Aspect Sensitivity Research Articles

Properties of midlatitude mesosphere summer echoes after three seasons of VHF radar observations at 54°N

During the three summer seasons of the years 1998, 2000 and 2001 mid‐latitude mesosphere summer echoes (MSE) were observed with the OSWIN VHF radar. The radar is located at Kühlungsborn (54.1°N, 11.8°E). Based on nearly continuous operation of the radar a large data set with altogether more than 200 hours of MSE observations with signal‐to‐noise ratios greater than 0 dB has been obtained. We present and discuss the results of the three years observation. Mid‐latitude mesosphere summer echoes occur much more seldom than their corresponding polar mesosphere summer echoes (PMSE) in polar regions. Both phenomena are characterized by very strong radar returns with a high aspect sensitivity and a restriction to the summer mesosphere. But beside these analogies their main differences will be discussed also. MSE occur in a shorter time interval in the summer months and normally only during daytime. They are still observable although the low temperatures and the sufficient degree of saturation necessary for the existence of ice particles cannot be reached at all times. Furthermore we show MSE distribution, scattering characteristics, aspect sensitivity, and turbulence characteristics as functions of height.

Simultaneous MST radar and radiosonde measurements at Gadanki (13.5°N, 79.2°E) 1. Causative mechanism and characteristics of radar backscatterers at VHF

Simultaneous MST radar and radiosonde observations have been carried out from Gadanki (13.5°N, 79.2°E), a tropical station in India. The radar measurements are used to obtain winds, vertical shear of horizontal winds (S), reflectivity (η), aspect sensitivity and horizontal correlation length (ζ) of the radar backscatterers. The radiosonde measurements of pressure, temperature and humidity are utilized for the computations of Brunt Vaisala frequency (N) and potential refractive index gradient (M). These measurements are used to understand the characteristics of the radar backscatterers in terms of prevailing background atmospheric conditions. Observations show strong vertical shears of horizontal winds in the height range of 16–18 km, at the upper edge of the tropical easterly jet (TEJ) associated winds. Results show that these shears and N2 both contribute to enhance the echo power in oblique directions (χ = 10°). The oblique beam observed enhancement of echo power, due to enhanced wind shears, is noticed to be confined to a narrow height range and associated backscatterers are more or less isotropic. At the height where N2 (or MD2) contribution to oblique echo power is significant, the radar backscatterers appear to be relatively more anisotropic with the horizontal correlation length (ζ) of 16–20 m. Present observations thus bring out clearly that even at oblique beam with zenith angle χ = 10° contribution of enhanced N2 to radar reflectivity (η) from refractivity structure with correlation length of ∼10–20 m can be very significant especially at the height above 17.5 km. These results have to be necessarily taken into account while interpreting the observed radar reflectivity observations.

Aspect sensitivity measurements of polar mesosphere summer echoes using coherent radar imaging

Abstract. The Esrange VHF radar (ESRAD), located in northern Sweden (67.88° N, 21.10° E), has been used to investigate polar mesosphere summer echoes (PMSE). During July and August of 1998, coherent radar imaging (CRI) was used to study the dynamic evolution of PMSE with high temporal and spatial resolution. A CRI analysis provides an estimate of the angular brightness distribution within the radar’s probing volume. The brightness distribution is directly related to the radar reflectivity. Consequently, these data are used to investigate the aspect sensitivity of PMSE. In addition to the CRI analysis, the full correlation analysis (FCA) is used to derive estimates of the prevailing three-dimensional wind associated with the observed PMSE. It is shown that regions within the PMSE with enhanced aspect sensitivity have a correspondingly high signal-to-noise ratio (SNR). Although this relationship has been investigated in the past, the present study allows for an estimation of the aspect sensitivity independent of the assumed scattering models and avoids the complications of comparing echo strengths from vertical and off-vertical beams over large horizontal separations, as in the Doppler Beam Swinging (DBS) method. Regions of enhanced aspect sensitivity were additionally shown to correlate with the wave-perturbation induced downward motions of air parcels embedded in the PMSE.Key words. Ionosphere (polar ionosphere) Meteorology and Atmospheric Dynamics (middle atmosphere dynamics) Radio Science (Interferometry)

Investigation on horizontal wind estimation around high aspect sensitivity

In the present paper, simultaneous radar observations in the Doppler beam swinging (DBS), spatial interferometry (SI), and spaced antenna (SA) modes with the middle and upper atmosphere (MU) radar are shown. High aspect sensitivities were found after a passage of a cutoff low and an incidental trough owing to observations in the DBS mode. Because it is known that tilted aspect sensitivities introduce a bias in radial velocity measurements, we corrected the horizontal wind directly measured by oblique beams in the DBS mode by taking account of the arrival angle of the oblique beams estimated by the SI technique. On the other hand, the aspect angle is derived from the echo power ratio of the vertical to the oblique beams with the DBS mode, which allows us to estimate an effective pointing angle, and the Doppler wind is corrected by using this effective angle. In addition, the SA technique is performed simultaneously for estimating the horizontal wind. The above four estimating processes are used to determine the horizontal wind and compared for the aspect sensitivity. Finally, a simple computer simulation is performed for the SA technique for constant and variable wind conditions during incoherent integrations.

VHF tropospheric scatterer anisotropy at Resolute Bay and its implications for tropospheric radar‐derived wind accuracies

Knowledge about the anisotropy of VHF radio wave scatterers in the atmosphere is important when VHF wind profiler radars are used to measure atmospheric winds by Doppler beam‐swinging methods, since appropriate correction factors must be employed. In this study, the anisotropy of the scatterers is determined using two different methods, over the course of a full year, for a VHF radar at Resolute Bay in northern Canada (75°N). The first method utilizes direct comparisons of the radar‐derived winds with those of radiosondes launched from close to the site, while the second uses comparisons of powers received on vertical and off‐vertical beams. The study is unique in that a full year of radiosonde data were available from balloons which were launched from a site only 4 km from the radar. The two methods for estimating the anisotropy at first appear to give slightly different conclusions, but by properly considering the errors associated with both radar and radiosonde measurements it is possible not only to reconcile the two sets of measurements but also to give reasonable estimates of the errors associated with both the sonde and radar winds. An important result which arises is that the radar data have very similar accuracies to the radiosonde data. Furthermore, it is demonstrated that in the past, radar wind measurements may have been overcorrected, because the errors in the sonde measurements may not have been properly considered in determining the correct conversions. Seasonal variations in the aspect sensitivity are found but are not large.

A comparison of tropospheric VHF Doppler beam steering and full correlation analysis measurements of aspect sensitivity

Tropospheric VHF radar data collected using both spaced antenna (SA) and Doppler beam steering (DBS) methods are presented. The SA data are analyzed using full correlation analysis (FCA) on vertical beam data, while DBS analysis is applied to vertical beam data and off‐zenith beam data at 3.6°, 7.2°, and 10.8°. A comparison of meridional velocities using SA FCA and DBS analysis is presented. These results indicate good agreement between the DBS and FCA velocities on all off‐zenith beams, particularly at 10.8°. The main aim of the study is to compare the aspect sensitivity parameter θs,using the spatial correlation method for the FCA data [e.g., Lesicar and Hocking, 1992] and the power method of Hocking et al. [1986] for the DBS data. This comparison reveals good general agreement between isotropic values of θs with both methods, and aspect sensitive values of θs values with both methods, although there are times when one method produces isotropic values and the other produces aspect sensitive values and visa versa. During aspect sensitive periods the FCA values agree best with the DBS method using the 0.0°–3.6° beam combination.

Wavelet analysis of high‐resolution temperature data with applications to radio wave scattering

High‐resolution balloon‐borne temperature measurements have been made in the troposphere and stratosphere during late fall over the western plains of the United States. In one such experiment the data are of remarkable quality and quite suitable for investigating methods of separating organized and turbulent features from a geophysical data stream, exploring atmospheric dynamics, and estimating VHF radar backscatter. We are particularly interested in the mechanisms creating aspect sensitivity, i.e., a nonunity ratio of vertical to well off‐vertical radar backscatter. We find that very steep positive vertical temperature gradients, as high as 40.0 K/km, can be supported. On the other hand, negative gradients are limited to values near the marginal stability boundary. The structures are thus anisotropic and similar to ramp cliff features found in other fluids. We use wavelet analysis to isolate the organized components of the signal and, after subtraction, the residual signal is investigated to determine its character. The Fourier transform of the residual is Kolmogorov in nature, unlike that of the original data stream, and yields Cn2, the refractive index structure parameter, in good agreement with the higher‐order structure function approach; this supports the success of our partition. We calculate the Fresnel reflection coefficient using the wavelet coefficients; the turbulent scatter is found using Cn2. The ratio between the Fresnel scatter and the turbulent scatter, i.e., the aspect sensitivity, is ∼10 decibels in the troposphere and over 20 decibels in the stratosphere, in agreement with published observations and supporting our assumption that the organized features have horizontal extent exceeding the Fresnel scale.

Temperature sheets and aspect sensitive radar echoes

Abstract. here have been years of discussion and controversy about the existence of very thin and stable temperature sheets and their relationship to the VHF radar aspect sensitivity. It is only recently that very high-resolution in situ temperature observations have brought credence to the reality and ubiquity of these structures in the free atmosphere and to their contribution to radar echo enhancements along the vertical. Indeed, measurements with very high-resolution sensors are still extremely rare and rather difficult to obtain outside of the planetary boundary layer. They have only been carried out up to the lower stratosphere by Service d’A´ eronomie (CNRS, France) for about 10 years. The controversy also persisted due to the volume resolution of the (Mesosphere)-Stratosphere-Troposphere VHF radars which is coarse with respect to sheet thickness, although widely sufficient for meteorological or mesoscale investigations. The contribution within the range gate of many of these structures, which are advected by the wind, and decay and grow at different instants and could be distorted either by internal gravity waves or turbulence fields, could lead to radar echoes with statistical properties similar to those produced by anisotropic turbulence. Some questions thus remain regarding the manner in which temperature sheets contribute to VHF radar echoes. In particular, the zenithal and azimuthal angular dependence of the echo power may not only be produced by diffuse reflection on stable distorted or corrugated sheets, but also by extra contributions from anisotropic turbulence occurring in the stratified atmosphere. Thus, for several years, efforts have been put forth to improve the radar height resolution in order to better describe thin structures. Frequency interferometric techniques are widely used and have been recently further developed with the implementation of high-resolution data processings. We begin by reviewing briefly some characteristics of the ST radar echoes with a particular emphasis on recent works. Their possible coupling with stable sheets is then presented and their known characteristics are described with some hypotheses concerning their generation mechanisms. Finally, measurement campaigns that took recently place or will be carried out in the near future for improving our knowledge of these small-scale structures are presented briefly.Key words. Meteorology and atmospheric dynamics (turbulence; instruments and techniques) – Radio Science (remote sensing)

MST radar and polarization lidar observations of tropical cirrus

Abstract. Significant gaps in our understanding of global cirrus effects on the climate system involve the role of frequently occurring tropical cirrus. Much of the cirrus in the atmosphere is largely due to frequent cumulus and convective activity in the tropics. In the Indian sub-tropical region, the deep convective activity is very prominent from April to December, which is a favorable period for the formation of deep cumulus clouds. The fibrous anvils of these clouds, laden with ice crystals, are one of the source mechanisms for much of the cirrus in the atmosphere. In the present study, several passages of tropical cirrus were investigated by simultaneously operating MST radar and a co-located polarization lidar at the National MST Radar Facility (NMRF), Gadanki (13.45° N, 79.18° E), India to understand its structure, the background wind field and the microphysics at the cloud boundaries. The lidar system used is capable of measuring the degree of depolarization in the laser backscatter. It has identified several different cirrus structures with a peak linear depolarization ratio (LDR) in the range of 0.1 to 0.32. Simultaneous observations of tropical cirrus by the VHF Doppler radar indicated a clear enhancement of reflectivity detected in the vicinity of the cloud boundaries, as revealed by the lidar and are strongly dependent on observed cloud LDR. An inter-comparison of radar reflectivity observed for vertical and oblique beams reveals that the radar-enhanced reflectivity at the cloud boundaries is also accompanied by significant aspect sensitivity. These observations indicate the presence of anisotropic turbulence at the cloud boundaries. Radar velocity measurements show that boundaries of cirrus are associated with enhanced horizontal winds, significant vertical shear in the horizontal winds and reduced vertical velocity. Therefore, these measurements indicate that a circulation at the cloud boundaries suggest an entrainment taking place close to these levels. The analysis of simultaneous lidar and MST Radar observations can thus yield valuable information on the structure and dynamics of the cirrus, specifically near the boundaries of such clouds.Key words. Atmospheric composition and structure (cloud physics and chemistry; instruments and technique) - Meteorology and atmospheric dynamics (tropical meteorology)

Anisotropy of Doppler spectral parameters in the VHF radar observations at MU and White Sands

Abstract. Significant differences are found between the mean spectral widths from beams in the meridional plane and in the zonal plane at both the MU and the White Sands VHF radars. The spectral width in the beam directed perpendicular to the prevailing wind is greater than that in the beam parallel to the wind. The magnitudes of the differences in spectral width show a linear relationship with wind speed, with the greatest differences at the greatest wind speeds. The differences in spectral width show a positive correlation with the differences in backscattered power. The anisotropy in backscattered power is well-known and is usually attributed to aspect sensitivity effects. However, the anisotropy in spectral width does not appear to be due to the same mechanism, and while several hypotheses to account for this are considered, including finite range-volume effects, effects from the tilting of isentropic layers, and anisotropic turbulence effects, it is seen that each of these suggestions has its shortcomings.Key words. Meteorology and atmospheric dynamics (turbulence; instruments and techniques)

VHF radar observation of atmospheric winds, associated shears and <b>C<sup>2</sup></b><b><sub>n</sub></b> at a tropical location: interdependence and seasonal pattern

Abstract. The turbulence refractivity structure constant (C2n ) is an important parameter of the atmosphere. VHF radars have been used extensively for the measurements of C2n. Presently, most of such observations are from mid and high latitudes and only very limited observations are available for equatorial and tropical latitudes. Indian MST radar is an excellent tool for making high-resolution measurements of atmospheric winds, associated shears and turbulence refractivity structure constant (C2n). This radar is located at Gadanki (13.45° N, 79.18° E), a tropical station in India. The objective of this paper is to bring out the height structure of C2n for different seasons using the long series of data (September 1995 – August 1999) from Indian MST radar. An attempt is also made to understand such changes in the height structure of C2n in relation to background atmospheric parameters such as horizontal winds and associated shears. The height structure of C2n, during the summer monsoon and post-monsoon season, shows specific height features that are found to be related to Tropical Easterly Jet (TEJ) winds. It is important to examine the nature of the radar back-scatterers and also to understand the causative mechanism of such scatterers. Aspect sensitivity of the received radar echo is examined for this purpose. It is observed that radar back-scatterers at the upper tropospheric and lower stratospheric heights are more anisotropic, with horizontal correlation length of 10–20 m, as compared to those observed at lower and middle tropospheric heights.Key words. Meteorology and atmospheric dynamics (climatology; tropical meteorology; turbulence)

DIFFERENTIAL DIAGNOSIS OF SHOULDER INJURIES IN SPORTS

Shoulder injuries are common problems in all types of sports. There can be a good deal of variability in how symptoms are expressed from individual to individual, and part of the challenge in understanding the injury is being able to interpret the athlete's complaints as they relate to examination findings and the conditions that produce the symptoms. The history is particularly important because the sensitivity and specificity of many aspects of the examination are variable. A relaxed patient and a systematic evaluation is the best way to be thorough. Radiographs and even MRIs are frequently negative in soft tissue problems in the young athlete. This underscores the importance of a good examination. There may be some utility in using contrast-enhanced MRIs to look at labral pathologic conditions, cysts, or ganglions, but this information is certainly less useful than the patient's description of symptoms and the examiner's findings. With this awareness, we can manage these injuries with more confidence.

Multi-beam radar observations of polar mesosphere summer echoes during the MIDAS/DROPPS/MiniDUSTY campaign at Andenes, Norway in July 1999

The dynamical and physical parameters of polar mesosphere summer echoes (PMSE) and noctilucent clouds (NLC) have been studied using co-ordinated rocket, radar, and lidar experiments (ALWIN MST radar, ALOMAR MF radar, and ALOMAR RMR lidar) at Andenes, Norway during the MIDAS/DROPPS/Mini-DUSTY campaign. A strong, long lasting PMSE event connected with a weak NLC was present during the first salvo on July 5/6, 1999. The ALWIN MST radar (53.5 MHz) performed spaced antenna measurements and Doppler beam steering observations using nine beam directions (vertical beam and oblique beams at 7° and 14° off-zenith). For the first time radar observations were possible along rocket trajectories allowing common-volume observations on intersections at altitudes between 75 and 90 km. The observed PMSE characteristics (echo power, aspect sensitivity, spatial coherence, wind field) are discussed with particular emphasis to the times of rocket soundings.

Three-dimensional coherent radar imaging at Jicamarca: comparison of different inversion techniques

We have implemented an eight-antenna module configuration at Jicamarca to perform a three-dimensional imaging of the atmospheric brightness distribution. As an initial test, measurements have been made in the troposphere (5– 10 km ) and in the equatorial electrojet (EEJ) (95– 105 km ). Even though the EEJ irregularities are aligned with the magnetic field, i.e., only a two-dimensional approach is needed, we decided to make these observations (1) to calibrate our eight-module configuration, and (2) to study the performance of different imaging methods. These two goals are feasible, given the fact that we have a good knowledge of the EEJ's mean position, orientation, and aspect sensitivity. Four different methods have been implemented to solve for the inverse problem, i.e., to get the atmospheric brightness from the visibility samples on the ground. We used Fourier-based, Capon, and maximum entropy (MaxEnt) methods that were originally used in radar astronomy. In addition, we implemented a fitting technique where a brightness distribution, characterized by a number of anisotropic Gaussian blobs, is assumed. From the EEJ results, (1) there is good agreement between Capon and MaxEnt methods, particularly when the signal-to-noise ratio (SNR) is high, (2) under low SNR, MaxEnt works better than the other techniques, and (3) our fitting technique using two Gaussian blobs seems to work very well, but it is very sensitive to the initial parameters needed to start the fitting procedure. Finally, the tropospheric images indicate that the troposphere over Jicamarca was too homogeneous and no significant gain in information was attained by using more than three-receiving antennas, particularly when long integration times were used ( >2 min ). The situation could be different in a troposphere less stable than the one at Jicamarca, where the Peruvian coast temperature inversion and stability are prevalent. In the future, efforts will be made to extend the three-dimensional approach to the tropopause and lower stratosphere, where we expect to get more interesting images.

Scattering properties of PMSE irregularities and refinement of velocity estimates

Observations of polar mesosphere summer echoes (PMSE) were performed with the ALOMAR SOUSY VHF radar in the Doppler beam swinging (DBS) and the spaced antenna (SA) mode. This radar is located in polar regions at 69°N, 16°E in Andenes, Norway. After a summary of the basic features of PMSE and the system lay-out, we will come to the analysis of PMSE irregularity structures. Differences of the signal power observed with the DBS and the smaller SA antenna configuration point to a dominance of a scattering process of PMSE irregularities rather than to specular reflection. The SA method yields a more pronounced aspect sensitivity than the DBS method which points towards fairly anisotropic PMSE irregularities in the smaller volume sensed by the SA set-up. These anisotropic irregularity layers are frequently inclined towards the horizontal by a few degrees. There are wave and instability events which change the layer structure, their inclination and also the velocity components. When the layer inclination is not horizontal, a leakage of the horizontal velocity into the vertical velocity is manifest. It is shown that this can be corrected by using the measured zenith and azimuth angles of signal arrival and by calculating the leakage of the horizontal velocity components. This procedure yields improved estimates of the velocity components in vertical direction. Finally, some thoughts how to explain such observations of highly anisotropic and inhomogeneous PMSE irregularity structures are summarized.

RADAR TARGET RECOGNITION BASED ON PARAMETERIZED HIGH RESOLUTION RANGE PROFILES

A new scheme of radar target recognition based on parameterized high resolution range profiles (PHRRP) is presented in this paper. A novel criterion called generalized-weighted-normalized correlation (GWNC) is proposed for measuring the similarity between PHRRP's. By properly choosing the parameter of the mainlobe width in GWNC, aspect sensitivity of PHRRP's can be reduced without sacrificing their discriminative power. Performance of the scheme is evaluated using a dataset of three scaled aircraft models. The experimental results show that by using GWNC, only a small number of most dominant scatterers can achieve the same recognition rates as HRRP's, thus leading to a significant data reduction for the recognition system.

Some thoughts on the evolution and ecology of recalcitrant seeds

AbstractThere are substantial scientific reports on the basic physiology and desiccation sensitivity of recalcitrant seeds, but ecological and evolutionary aspects of their biology have received scant attention. Recalcitrant seeds are shed hydrated, are desiccation sensitive and have a short lifespan. In vegetative tissue, desiccation sensitivity is probably the ancestral state, but tolerance is thought to have evolved early and a number of times independently. It is difficult to see evolutionary relationships among species producing recalcitrant seeds. However, it is suggested that early evolved seeds were desiccation sensitive and that desiccation tolerance is a derived characteristic. Desiccation sensitivity and short lifespan of recalcitrant seeds places constraints on the range of environmental conditions in which reproductive success can occur. Species producing recalcitrant seeds are common in humid tropical forests, where the seeds of climax species germinate and form a seedling bank, rather than contributing to the soil seed bank. However, there is a wide range in post‐shedding physiology among recalcitrant seed species, and recalcitrant seeds do occur in habitats with seasonal climates. Here, regeneration strategies may be more specialized.

The generation of non aspect sensitive plasma density irregularities by field aligned drifts in the lower ionosphere

Abstract. A theory of the generation of plasma density irregularities with virtually no aspect sensitivity, in the lower ionosphere at high latitudes, by electron drifts aligned with the geomagnetic field, is presented. The theory is developed through fluid equations in which the destabilising mechanism involves positive feedback from electron collisional heating. When field aligned electron drift speeds exceed a few km s-1, this effect destabilises waves with wavelengths in excess of a few tens of metres in the lower E-region, where collisional effects are sufficiently large. Furthermore, the threshold conditions are almost independent of the wave propagation direction and the unstable waves propagate at speeds well below the ion acoustic speed. The role that this new instability may play in recent radar backscatter observations of short scale irregularities propagating in directions close to that of the geomagnetic field, in the lower E-region is also considered.Key words: Ionosphere (auroral ionosphere; ionospheric irregularities; plasma waves and instabilities)

The generation of non aspect sensitive plasma density irregularities by field aligned drifts in the lower ionosphere

A theory of the generation of plasma density irregularities with virtually no aspect sensitivity, in the lower ionosphere at high latitudes, by electron drifts aligned with the geomagnetic field, is presented. The theory is developed through fluid equations in which the destabilising mechanism involves positive feedback from electron collisional heating. When field aligned electron drift speeds exceed a few km s-1, this effect destabilises waves with wavelengths in excess of a few tens of metres in the lower E-region, where collisional effects are sufficiently large. Furthermore, the threshold conditions are almost independent of the wave propagation direction and the unstable waves propagate at speeds well below the ion acoustic speed. The role that this new instability may play in recent radar backscatter observations of short scale irregularities propagating in directions close to that of the geomagnetic field, in the lower E-region is also considered.Key words: Ionosphere (auroral ionosphere; ionospheric irregularities; plasma waves and instabilities)

The 30‐MHz radar interferometer studies of midlatitude E region irregularities

The 30‐MHz radar observations of coherent scatter from midlatitude E region plasma irregularities are presented and interpreted. Our analysis indicates that quasiperiodic (QP) echoes can be interpreted as quasi‐point target scatterers drifting through the radar beam at approximately constant altitude. Interferometry data collected with multiple baselines show that the patchy scatterers are not organized along wave fronts but instead form drift and decay independently of one another. An analysis of the intensity of the echoes versus range reveals that the morphology of the QP echo striations is consistent with expectations for drifting field‐aligned irregularities with finite aspect angle sensitivity. An analysis of the effects of refraction, aspect sensitivity, and scattering geometry shows that the plasma irregularities in question reside at altitudes between 95 and 120 km. Type I echoes observed on one occasion imply the existence of strong zonal polarization electric fields with magnitudes of about 20 mV/m. A numerical model of the E region current system demonstrates how such electric fields can be generated by elongated, patchy sporadic E layers electrically coupled to the F region dynamo.