meteorological wind potential in alpine settings compared to flatland and offshore sites are studied. Four data sources are used: Global reanalysis ERA40 from ECMWF, long‐term stations in the Tyrolean Alps, spatially dense measurements near the best site and Doppler sodar wind profiles. Due to the decrease of density with height, alpine sites suffer from a nearly linear decrease of harvestable power with altitude, which is more than offset by the increase of wind speed at altitudes above 1.5 km MSL. ERA40 data show higher potential on the northern than on the southern side of the Alps. The best locations are not isolated peaks but ridges within wide orographic channels. The best potential sites in the Tyrolean part of the Alps have median wind speeds of up to 7.1 m s−1 and extractable potentials between 2900 and 1600 kWh per year and per square meter of rotor area. The profile of horizontal wind speed at ridge sites is often not logarithmic but approximately constant within the height of a wind turbine due to a (nearly) complete absence of upwind fetch. Also, the turbulence intensity is independent of height. Icing can cause considerable downtimes. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Effect of the annular solar eclipse of 15 January 2010 on the lower atmospheric boundary layer over a tropical rural station

This paper documents the effect of the annular solar eclipse of 15 January 2010 on the lower atmospheric boundary layer dynamics over a complex terrain environment at Gadanki (13.5°N, 79.2°E,) using a suite of instruments namely automatic weather station, mini boundary layer mast (15 m), Doppler SODAR, GPS radiosonde and ozonesonde observations. The net heating rates are estimated using radiative transfer algorithm before, during and after the eclipse. Effect on soil temperature is seen clearly up to 20 cm depth and at all the levels up to 15 m. Decrease in the thermal plume level, a dip in the surface layer and a strong vertical downdrafts (subsidence) are noticed during the peak eclipse. Upper layer winds did not show any variation during the eclipse. It is also found to have pronounced effect on all the surface meteorological parameters for a two-day period.