Imaging Doppler Interferometry Research Articles

On the role of anisotropic MF/HF scattering in mesospheric wind estimation

The Saura radar is designed and used to measure winds and electron densities at polar latitudes (69^circ N) within the D region, namely between 50 and 100 km altitude. A relatively narrow radar beam can be generated and steered into distinct pointing directions as a rather large antenna array is used. From the observed radial velocities of the individual pointing directions, the horizontal and vertical wind fields can be obtained using the Doppler beam swinging (DBS) method. With recent upgrades to the radar, the interferometric capabilities are largely improved allowing simultaneous application of different wind estimation techniques now, and also echo localization. In recent studies, Saura DBS winds assuming isotropic scattering were found to be underestimated in comparison with highly reliable winds observed with the MAARSY MST radar in the presence of polar mesospheric summer echoes (PMSE). This underestimation has been investigated by analyzing the scattering positions as well as applying the imaging Doppler interferometry technique. Besides this, Saura winds derived with the classical DBS method seem to be error prone at altitudes above 90 km and even below this altitude for periods of enhanced ionization, e.g., particle precipitations. Various methods taking into account the scattering positions have been used to correct the wind underestimation. These winds are compared to MST radar winds during PMSE, and an optimal combination of these methods for the Saura radar is presented. This combined wind data appears to be reliable; it shows reasonable amplitudes as well as tidal structures for the entire altitude region.

Mesospheric and lower thermospheric turbulence over Bear Lake, Utah, 1999–2003

Bear Lake Observatory (41.9°N, 111.4°W) dynasonde imaging Doppler interferometry (IDI) data for the years 1999 through 2002 are analyzed to produce not only mesospheric–lower thermospheric (MLT) mean winds, tides and gravity waves, but also estimates of turbulence parameters—characteristic velocities, energy dissipation rates, eddy diffusion coefficients and buoyancy scale lengths. The zonal and meridional monthly mean winds and tides throughout the 75–114 km height range analyzed are characteristic of wind measurements at mid-latitudes. The diurnal tidal amplitude is less than 20 ms −1 below 100 km, with peaks at 105 km of 70, 65, and 50 ms −1 in February–March, June–July and September, respectively. The semidiurnal tide shows distinct differences between the mesosphere and lower thermosphere. The turbulence results are somewhat surprising in that they show coherence from month to month at certain altitudes, particularly in the delineation of the mesopause at around 87 km. The coherence between the vertical gravity wave and turbulence velocities ( R=0.8±0.1) confirms gravity waves as the source of the turbulent energy. In the upper mesosphere, the turbulent dissipation rate ε increases from some 20 mW kg −1 in January to peaks of 30 mW kg −1 in May, July and September–October, decreasing to 25 mW kg −1 in December. The major feature is an increase from a background at 105 km of some 25 mW kg −1 to a peak of 55 mW kg −1 in August–September, which descends to around 95 km in October. The 1999 through 2002 data shows a surprisingly small interannual variability. If the IDI technique, as implemented on the NOAA dynasonde, were to be adapted to other digital ionosondes, a truly global mesosphere–lower thermosphere wind measuring system would be created.

Diurnal variations in the rate of dissipation of turbulent energy in the equatorial upper mesosphere–lower thermosphere

The diurnal variation of the rate of dissipation of turbulent energy over Arecibo (18°N, 67°W) for the 1989 Arecibo Initiative on Dynamics of the Atmosphere (AIDA '89) campaigns from 1–10 April and 2–9 May 1989 have been estimated using the random velocity field deduced from imaging Doppler interferometry data. A method for extraction of the vertical component of the gravity wave spectrum has been applied to the random velocity. There is a tendency for the turbulence to maximize at dawn and dusk. Estimates of the RMS vertical gravity wave component range from 2 to 5 m s−1, and estimates of the RMS turbulent energy range from 5 to 100 mW kg−1. Estimates of the vertical eddy diffusivity have also been calculated, ranging from <50 to 300 m2 s−1. The buoyancy scale ranges from <100 m to 1 km. A cautionary note is added regarding the applicability of the Kolmogorov theory above 100 km.

Comparisons of full correlation analysis (FCA) and imaging Doppler interferometry (IDI) winds using the Buckland Park MF radar

Abstract. We present results from three years of mesospheric and thermospheric wind measurements obtained using full correlation analysis (FCA) and imaging Doppler interferometry (IDI) for the Buckland Park MF radar. The IDI winds show excellent agreement with the FCA winds, both for short (2-min) and longer term (hourly, fortnightly) comparisons. An extension to a commonly used statistical analysis technique is introduced to show that the IDI winds are approximately 10% larger than the FCA winds, which we attribute to an underestimation of the FCA winds rather than an indication that IDI overestimates the wind velocity. Although the distribution of IDI effective scattering positions are shown to be consistent with volume scatter predictions, the velocity comparisons contradict volume scatter predictions that the IDI velocity will be overestimated. However, reanalysis of a 14-day data set suggests the lack of overestimation is due to the radial velocity threshold used in the analysis, and that removal of this threshold produces the volume scatter predicted overestimation of the IDI velocities. The merits of using hourly IDI estimates versus hourly averaged 2-min IDI estimates are presented, suggesting that hourly estimated turbulent velocities are overestimated.

The Buckland Park MF radar: routine observation scheme and velocity comparisons

Abstract. This paper describes the routine observations scheme implemented for the Buckland Park medium frequency (BPMF) radar. These observations are rare among current MF/HF radar observations in that they are made using a relatively narrow transmit polar diagram. The flexibility of the radar allows a number of analyses to be performed simultaneously. The analyses described include the full correlation analysis (FCA), spatial correlation analysis (SCA), hybrid Doppler interferometry (HDI) and imaging Doppler interferometry (IDI) for observations of mesospheric dynamics and the temporal and spatial characteristics of their scatterers, the differential absorption experiment (DAE) for the estimation of electron densities and collision frequencies, and meteor analysis for estimation of meteor height, time and angle of arrival (AOA) distributions. Intercomparisons between wind velocities estimated using the FCA with SCA, HDI and IDI techniques are presented. The FCA velocities exhibit the well-known "triangle size effect" (TSE), whereby the wind velocity is underestimated at smaller antenna spacings. Although the SCA, IDI and HDI techniques were not applied concurrently, comparisons using FCA as a reference suggest these techniques produce velocities in good agreement.

Initial observations of mesospheric winds using IDI Radar measurements at the Bear Lake Observatory

The adaptation of Imaging Doppler Interferometry (IDI) to the dynasonde deployed at the Bear Lake Observatory (41.9°N, 111.4°W) in northeastern Utah enables the routine measurement of mesospheric dynamics. Influenced by the direction of the prevailing wind, a clear seasonal variation is seen in the mesospheric echo numbers. The technique also provides a monitor for the amplitude of the main tidal modes as well as for studying planetary waves with periods of several days. The observations are found to be consistent with those from different instruments based at the same site thus confirming the IDI characterisation of the mesospheric wind field.

On the radar estimation of turbulence parameters in a stably stratified atmosphere

Four estimators of the rate of dissipation of turbulent energy based on the velocity structure function, rms turbulent velocities, Brunt‐Väisälä frequencies, and ground diffraction pattern fading rates are discussed and compared using Arecibo Initiative on the Dynamics of the Atmosphere MF radar data in both the imaging Doppler interferometry (IDI) and spaced antenna modes. A consistent set of empirical equations is developed which define and describe the relationships between the turbulent velocity, the Brunt‐Väisälä frequency, the buoyancy length scale, the rate of dissipation of turbulent energy, the eddy diffusivity, and the time constant of the gravity wave generated, intermittent, decaying, coherent structures responsible for the scatterers detected by the IDI technique. It should be noted that the constants of proportionality are significantly different from the currently accepted values.

Further direct comparisons of incoherent scatter and medium frequency radar winds from AIDA '89

Abstract Mesopause level winds measured over Arecibo (18°N, 67°W) for the interval 1200–1416′ h 10 April 1989 by the incoherent scatter (ISR) and the MAPSTAR MF radars during AIDA '89 are compared. Imaging Doppler interferometry (IDI), full correlation and three dimensional interferometry analyses have been applied to the MAPSTAR radar data. To determine the measure of agreement at time scales less than the two hour integration intervals previously employed for published AIDA data, the overall interval was split into four 34 min segments, and each analyzed individually. The agreement at the shorter timescales was found to be as good as that for the 2 h interval with, as has been noted previously, the zonal component agreement being better than that for the meridional. The IDI and Full Correlation Analysis (FCA) apparent velocities are in better agreement with the ISR winds than is the FCA true velocity. Sky maps of the distribution of IDI measured scattering point velocities are what one would expect if these represent actual line of sight wind velocities. We have looked at scattering point distributions, and find no evidence in our data for the scattering point alignment predicted by the volume scattering hypothesis.

On the reality of upper mesospheric/lower thermospheric turbulent “eddies”

The structure of atmospheric turbulence and its relationship to the refractive index discontinuities responsible for the scattering of radio waves are discussed. Emphasis is placed more on the nature of the intermittent scatterers observed at upper mesospheric/lower thermospheric altitudes than on volume scattering. The preferred model is of transient patches composed of vortex strings. An overview of the methods used to estimate the rate of dissipation of turbulent energy using MF radar data which emphasizes the advantages of imaging Doppler interferometry is presented, together with a review of pertinent laboratory investigations of coherent structures. An attempt is made to remove the contamination by gravity waves from imaging Doppler interferometry fluctuating velocity data. Emphasis is given to the fact that all radar measurements of turbulent intensity to date are upper estimates, not only because of the presence of gravity wave fluctuations contaminating the data, but also because the turbulence itself is not stationary and homogeneous but intermittent in both space and time. The intermittent nature of the turbulence, in which the refractive index discontinuities responsible for the radar backscatter are associated with decaying, homogeneous turbulent coherent structures, is in contrast to the theory of volume scattering, which assumes the radar pulse volume to be partially filled with stationary, homogeneous turbulence.

Imaging Doppler interferometry and the measurement of atmospheric turbulence

Data taken by an imaging Doppler interferometer (IDI) 3.175‐MHz radar during the Arecibo Initiative on Dynamics of the Atmosphere in 1989 is interpreted in terms of turbulence parameters. The rms random velocity determined from scatterers within 2° of the zenith for the interval 1200–1416AST on April 10, 1989, is used to calculate the rate of dissipation of turbulent energy using two formulations, one based on an estimate of the lifetime (∼100 s) of the intermittent scatterers identified by the IDI technique, and another based on the turbulent structure function. Both techniques yield upper limit values of 0.01–0.06 W kg−1 for the rate of dissipation of turbulent energy, increasing with height over the range from 72 to 90 km. The energy‐bearing eddy scale of turbulence, neglecting buoyancy, is found to vary with height, being a minimum at 75 km (200 m) and a maximum at 90 km (400 m). The effects of buoyancy are discussed in detail, and the buoyancy, or outer scale, is found to vary from 300 m at 75 km to a maximum of 900 m at 90 km. The Kolmogorov microscale, or inner scale, is found to increase from 1 m at 72 km to 10 m at 93 km. Estimates of the vertical eddy diffusion coefficient are also made, and the values obtained are shown to be highly dependent on the assumptions underlying the calculations.

Rocket vapor trail releases revisited: Turbulence and the scale of gravity waves: Implications for the imaging Doppler interferometry/incoherent scatter radar controversy

The rate of diffusion of a trimethyl aluminum trail released by a Skylark rocket over Woomera, Australia (31°S, 137°E), in 1969 is interpreted in terms of molecular and turbulent diffusion, and is related to the scale of internal atmospheric gravity wave winds. Implications for disagreement between imaging Doppler interferometry and incoherent scatter radar winds are explored.

Distortion of gravity wave spectra of imaging Doppler interferometry horizontal winds

We elsewhere suggest (Rastogi et al., this issue) that an analytic model for spectral distortion in the Doppler beam steering (DBS) horizontal wind estimates should also qualitatively hold for the imaging Doppler interferometry (IDI) and spaced antenna (SA) methods. Here we give further support for this suggestion through a statistical model for spectral distortion in the IDI horizontal wind estimates. In IDI the signals observed with a single or multiple receivers at ground are used to identify the locations and radial velocities of virtual atmospheric targets; wind components are then estimated by regression. The statistical model includes the effect of isotropic target configurations as a spatial filter on an isotropic ensemble of atmospheric gravity waves. It shows that the frequency spectrum of IDI horizontal wind estimates is enhanced beyond a knee, located at a ∼0.7‐hour period in the upper mesosphere, and then drops near the buoyancy frequency. The occurrence of the knee and its location is similar to the spectral distortion in the DBS horizontal wind estimates modelled earlier.

Distortion of gravity wave spectra of horizontal winds measured in atmospheric radar experiments

Large disparities in upper mesospheric winds measured with different radar techniques have been attributed earlier to the possibility that MF radars are sensitive to the phase speed of waves, rather than to true winds. An alternative explanation attributes these disparities to biases due to horizontal gradients in the wind field. These biases are common to the Doppler beam swinging (DBS), spaced antenna (SA), and imaging Doppler interferometry (IDI) methods and arise because of spatial filtering of the wind field by the beam configuration. We develop a theoretical model for intrinsic frequency and vertical wave number spectra of DBS wind component estimates that includes the effect of spatial filtering on an ensemble of gravity waves. The model shows a strong enhancement of frequency components above a knee, at ∼1 hour intrinsic period. A similar model for the SA experiment is inherently intractable. We suggest, instead, that the DBS model should also hold for the spectra of SA wind component estimates. The knee is clearly evident in recently observed SA wind component spectra, but is less pronounced and occurs at a frequency lower than the DBS model predicts. Better agreement is expected with a model refined to include the effect of Doppler shifts due to background winds. Occurrence of the knee in observed SA wind spectra implies that MF radars are sensitive to winds rather than to phase speed of atmospheric waves.

High‐resolution wind profiling using combined spatial and frequency domain interferometry

A novel approach to wind profiling is presented which is based on the hybrid use of spatial interferometry (SI) and frequency domain interferometry (FDI). Many algorithms exist that can be used to determine the wind field using SI. However, the imaging Doppler interferometry (IDI) technique is somewhat unique in that the wind field within the radar beam is angularly “imaged” using the Doppler sorting effect. The spatial locations of scatterers are determined by assuming a wind field across the beam and Fourier analyzing signals to sort Doppler velocities. Pulsed radar systems are limited in range resolution by the length of the transmitted pulse, and wind estimates are obtained for a discrete set of altitudes determined by sampling the continuous stream of signals. Frequency domain interferometry (FDI) can be used to determine the radial location of scattering layers within the resolution volume. Thus the combined use of FDI and IDI can provide the radial and angular location of particular scattering points. Using the Doppler sorting idea, a new wind profiling technique is presented which uses FDI to increase the altitude resolution of wind estimates obtained from IDI. Experimental data that illustrate the implementation of the algorithm are presented from the Middle and Upper (MU) Atmosphere radar.

Weighted imaging Doppler interferometry

Development of a simplified, multiple‐receiver, wind estimation scheme is presented. The new algorithm is a modification of the three‐receiver, imaging Doppler interferometry (IDI3) technique, which estimates the location of scattering centers and their corresponding radial velocities. Using the estimated location of the scattering centers, a solution for the three components of the wind vector is obtained. The modification presented is based on a weighted least squares solution of the radial velocity equation. Many IDI3‐like analyses are hampered by the use of a threshold that determines whether a scattering center is “good” or “bad.” Using the weighted least squares solution simplifies the IDI3 procedure by eliminating the need for a threshold. It should be noted that the results obtained by IDI3 are extremely sensitive to the thresholding scheme used. In addition to eliminating the need for the thresholding procedure, the new algorithm greatly reduces the measurement variance in the horizontal wind estimates. Data collected from the Middle and Upper (MU) Atmosphere radar are used to substantiate the technique.

Comparison of time‐ and frequency‐domain techniques for wind velocity estimation using multiple‐receiver MF radar data

Multiple‐receiver MF radar returns from the mesosphere are used to investigate the relationship between spaced antenna (SA), radar interferometry (RI), and imaging Doppler interferometry (IDI) wind estimation techniques. Our results show that frequency‐domain (RI and IDI) and time‐domain (SA) techniques yield almost identical results under high SNR conditions suitable for SA full correlation analysis (FCA).

The imaging Doppler interferometer: Data analysis

Imaging Doppler interferometry has been developed as a pulsed‐radar technique for obtaining three‐dimensional images of mesospheric scattering. Here we discuss the analysis procedure and algorithms and show the results of processing synthetic data records for a variety of situations.