SOUSY VHF Radar Research Articles

The radar tropopause at 78°N, 16°E: Characteristics of diurnal variation

A continuous 5 year time series of tropopause altitude made by the SOUSY VHF radar on Svalbard (78°N, 16°E) is examined for diurnal variation. The data have a nominal time resolution of 1 h, allowing for investigation of intraday timescales not normally possible using radiosonde observations. Periodicities identified in tropopause altitude are found to vary with season. Surface air temperature measurements naturally exhibit diurnal variations which depend on season: however, it is demonstrated that, using a simple scenario of constant adiabatic lapse rate in the troposphere, the tropopause altitude variation that might be anticipated from that of the surface air temperature is not in agreement with the radar observation. Of several mechanisms that could be postulated for this discrepancy, it is demonstrated that a likely candidate is intraday top‐down forcing from the stratosphere.

Tropopause height at 78° N 16° E: average seasonal variation 2007–2010

Abstract. We present a seasonal climatology of tropopause altitude for 78° N 16° E derived from observations 2007–2010 by the SOUSY VHF radar on Svalbard. The spring minimum occurs one month later than that of surface air temperature and instead coincides with the maximum in ozone column density. This confirms similar studies based on radiosonde measurements in the arctic and demonstrates downward control by the stratosphere. If one is to exploit the potential of tropopause height as a metric for climate change at high latitude and elsewhere, it is imperative to observe and understand the processes which establish the tropopause – an understanding to which this study contributes.

Resonant nonlinear interactions between atmospheric waves in the polar summer mesopause region

Data obtained from the mobile SOUSY VHF radar at Andoya/Norway in summer 1987 have been used to study the nonlinear interactions between planetary waves, tides and gravity waves in the polar mesosphere, and the instability of background atmosphere above the mesopause. It is observed that 35-h planetary wave, diurnal, semidiurnal and terdiurnal tides are the prominent perturbations in the Lomb-Scargle spectra of the zonal wind component. By inspecting the frequency combinations, several triads are identified. By bispectral analysis it is shown that most bispectral peaks stand for quadratic coupling between tidal harmonics or between tide and planetary or gravity wave, and the height dependence of bispectral peaks reflects the variation of wave-wave interactions. Above the mesopause, the occurrence heights of the maximum L-S power spectral peaks corresponding to the prominent wave components ten to increase with their frequencies. This may result from the process in which two low frequency waves interact to generate a high frequency wave. Intensities of the planetary wave and tides increase gradually, arrive at their maxima, and then decay quickly in turn with increasing height. This kind of scene correlates with a “chain” of wave-wave resonant interactions that shifts with height from lower frequency segment to higher frequency segment. By instability analysis, it is observed that above the mesopause, theRichardson number becomes smaller and smaller with height, implying that the turbulent motion grows stronger and stronger and accordingly the background atmosphere more and more instable. It is suggested that the wave-wave sum resonant interaction and the wave dissipation due to instability are two dominant dynamical processes that occur in the mesopause region. The former invokes the energy transfer from lower frequency waves to higher frequency waves. The latter results in the heating of the atmosphere and accelerating of the background flow.

Short-term variability and temporary structures of tides and mean wind in the polar summer mesosphere

Data obtained from the mobile SOUSY VHF radar at Andoya/Norway (69°N, 16°E) during the MAC/SINE campaign in summer 1987 have been used to study the short-term variability of tides and mean wind in the polar mesosphere. It is shown that the spectral peak corresponding to the semidiurnal tide dominates and persists almost uninterruptedly in time. The spectral peak corresponding to the diurnal tide is usually secondary and interrupted in time. The terdiurnal tide rarely presents a prominent spectral peak. The frequency corresponding to each tidal component often shows a deviation from its definition value. This likely represents a biased or perturbed state of the tide. The frequency fluctuation of the semidiurnal tide is usually smaller than those of the diurnal and terdiurnal tides. At some heights, the time variation of the semidiurnal tide amplitude is similar to that of the 36-h mean wind, which may be ascribed to the nonlinear interaction between the tide and planetary waves. But for most heights, there is no clear correlation between the tide and the mean wind. The time variations in either of the zonal and meridional mean winds for two heights with a distance of 2.7 km show a considerable similarity, but the time variation trend in the zonal mean wind is distinct from that in the meridional mean wind, indicating an anisotropy in the variability of the horizontal mean wind. The time variations of the semidiurnal tidal amplitude in the zonal wind for two heights with a distance of 2.7 km exhibit a considerable difference, but they present somewhat similarity in the meridional wind. This is a manifestation of the anisotropy and localization in the tidal variability. Since the frequency values of each tidal component in the zonal and meridional winds are usually different, the hodograph of its wind vector with respect to time is no longer a closed ellipse in general, but still has a clockwise rotation. The semidiurnal tidal wind vector shows a clockwise rotation with increasing height, indicating a downward phase progression. The diurnal tide wind vector does not display certain rotation with height, implying that it is not a travelling wave in the vertical.

Boundary-layer convection and diurnal variation of vertical-velocity characteristics in the free troposphere

During the field experiment SOMARE-99 (Sounding System Multifrequency Atmospheric Radar Experiment 1999), the troposphere above the Harz Mountains in Northern Germany was observed with the SOUSY VHF radar, with four microbarographs, and with 26 radiosondes launched every 3 h from nearby the radar site. This paper examines the 72 h period between 0000 UTC 25 May and 0000 UTC 28 May. Time series, root-mean-square values, and power spectra of radar-observed vertical-wind, w, fluctuations in the free troposphere and of pressure, p, fluctuations at the surface are presented and discussed. A pronounced diurnal cycle was observed: standard deviations (from 1 h periods) were about 10 cm s−1 (w) and 2 Pa (p) at night and up to 60 cm s−1 and 6 Pa during daytime. This is seen as evidence for the relevance of boundary-layer convection for short-period gravity waves in the free troposphere. Frequency spectra of w show a power-law drop-off beyond the Brunt-Väisälä frequency with logarithmic slopes close to -5/3 at moderate and strong wind speeds and down to -3 or -4 at weak winds (smaller than 5 m s−1), supporting earlier work on the effects of Doppler shift on Eulerian gravity-wave spectra. The w spectra, which have been calculated from 8 h long w time series (sampling period 1 min), have a noise floor of 10−3 m2s−2Hz−1 or less. This corresponds to an uncorrelated noise with a standard deviation of 3 mm s−1 for the 1 min samples. The concept of gravity-wave intermittency is introduced, in analogy to a well-established concept in turbulence physics. It is demonstrated how this concept can be used to distinguish turbulence from gravity waves empirically.

Boundary‐layer convection and diurnal variation of vertical‐velocity characteristics in the free troposphere

AbstractDuring the field experiment SOMARE‐99 (Sounding System Multifrequency Atmospheric Radar Experiment 1999), the troposphere above the Harz Mountains in Northern Germany was observed with the SOUSY VHF radar, with four microbarographs, and with 26 radiosondes launched every 3 h from nearby the radar site. This paper examines the 72 h period between 0000 UTC 25 May and 0000 UTC 28 May. Time series, root‐mean‐square values, and power spectra of radar‐observed vertical‐wind, w, fluctuations in the free troposphere and of pressure, p, fluctuations at the surface are presented and discussed. A pronounced diurnal cycle was observed: standard deviations (from 1 h periods) were about 10 cm s−1 (w) and 2 Pa (p) at night and up to 60 cm s−1 and 6 Pa during daytime. This is seen as evidence for the relevance of boundary‐layer convection for short‐period gravity waves in the free troposphere. Frequency spectra of w show a power‐law drop‐off beyond the Brunt‐Väisälä frequency with logarithmic slopes close to ‐5/3 at moderate and strong wind speeds and down to ‐3 or ‐4 at weak winds (smaller than 5 m s−1), supporting earlier work on the effects of Doppler shift on Eulerian gravity‐wave spectra. The w spectra, which have been calculated from 8 h long w time series (sampling period 1 min), have a noise floor of 10−3 m2s−2Hz−1 or less. This corresponds to an uncorrelated noise with a standard deviation of 3 mm s−1 for the 1 min samples. The concept of gravity‐wave intermittency is introduced, in analogy to a well‐established concept in turbulence physics. It is demonstrated how this concept can be used to distinguish turbulence from gravity waves empirically.

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.

The Coupling of Vertical Velocity and Signal Power Observed with the SOUSY VHF Radar

Abstract The perturbations to the static stability (and hence to the radar reflectivity) and to the velocity in a vertically propagating gravity wave are correlated, and the sign of the correlation depends on whether the wave is propagating upward or downward. The wave-induced correlation between radar reflectivity and vertical velocity is the basis of a hypothesis to explain the downward bias in long-term averages of the vertical velocity seen at extratropical sites by wind profiler radars, and for predictions of biases in the horizontal wind speeds and in the vertical momentum flux seen by profiler radars. In this study, the hypothesis that mean vertical velocity is related to the correlation between perturbations to vertical velocity and signal power is tested. Observations with very high time and vertical resolution from the SOUSY VHF radar are used. It is found that the mean vertical velocity in the midtroposphere (2.4–6.3 km) is downward (upward) when the perturbations to vertical velocity and to ba...

VHF radar observations of turbulent structures in the polar mesopause region

Abstract. The mobile SOUSY VHF Radar was operated in the summer of 1987 during the MAC/SINE campaign in northern Norway to study the polar mesosphere summer echoes (PMSE). Measurements of the spectral width indicate that two types of structures occur. In general mesospheric layers are bifurcated exhibiting a narrow spectral width and a well-defined aspect sensitivity. However, for about 10% of the observation time cells of enhanced turbulence characterized by extremely broad spectral widths appear predominantly in the upper sublayer above 86 km. Identification and separation of beam and shear broadening allows a determination of the turbulence-induced component of the spectral width. This case study reveals that during several events these cloud-like structures of enhanced turbulence move with an apparent velocity of several tens of meters per second which is almost identical with the phase trace velocity of simultaneously observed waves. Since, at that time, the Richardson number was less than a quarter, it was concluded that these turbulent cells were generated by a Kelvin-Helmholtz mechanism. The horizontal extent of these structures was calculated to be less than 40 km. A general relation between spectral width and echo power was not detected. The turbulent component of the spectral width was used to calculate typical values of the energy dissipation rate at times where narrow spectral width dominates and during periods of enhanced turbulence. In addition, the outer scale of the inertial subrange (buoyancy scale) was estimated. For the first time the occurrence and motion of this type of structures of enhanced spectral width is analyzed and discussed in detail.

VHF radar observations of turbulent structures in the polar mesopause region

The mobile SOUSY VHF Radar was operated in the summer of 1987 during the MAC/SINE campaign in northern Norway to study the polar mesosphere summer echoes (PMSE). Measurements of the spectral width indicate that two types of structures occur. In general mesospheric layers are bifurcated exhibiting a narrow spectral width and a well-defined aspect sensitivity. However, for about 10% of the observation time cells of enhanced turbulence characterized by extremely broad spectral widths appear predominantly in the upper sublayer above 86 km. Identification and separation of beam and shear broadening allows a determination of the turbulence-induced component of the spectral width. This case study reveals that during several events these cloud-like structures of enhanced turbulence move with an apparent velocity of several tens of meters per second which is almost identical with the phase trace velocity of simultaneously observed waves. Since, at that time, the Richardson number was less than a quarter, it was concluded that these turbulent cells were generated by a Kelvin-Helmholtz mechanism. The horizontal extent of these structures was calculated to be less than 40 km. A general relation between spectral width and echo power was not detected. The turbulent component of the spectral width was used to calculate typical values of the energy dissipation rate at times where narrow spectral width dominates and during periods of enhanced turbulence. In addition, the outer scale of the inertial subrange (buoyancy scale) was estimated. For the first time the occurrence and motion of this type of structures of enhanced spectral width is analyzed and discussed in detail.

Field campaign for the comparison of SOUSY radar wind measurements with rawinsonde and model data

Abstract. A field campaign was carried out from 26 October to 7 November 1992, using the SOUSY-VHF radar and a mobile rawinsonde system installed and operated nearby to produce vertical wind profiles. The purpose of this campaign was to compare the two types of wind measurements with one another and with results from forecast models. Numerical algorithms were developed and applied to the radar data in order to eliminate random errors, correct for velocity aliasing, and calculate the effective zenith angle of the off-vertical beams. Differences between wind profiler data and rawinsonde or model results depend not only upon the errors of the different systems, but also on temporal and spatial variations of the wind field. Therefore, methods for the comparison of radar and rawinsonde data were developed which take into consideration these variations. The practical potential of these methods is demonstrated by comparisons of rawinsonde and radar wind profiles. The comparison of radar data and model output shows excellent agreement in the direction and in the speed of the wind at virtually all altitudes. An evaluation of the quality of wind profiler measurements is possible using the estimation of variance and variability of wind components.

Field campaign for the comparison of SOUSY radar wind measurements with rawinsonde and model data

A field campaign was carried out from 26 October to 7 November 1992, using the SOUSY-VHF radar and a mobile rawinsonde system installed and operated nearby to produce vertical wind profiles. The purpose of this campaign was to compare the two types of wind measurements with one another and with results from forecast models. Numerical algorithms were developed and applied to the radar data in order to eliminate random errors, correct for velocity aliasing, and calculate the effective zenith angle of the off-vertical beams. Differences between wind profiler data and rawinsonde or model results depend not only upon the errors of the different systems, but also on temporal and spatial variations of the wind field. Therefore, methods for the comparison of radar and rawinsonde data were developed which take into consideration these variations. The practical potential of these methods is demonstrated by comparisons of rawinsonde and radar wind profiles. The comparison of radar data and model output shows excellent agreement in the direction and in the speed of the wind at virtually all altitudes. An evaluation of the quality of wind profiler measurements is possible using the estimation of variance and variability of wind components.

Mesosphere and lower thermosphere (80–120 km) winds and tides from near Troms0 (70°N, 19°E) : comparisons between radars (MF, EISCAT, VHF) and rockets

Winds and tides from the MF radar near Tromsø are first compared with data from the SOUSY VHF radar near Andenes (69°N, 16°E) and from rocket chaff and falling sphere systems, both obtained during SINE (Summer in Northern Europe; 7 June–19 July 1987). Wind directions and tidal phases are in excellent agreement, but magnitudes and amplitudes consistently demonstrate that the MF winds are reduced by 35%. This is associated with reduced signal to noise for the MF radar scatter due to minimal systems (loops) used for the spaced antennas. The adjusted winds and tides are then compared with data obtained from the co-located EISCAT and the Saskatoon MF radars, for a total of six campaigns (1987–1989) covering each season of the year. The data almost overlap near 100 km for three campaigns and do overlap from 90 to 100 km for the other three. There is, in general, excellent agreement between the incoherent scatter and MF radars, and clear seasonal variations are revealed. Differences are attributed to low signal to noise conditions for either radar, lack of complete data overlap, and strong wind gradients in the lower thermosphere.

VHF radar observation of gravity wave critical layers in the mid‐latitude summer mesopause region

Observations with the SOUSY VHF Radar in the mesopause region during summer 1989 show layers of strong echo intensity and wind speeds increasing with height. Occasionally, quasi‐monochromatic internal gravity waves occur in the bottom side of the echo layer and vanish in the height interval around the echo intensity maximum which tends to increase with increasing wave amplitude. The data analysis can be performed using a Boussinesq and a WKB approximation. It shows that the horizontal phase trace velocity of the observed gravity waves is equal to the background wind velocity at the height of the echo intensity maximum. The observations thus represent nonlinear gravity wave‐critical level encounters producing strong turbulence in the neutral gas. But the estimated energy dissipation rates are too small to produce neutral gas turbulence at scales equal to half the radar wavelength yielding an additional indication for a yet unidentified mechanism generating small‐scale structures in the electron gas.

VHF radar echoes observed in the summer and winter polar mesosphere over Andøya, Norway

The mobile SOUSY VHF (53.5 MHz) Radar has been operated on a campaign basis on the island of Andøya in Northern Norway (69°17′N, 16°01′E), with the aim of investigating the backseattering structures and dynamics of the polar middle atmosphere, since November 1983. During winter the occurrence of mesospheric echoes at altitudes from 50 to 90 km is strongly correlated with radio wave absorption events measured with a riometer on 32.5 MHz. Individual structures are observed for up to 8 hours and, in general, appear as thin layers separated by 3–5 km, with typical maximum signal‐to‐noise ratios (SNRs) of 5–25 dB. Many of these are related to variations in the background wind produced by long‐period gravity waves and tidal period motions and, in as much, are similar to mesospheric layers at middle and low latitudes. In summer the echo region is largely restricted to a height interval from 75 to 95 km, with 80% of the echoes occurring in the 80‐ to 93‐km height range, and on average, it has a distinct maximum in SNR near 86 km. In the 83‐ to 91‐km height region, echoes are detected almost continuously, with a minimum occurrence rate of 85%. There is a low correlation (∼0.26) between echo strength and absorption. Many of the weaker echoing regions, those with SNRs up to 30 dB, are related to the background wind shear. Spectral analysis of time series of both SNR and zonal velocity for a period of 9 days reveals 54‐, 24‐, 12‐, and 8‐hour components, with the 12‐hour component dominating. Strong bursts in backscattered power tend to occur in the late afternoon and early morning hours, so they coincide with the time of maximum westward velocity of the semidiurnal tide. This suggests that the bursts are due to an increase in the turbulent intensity produced by dynamical instability of this mode. Layers consistent with dynamically unstable upward propagating inertio‐gravity waves are also often observed to move down through the 93‐ to 80‐km height range. Thus dynamical instability of the semidiurnal tide and long‐period gravity waves appears to play a major role in determining much of the structure observed in the backscatter region. However, the strongest summer polar mesopause echoes, those with peak SNRs greater than 30 dB, while showing modulation due to the background winds, often show no clear relation to the background wind shear. This supports the view that the production mechanism of the very strong SNRs associated with this layer is different in character from that of winter echoes observed in the polar mesosphere by VHF radars, and to those mesospheric layers observed at mid‐latitudes using the same technique.

Multibeam radar observations of winds in the mesosphere

We present the results of a statistical analysis of mesospheric wind fluctuations with periods in the range 5 min to 8 hours. The data were collected with the SOUSY VHF radar by observing line‐of‐sight Doppler velocities sequentially in three directions. The temporal spectrum is computed for the vertical fluctuations as well as for line‐of‐sight fluctuations on two oblique beams at 3.6° zenith angle separated by 90° in azimuth. The observed spectra are shown to be mutually consistent if interpreted in terms of a field of propagating gravity waves. Using the three‐beam data, the temporal spectrum for the underlying gravity waves is obtained; the height dependence of the line‐of‐sight component of the mean square wave amplitudes is also studied.

VHF radar observations of tides at polar latitudes in the summer mesosphere

Doppler measurements at mesospheric heights were carried out in June 1984 using the mobile SOUSY VHF radar at Andenes/Norway (69°N, 16°E). The temporal wind variations at heights between 80 and 95 km reveal wave motions with periods of about 12 h, 14.5 h, 24 h and 54 h. The propagating semi-diurnal tide has amplitudes up to 20 m s −1 and a vertical wavelength of about 30 km. The diurnal tide is much weaker and the phase shows a more complex variation with height. The 14.5 h period motion reveals a maximum amplitude of 15 m s −1 at about 84 km and a vertical wavelength of 15 km.

VHF radar measurements of the aspect sensitivity of the summer polar mesopause echoes over Andenes (69°N,16°E), Norway

The mobile SOUSY VHF (53.5 MHz) radar, located near Andenes on the island of Andoya in Northern Norway, has been used to measure the aspect sensitivity of the very strong radar returns obtained from near the summer polar mesopause. This so‐called Polar Mesopause Summer Echo (PMSE) exhibits substantial aspect sensitivity, with angular polar diagrams of the backscatter of 2‐10° half‐width for a two day average. During one three hour period, in which the power received in the antenna vertical beam was particularly intense, the grating lobes of the antenna polar diagram were utilized as additional beams, and the power returned at angles of 35 and 38° off‐zenith was obtained. These measurements provide an estimate of the ratio of the specular to the turbulent contributions to the backscatter.

VHF radar measurements of momentum flux in the summer polar mesosphere over Andenes (69°N,16°E), Norway

The mobile SOUSY VHF (53.5 MHz) radar, located near Andenes on the island of Andoya in Northern Norway, has been used to measure the upward flux of horizontal momentum per unit mass associated with gravity waves and turbulence near the summer polar mesopause. Here we describe the experiment and present some preliminary results. When averaged over a period of about three hours, magnitudes of are found to reach a maximum value of 66 m² s−2 near the peak of the so‐called Polar Mesopause Summer Echo (PMSE). The PMSE itself appears to exhibit substantial aspect sensitivity.

Systematic behavior of signal statistics of MST radar echoes from clear air and their interpretation

Signal statistics of data from the SOUSY‐VHF radar were analyzed. Mean, variance, skewness and kurtosis of the distribution of the quadrature components of the signals as well as Nakagami m coefficient of the amplitude distribution were calculated. We found a systematic behavior in the correlation between kurtosis and Nakagami coefficient, and we propose a stochastic model for its explanation. These stochastic effects on the signal are generated in the variable echoing process itself. Numerical simulation and analytical calculation of the model were carried out for comparison with the data, and the results are encouraging.