Mesospheric winds measured by medium-frequency radar with full correlation analysis: error properties and impacts on studies of wind variance

The mesosphere is one of the most difficult parts of the atmosphere to sample; it is too high for balloon measurements and too low for in situ satellites. Consequently, there is a reliance on remote sensing (either from the ground or from space) to diagnose this region. Ground-based radars have been used since the second half of the 20th century to probe the dynamics of the mesosphere; medium-frequency (MF) radars provide estimates of the horizontal wind fields and are still used to analyse tidal structures and planetary waves that modulate the meridional and zonal winds. The variance of the winds has traditionally been linked qualitatively to the occurrence of gravity waves. In this paper, the method of wind retrieval (full correlation analysis) employed by MF radars is considered with reference to two systems in Antarctica at different latitude (Halley at 76∘ S and Rothera at 67∘ S). It is shown that the width of the velocity distribution and occurrence of “outliers” is related to the measured levels of anisotropy in the received signal pattern. The magnitude of the error distribution, as represented by the wind variance, varies with both insolation levels and geomagnetic activity. Thus, it is demonstrated that for these two radars the influence of gravity waves may not be the primary mechanism that controls the overall variance.

Mesospheric winds measured by MF radar with Full Correlation Analysis: error properties and impacts on studies of wind variance

The mesosphere is one of the most difficult parts of the atmosphere to sample; too high for balloon measurements and too low for in-situ satellites. Consequently there is a reliance on remote sensing (either from the ground or from space) to diagnose this region. Ground based radars have been used since the second half of the 20th century to probe the dynamics of the mesosphere; Medium Frequency (MF) radars provide estimates of the horizontal wind fields and are still used to analyse tidal structures and planetary waves that modulate the meridional and zonal winds. The variance of the winds has traditionally been linked qualitatively to the occurrence of gravity waves. In this paper the method of wind retrieval (full correlation analysis) employed by MF radars is considered with reference to two systems in Antarctica at different latitude (Halley at 76° S and Rothera at 67° S). It is shown that the width of the velocity distribution and occurrence of ‘outliers’ is related to the measured levels of anisotropy in the received signal pattern. The magnitude of the error distribution, as represented by the wind variance, varies with both insolation levels and geomagnetic activity. Thus it is demonstrated that for these two radars the influence of gravity waves may not be the primary mechanism that controls the overall variance.

Simulation of Spaced Antenna Wind Retrieval Performance on an X-Band Active Phased Array Weather Radar

Spaced antenna baseline wind retrievals, in conjunction with traditional Doppler measurements, are a potential means of fine angular resolution weather radar wind vector retrieval. A spaced antenna implementation on an X-band active phased array architecture is investigated via Monte Carlo simulations of the backscattered electric fields at the antenna array. Several retrieval methods are exercised on the data produced by the simulator. Parameters of the X-band spaced-antenna design are then optimized. Benefiting from the parametric fitting procedure inherent in the time domain slope at zero lag and full correlation analysis, the study finds both of these algorithms to be more immune to thermal noise than the spectral retrieval algorithms investigated. With appropriately chosen baselines, these time domain algorithms are shown to perform adequately for 5-dB SNR and above. The study also shows that the Gaussian slope at zero lag (G-SZL) algorithm leads to more robust estimates over a wider range of beamwidths than the Gaussian full correlation analysis (G-FCA) algorithm. The predicted performance of the X-band array is compared to a similar spaced antenna implementation on the S-band National Weather Radar Testbed (NWRT). Since the X-band signal decorrelates more rapidly (relative to S band), the X-band array accumulates more independent samples, thereby obtaining lower retrieval uncertainty. However, the same rapid decorrelation also limits the maximum range of the X-band array, as the pulse rate must be sufficiently high to sample the cross-correlation function. It also limits the range of tolerable turbulence velocity within the resolution cell.

Aspect sensitivity of polar mesosphere summer echoes based on ESRAD MST radar measurements in Kiruna, Sweden in 1997–2010

Aspect sensitivities of polar mesosphere summer echoes (PMSE) measured with the ESRAD 50 MHz radar in 1997–2010 are studied using the full correlation analysis technique. Half of PMSE detected each year are found to be highly aspect sensitive. Yearly median values of the aspect sensitivity parameter θs, characterising the half-width of the scatterers' polar diagram, are 2.9–3.7° depending on the year. The other half of the PMSE have θs values larger than 9–11° and cannot be evaluated using the ESRAD vertical beam only. PMSE aspect sensitivity reveals an altitude dependence, namely, the scatter becomes more isotropic with increasing height. This result is consistent with that reported in other studies. No dependence of PMSE aspect sensitivity on backscattered power for any year was identified. In the paper the limitations of the in-beam and off-vertical beam methods for estimation of PMSE aspect sensitivity are discussed. We conclude that both methods should be combined in order to get complete information about PMSE aspect sensitivity and to estimate correctly PMSE absolute strength.

Variance of wind estimates using spaced antenna techniques with the MU radar

Variance of horizontal wind estimates in conditions of anisotropic scattering are obtained for the Spaced Antenna (SA) Full Correlation Analysis (FCA) method of Holloway et al. (1997b) and Doviak et al. (1996), but are equally applicable to the Briggs method of FCA. Variance and covariance of cross-correlation magnitudes are theoretically estimated, and the standard theory of error propagation is used to estimate the variance of the wind components for the infinite SNR case. The effect of baseline orientation is investigated, and experimental data from the MU radar in Japan is presented.

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

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

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.