Radio Acoustic Sounding System Research Articles

A novel idea of purposefully affecting radio wave propagation by coherent acoustic source‐induced atmospheric refractivity fluctuation

AbstractThe mechanism generating the array‐distributed atmospheric refractivity fluctuation by a coherent acoustic source is analyzed. The theoretical model is established, which is used to quantifiably analyze the array structure of the artificial dielectric irregularities. It is qualitatively validated that the array‐distributed artificial dielectric irregularities really exist and that the array structure of the artificial dielectric irregularities and the scattering effect of the artificial dielectric irregularities on a radio wave can be controlled by adjusting and selecting the optimized parameters of the transmitted acoustic wave and the adopted acoustic antenna array. It can be concluded that the array‐distributed artificial dielectric irregularities can be used to purposefully affect radio wave propagation. After radio acoustic sounding system, the idea of this paper is a novel development in the field of the tropospheric atmospheric refractivity artificial abnormality technique and its applications.

Orographic Precipitation Forcing along the Coast of Northern California during a Landfalling Winter Storm

Abstract This study documents orographic precipitation forcing along the coastal mountains of Northern California during the landfall of a significant winter storm over the period 16–18 February 2004. The primary observing asset is a scanning X-band Doppler radar deployed on the coast at Fort Ross, California, which provides low-level (e.g., below 1 km MSL) horizontal and vertical scans of radial velocity and reflectivity to characterize airflow and precipitation structures. Further context is provided by a wind-profiling radar, a radio acoustic sounding system (RASS), balloon soundings, buoys, a GPS receiver, and surface meteorological sensors. The winter storm is divided into two episodes, each having pre-cold-frontal low-level jet (LLJ) structures and atmospheric river characteristics. Episode 1 has a corridor of terrain-trapped airflow (TTA) that forms an interface with the LLJ. The interface extends ~25 km offshore in a ~0.5-km vertical layer, and the western edge of this interface near the ocean surface advances toward the coast over the course of ~5 h. The TTA acts as a dynamically driven barrier, so that the incoming LLJ slopes upward offshore below 1.5 km MSL and precipitation is enhanced over the ocean and near the coast. The absence of a TTA in episode 2 allows the cross-barrier flow to slope upward and enhance precipitation directly over the coastal mountains. A theoretical analysis favors the hypothesis that a gap flow exiting the Petaluma Gap forces the TTA.

Observational Characteristics of Low-Level Jets in Central Western Mexico

During field experiments using surface weather stations and a Sodar-RASS (radio acoustic sounding system) combination, low-level jets were observed in the Colima Valley of central western Mexico. These low-level jet events have not been previously observed in this part of the tropics. The present study focuses on events from one specific field period from December 2011 to April 2013 performed at a specific location. From this dataset, it was found that these phenomena typically develop at night at around 0300 local time and they reach their maximum speed 20 min after sunrise and end at 1100 local time. For the considered period, low-level jet events occurred during 88 % of nights, and most of these events were fully characterized; thus, 73 % of the nights presented with categorized low-level jets following the classical literature on this mesoscale phenomenon in other parts of the world. The low-level wind maxima occurred with events ranging from category sub-zero to category 3. The strongest category 3 events are around 25 m s $$^{-1}$$ and the longest event was 57.5 h, but they are more typical of category 1. Their average speed is 11.7 m s $$^{-1}$$ and their height distribution is bimodal, with a group of events occurring at lower levels, with an average height of 190 m; higher levels have a mean height of 480 m. The higher cases are more typical of the rainy season. During all seasons, their direction is predominantly from the north-north-west to north-north-east with some dispersion occurring during the rainy season. The region of study is typically considered to have low wind speeds in the atmospheric boundary layer, making these events super-geostrophic.

A numerical study of effects on detection height of a radio acoustic sounding system influenced by atmospheric wind and temperature

Radio acoustic sounding system (RASS) is a detection technique using the interaction between radio wave and acoustic wave to remotely measure vertical profiles of the atmospheric temperature, and usually composed of a Doppler radar with fixed beam (monostatic or bistatic) and an acoustic source with high power. By combining acoustic propagation equation and radio wave propagation equation in a disturbance medium and using a finite-difference time-domain method, a numerical model describing the interaction between acoustic wave and electric wave is constructed, and the model is used to analyze the effects of wind and temperature on detection height of RASS. In the atmospheric temperature background, the propagations of a single frequency acoustic wave packet under different wind conditions are simulated, and the scattering propagation of electric wave packets corresponding to the acoustic scatterer are analyzed and compared. Besides, the entire physical process are described from the angle of energy density. The numerical simulation results show that the propagation trajectories of both acoustic wave and radio wave backscattering echo are changed due to the existence of wind field and temperature profile. The presence of wind field results in an offset of acoustic wave front, reducing the strength and changing the trajectory of radio wave backscattering echo, so that the detection height is limited due to the reduction of receiving data. The simulation results of the acoustic wave reveal that the temperature profile mainly affects the propagation velocity of acoustic wave, while the presence of wind field may result in shifts of propagation trajectory and acoustic wave front, and the greater the wind speed, the more the horizontal shift of acoustic wave front is. The numerical analyses of scattering propagations of radio wave with the acoustic scatterer at the same height under different background atmospheric conditions manifest that the stronger the wind speed, the more the deviation of electric wave echo from the receive antenna is, and the smaller the echo intensity is when the scattering echo propagates to the same position. The theoretical calculations with the acoustic wave scatterer at different heights under the same atmospheric wind field (strong wind) background demonstrate that if the height of scattering point is reduced, the offset of the scattering echo “bunching point” at the same altitude will be greatly improved and the intensity will be enhanced, but it also means the decline of detection height. In order to improve the detection height under the background of wind field, some methods are adopted, such as using a bistatic radar antenna or increasing the reception antenna area.

ATMOSPHERIC RADIO ACOUSTIC SOUNDING SYSTEMS WITH CONTROLLED ARRAY ANTENNAS. PART 1: PERFORMANCE EVALUATION OF CONTROLLED AA

Radio Acoustic Sounding Systems (RASS) reveal the principal possibility of remote sensing for basic meteorological parameters in the atmospheric boundary layer. However, the lack of effective means for wind shear compensation of sounding packet imposes limitations on implementing such systems into meteorological observations. In this part of the study we qualitatively analyzed the prospects for creating new RASS based on the opportunities offered by modern hardware components. Here we defined the issues for further studies.

An Adjustable Radar Cross Section Doppler Calibration Target

This paper describes the development of a simple low-cost adjustable radar cross section (RCS) Doppler calibration target. It consists of a small metal sphere attached to the diaphragm of an ultrasound transducer. It is shown that the effective RCS of the target is a function of the physical RCS determined by the size of the sphere, and the magnitude of the displacement introduced by the transducer. A simple Michelson's interferometer was constructed to determine the magnitude of these displacements, and a linear relationship between the applied sinusoidal voltage (at resonance) and the displacement is determined. Measurements made using a sensitive Doppler radar confirm the calculated RCS to an accuracy of better than 2 dB, and show that it can be adjusted over 20 dB by adjusting the transducer excitation voltage through one decade. These low RCS targets are ideal for testing radio acoustic sounding systems and entomological radars.

Modeling the eddy transport of momentum and heat: Comparison with direct measurements in free atmosphere

Direct measurements of eddy diffusivities for momentum Km and heat Kh by Doppler radar and by a radio acoustic sounding system in the upper troposphere and lower stratosphere were used to examine the applicability of three Reynolds-averaged Navier-Stokes (RANS) schemes of stratified turbulence in the environment: the E — ɛ turbulence scheme modified for stratified flows, the algebraic two-parameter E — ɛ Reynolds-stress scheme, and the three-parameter \(E - \varepsilon - \overline {\theta ^2 } \) turbulence scheme. All turbulence parameters-the turbulent kinetic energy (E), the dissipation rate (ɛ), and vertical profiles of potential temperature (atmospheric stability) and mean wind velocity-were derived from direct measurements for all three turbulence schemes. It is shown that the profile of the vertical diffusivity of momentum (Km) obtained from the three-parameter RANS turbulence scheme agrees well with its directly measured analog. The profile of Km calculated by the two-parameter turbulence schemes fits measurements rather qualitatively.

АлгоритМ управлениЯ параметрамИ систеМ радиОакустическОгО зондированиЯ атмосферЫ

Выполнен анализ известных методов адаптации систем радиоакустического зондирования атмосферы к изменяющейся метеорологической обстановке, показаны их недостатки. Предложен новый усовершенствованный метод радиоакустического зондирования атмосферы, который учитывает влияние метеорологической обстановки на работу системы и обеспечивает повышение основных качественных показателей систем зондирования.

Verification of RASS-measured temperature profiles using a radio-controlled model glider

Potential sonic temperature profiles from a continuous electromagnetic, pulsed acoustic, radio acoustic sounding system (RASS) were compared with profiles recorded by a commercially available temperature, relative humidity and pressure recorder mounted in a radio-controlled model motor-glider. The glider profiles covered the period of the morning transition in the lowest 200 m of an initially stably stratified boundary layer. Comparison of the profiles shows that the sonic temperature can be calculated based on the average speed of sound in the boundary layer; this removes the need to correct for vertical velocity in each temperature profile, thus avoiding the possibility of contaminating the temperatures with measurement noise from the vertical velocity profiles. Further, the low-level cold bias that occurs with the spatially separated transmit and receive antennas of a bistatic RASS system was not significant for the present measurements as the separation between the antennas was minimised. The comparison of RASS and glider temperatures gives confidence in the use of RASS-derived temperatures for investigating the performance of boundary layer models.

Eddy Diffusivities for Momentum and Heat in the Upper Troposphere and Lower Stratosphere Measured by MU Radar and RASS, and a Comparison of Turbulence Model Predictions

Abstract Recently, middle- and upper-atmosphere Doppler radar (MU radar) has enabled the measurement of middle-atmosphere turbulence from radar backscatter Doppler spectra. In this work, eddy diffusivities for momentum Km in the upper troposphere and lower stratosphere during clear-air conditions were derived from direct measurements of the Reynolds stress and vertical gradient of mean wind velocity measured by MU radar. Eddy diffusivity for heat Kh below 8 km was determined from measurements of temperature fluctuations by the Radio Acoustic Sounding System (RASS) attached to the MU radar. The eddy diffusivity for momentum was on the order of 10 m2 s−1 in the upper troposphere and decreased gradually in the stratosphere by an order of magnitude or more. The eddy diffusivity for heat was almost of the same order of magnitude as Km. Estimates of eddy diffusivity from the radar echo power spectral width give fairly good values compared with the direct measurement of Km. Applicability of three turbulence models—the spectral width method, the k–ɛ model modified for stratified flows, and the algebraic stress model—were also examined, using radar observation values of turbulent kinetic energy k and turbulent energy dissipation rate ɛ together with atmospheric stability observations from rawinsonde data. It is concluded that the algebraic stress model shows the best fit with the direct measurement of Km, even in the free atmosphere above the atmospheric boundary layer once k and ɛ values are obtained from observations or a model.

Features scattering of radio waves by complex acoustic signals to radio acoustic sounding of the atmosphere

The characteristics of the radio waves scattering over the acoustic wave packets created by sophisticated acoustic sounding signals in radio acoustic sounding systems of atmosphere are analyzed. In this case, there is an expansion of the scattering body along spatial frequencies, expanding the wave numbers range of effective interaction acoustic sounding and electromagnetic signals, and a decrease in the duration of the scattered radio signals is also observed. The physical reason signal length decrease is the signal length duration spatial domain decrease of acoustic and electromagnetic waves, due to the presence of modulation parameters in space. The form and parameters of the spatial spectrum, that is embedded in the scattering signal, in a great degree acts on the quality characteristics resulting estimates of signal parameters and atmospheric parameters. The presence of specific errors in the estimation of the Doppler frequency information parameters and radio signals time delay, scattered on the complex acoustic sounding signals is shown. The use of sound oscillations can significantly increase the efficiency of the radio acoustic sounding stations when measuring vertical temperature profiles of the atmosphere

Development and application of an algorithm for the estimation of mixing height with the use of a SODAR-RASS remote sensing system

This work introduces a newly developed algorithm for the estimation of mixing height with the use of a SODAR-RASS system (sound detection and ranging; radio acoustic sounding system) and in-situ instrumentation. The algorithm utilizes the temperature, acoustic back-scattered signal and the Richardson number profiles, estimated by the remote sensing system, and incorporates information on the basic meteorological parameters related to the surface flow given by in-situ measurements. It was applied to a two-year period (2007–2008) data set of a SODAR-RASS installed in the Messogia Plain, a coastal region of the eastern Mediterranean Sea. The profiles of the parameters used by the algorithm showed good agreement among the different criteria used for various atmospheric conditions. The mixing height estimates are analysed with respect to the cold and warm seasons of the two-year period, and the frequency distributions of the estimated 30-min mixing height values from the entire time series show clear peaks, associated with certain synoptic or meso-scale phenomena observed over the area. The mean diurnal cycle of the mixing height during the warm period exhibited higher daytime values compared with the corresponding values during the cold period. The dominating synoptic and meso-scale phenomena over the area were analysed, and selected plots of mixing height estimates together with time–height distributions of SODAR-RASS parameters are given. Mixing height values during the evolution of low-level jets show weak diurnal variability, following the lift or the descent of the core of the jet, and in most cases mixing height estimates nearly coincided with the height of the jet's core, while night-time values were kept high due to the increased mixing. For the case of frontal passages, the mixing height follows the intense wind shift line, with values close to the temperature inversion height that separates the two air masses. Under sea–land breeze conditions, mixing height obtains low daytime values due to the elevated inversion that is formed at the upper part of the lower current of the sea breeze cell. The algorithm exhibited unsatisfactory performance during the transition periods (early morning and night-time hours – neutral conditions) of the diurnal cycle.

CO2 dilution in the lower atmosphere from temperature and wind speed profiles

Temperature and wind speed profiles obtained from 3 years of radio acoustic sounding system sodar measurements at a rural site in the northern Spanish plateau were fitted to polynomial functions. Depending on the extrema of these fits, several groups of profiles were considered. Daily evolution of temperature profiles corresponded to the lower boundary layer evolution. However, wind speed profiles revealed a frequent low-level jet during the whole day. CO2 surface concentrations were analysed, and surface CO2 dilution was also considered by selection of thin canopies with variable depth, resulting in dilution rates of 7 and 18 ppm when the layer increased 100 m for the 95th percentile and temperature and wind speed profiles, respectively.

Diurnal Evolution and Annual Variability of Boundary-Layer Height and Its Correlation to Other Meteorological Variables in California’s Central Valley

One year of observations from a network of five 915-MHz boundary-layer radar wind profilers equipped with radio acoustic sounding systems located in California’s Central Valley are used to investigate the annual variability of convective boundary-layer depth and its correlation to meteorological parameters and conditions. Results from the analysis show that at four of the sites, the boundary-layer height reaches its maximum in the late-spring months then surprisingly decreases during the summer months, with mean July depths almost identical to those for December. The temporal decrease in boundary-layer depth, as well as its spatial variation, is found to be consistent with the nocturnal low-level lapse rate observed at each site. Multiple forcing mechanisms that could explain the unexpected seasonal behaviour of boundary-layer depth are investigated, including solar radiation, precipitation, boundary-layer mesoscale convergence, low-level cold-air advection, local surface characteristics and irrigation patterns and synoptic-scale subsidence. Variations in solar radiation, precipitation and synoptic-scale subsidence do not explain the shallow summertime convective boundary-layer depths observed. Topographically forced cold-air advection and local land-use characteristics can help explain the shallow CBL depths at the four sites, while topographically forced low-level convergence helps maintain larger CBL depths at the fifth site near the southern end of the valley.

Evaluation of the Interpretation of Ceilometer Data with RASS and Radiosonde Data

Since 2006 different remote monitoring methods for determining mixing-layer height have been operated in parallel in Augsburg (Germany). One method is based on the operation of eye-safe commercial mini-lidar systems (ceilometers). The optical backscatter intensities recorded with ceilometers provide information about the range-dependent aerosol concentration; gradient minima within this profile mark the tops of mixed layers. Special software for these ceilometers provides routine retrievals of lower atmospheric layering. A second method, based on sodar observations, detects the height of a turbulent layer characterized by high acoustic backscatter intensities due to thermal fluctuations and a high variance of the vertical velocity component. This information is extended by measurements with a radio-acoustic sounding system (RASS) that directly provides the vertical temperature profile from the detection of acoustic signal propagation and thus temperature inversions that mark atmospheric layers. Ceilometer backscatter information is evaluated by comparison with parallel measurements. Data are presented from 2 years of combined ceilometer and RASS measurements at the same site and from comparison with a nearby (60 km) radiosonde for larger-scale humidity information. This evaluation is designed to ensure mixing-layer height monitoring from ceilometer data more reliable.

Measurements of Detailed Temperature Profiles within the Radar Range Gate Using the Range Imaging Technique

Abstract The present study applies the range imaging (RIM) technique to radio acoustic sounding system (RASS) measurements in an attempt to improve the vertical resolution of temperature profiles obtained by RASS measurements. When an FM-chirped acoustic wave is used for RASS observations, the transmitted radio wave is backscattered from the section of the FM-chirped acoustic pulse where the Bragg condition between the radar and acoustic wavelengths is satisfied. The Bragg resonance region propagates upward with the upward movement of the acoustic pulse. To improve the height resolution of the RIM results, complex time series are extracted for an adequate period corresponding to the effective scattering length of the RASS echo. In the RIM analysis for RASS echo, a model temperature profile is required to compensate the Doppler shift bias due to the shape of range-gate weighting. To remove this bias, an iteration algorithm was developed so that the temperature profiles of the previous RIM results could be used for the bias correction of the next RIM step. The RIM technique was applied to RASS imaging measurements performed using the middle and upper atmosphere (MU) radar on 29–31 October 2006 in an attempt to improve the height resolution to approximately 60 m from the nominal range gate width of 150 m. The temperature profiles inside the radar range volume with a temporal resolution of 26 min were successfully retrieved. The detailed temperature structure, which cannot be revealed by conventional RASS observations, was clarified in the RIM results. In particular, the small-scale inversion layers inside the radar range gate were clearly revealed by the MU radar–RASS imaging data. These detailed temperature variations within the radar range gate agreed well with the simultaneous rawinsonde results.

Observations of temperature profiles by 443 MHz wind profiling radar using a radio acoustic sounding system in Okinawa

The present study is devoted to developing a radio acoustic sounding system (RASS) for the 443 MHz wind profiling radar (443 MHz WPR) installed at the Ogimi Wind Profiler Facility of the National Institute of Information and Communications Technology (NICT) in Okinawa, Japan. We used four fixed horns attached to the sides of the 443 MHz WPR antenna. Additional 15 portable horn speakers were also installed to transmit acoustic waves at frequencies of 900–1050 Hz, which is suitable for the 443 MHz WPR with RASS (443 MHz WPR/RASS) observation. We also developed the software to switch active speakers adaptively by considering the real-time raytracing results of acoustic wavefronts. In the software, four fixed speakers are always activated and several portable speakers are selectively used in order to efficiently obtain strong RASS echoes. Aiming at unmanned operation of the 443 MHz WPR/RASS, a remote control and monitoring system for 443 MHz WPR/RASS observation and a data processing system to derive virtual temperature profiles were developed. Virtual temperature profiles obtained using the 443 MHz WPR/RASS were evaluated by comparison with the data from radiosondes launched from the radar site in 2006.

Characteristics of atmospheric waves in the upper troposphere observed with the Gadanki MST Radar—RASS

Indian MST Radar at Gadanki (13.46ºN, 79.17ºE) working at 53 MHz was operated continuously in Radio Acoustic Sounding System (RASS) mode with six distributed acoustic exciters during 22–25 August 2007 for about 69 h in RASS and turbulence echo modes alternately, with one cycle spanning about 20 min. Temperature and wind velocity were observed in the altitude range 3.6–12 km most of the time and 3.6–20 km, respectively, with a range resolution of 150 m. Presence of an inertia–gravity wave in the lower stratosphere was identified. Perturbations in the wind and temperature fields in the troposphere, however, indicated a mixture of waves with a wide frequency range, within which a dominant periodicity of ∼8 h with a vertical wavelength of ∼4 km was revealed by two-dimensional spectral analysis. Further, signatures of diurnal tide were also observed and the temperature phase profile was found to exhibit a close match with the Global Scale Wave Model (GSWM02). From the RASS virtual temperature profiles, time–height section of Brunt-Väisälä frequency squared was computed to deduce the background atmospheric stability, which showed stable layered structures with slow downward phase progression. Outgoing Longwave Radiation (OLR) over Gadanki and TRMM precipitation data over peninsular India was used as an indicator of convective activity. During the beginning of the observation period, even though lower OLR was seen, corresponding convective activity or precipitation was not seen in the TRMM data. On 24 August, enhanced temperature perturbations could be related to widespread precipitation shown by TRMM.

Meso-γ-scale convective systems observed by a 443-MHz wind-profiling radar with RASS in the Okinawa subtropical region

We observed a meso-γ-scale convective system in July 2007 using a 443-MHz wind profiler radar (WPR) with a radio acoustic sounding system (RASS) at the NICT Ogimi observatory in Okinawa, Japan. We analyzed the virtual temperature, T v, the Brunt–Vaisala frequency squared, N 2, and three components of wind velocity profiles from the WPR-RASS data. We also employed a non-hydrostatic meso-scale (NHM) numerical model. Although the island of Okinawa was covered with a Pacific high-pressure system from 21–26 July, the atmospheric condition was convectively unstable below about 5 km. A number of convective clouds generally appeared from 11:00–18:00 in local time (i.e., Japan Standard Time; JST) with a typical horizontal scale of 10 km and temporal scale of 40–60 min. We focused on the convective system that passed over the Ogimi radar site on 23rd and 25th July. Just before rain occurred on these days, a low N 2 region extended upward to 2.0 km, and this characteristics is also commonly seen around a convective cloud in the NHM model. The cloud water content from the NHM model indicated that the cloud top height correlates with the low N 2 structure. Before the convective system was generated, N 2 decreased below an altitude of about 1 km, because air with low T v intruded at 1–3 km, and the surface temperature increased due to solar radiation. The sea-breeze from both the east and west coasts of Okinawa collided to force the convergence below 1 km. Thus, the synergetic effects of the low static stability and convergence seemed to trigger the generation of a convective system, which eventually grew to 11 km over the radar site.

Comparison of Boundary-Layer Profiles and Layer Detection by AMDAR and WTR/RASS at Frankfurt Airport

At Frankfurt airport (EDDF) vertical soundings of the lower atmosphere from two independent sources are available. One of them is a wind and temperature profiler (wind temperature radar and radio acoustic sounding system, WTR/RASS) located at the western end of the main pair of runways. The second source is aircraft meteorological data relay (AMDAR), i.e. measurements operationally collected by approaching and departing aircraft. Together, both offer a rare opportunity to compare the performance of these widely used systems. We use 1 year of continuous data to calculate systematic temperature and wind vector differences between both measurement systems. The differences show a clear season-dependent structure in conjunction with typical inversion heights. Possible causes for this behaviour are discussed. Second, we compare the ability of both systems to identify inversion and wind-shear layers above the airport. AMDAR-detected layers are typically higher than wind profiler detections. The layer base is usually detected with more agreement than the top. The mutual probability of detection of inversions is found to be mostly between 40 and 60%.