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Schematic diagram showing a 5-beam radar arrangement, and scattering layers tilted in a N±S direction, but not E±W, which would give an echo power imbalance between N and S beams, but not between E and W beams
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A simple method is described, based on standard VHF wind-profiler data, where imbalances of echo power between four off-vertical radar beams, caused by mountain waves, can be used to calculate the orientation of the wave pattern. It is shown that the mountain wave azimuth (direction of the horizontal component of the wavevector), is given by the ve...
Contexts in source publication
Context 1
... that, even in Fig. 1b where the layers are on average horizontal, there are small regions with larger tilt angles from horizontal; according to WT97, there is a small specular contribution to radar echoes from beams with large zenith angles, e.g. 12 , implying that some scattering layers are tilted to similarly large angles from horizontal. Figure 2 shows a typical radar 5-beam arrangement, with four beams pointed o-vertical by an equal angle towards north, south, east and west. The beams probe a region of atmosphere where scattering layers are tilted in a N±S direction by an angle d from horizontal, but are not tilted in the E±W direction. ...
Context 2
... in the E±W direction. This situation could occur if there is a mountain wave pattern above the radar, with its horizontal wave vector in the N±S azimuth, i.e. its phase fronts and cloud bands aligned E± W. The magnitude and sign of d depend on the phase of the region of mountain wave above the radar; d can be positive, negative or zero, and Fig. 2 represents a `snapshot' where d takes one particular (non-zero) ...
Context 3
... power in Fig. 2 would be increased in the S beam, and decreased in the N beam, compared to the radar echoes from horizontal scattering layers, as discussed for Fig. 1. The power imbalance (P N À P S ) would be negative (P N , P S , P E , P W are echo powers, measured in dB, for the beams pointed towards north, south, east and west respectively.) In ...
Context 4
... radar beam pointed away from the zenith, towards north (e.g. thèN' beam in Fig. 2), has the unit vector 0Y sin hY cos h, where h is its zenith angle. Cartesian co- ordinates (xY yY z) are used, with positive x towards east, positive y towards north, and positive z upwards. For a tilted plane, with its north side inclined upward from the horizontal by an angle d, and then rotated (about a vertical axis) through an ...
Context 5
... hY cos h, where h is its zenith angle. Cartesian co- ordinates (xY yY z) are used, with positive x towards east, positive y towards north, and positive z upwards. For a tilted plane, with its north side inclined upward from the horizontal by an angle d, and then rotated (about a vertical axis) through an azimuth angle / clockwise from north ( Fig. 2 shows the case where / 0), the unit vector perpendicular to the plane is À sin / sin dY À cos / sin dY cos d. The scalar product of these two unit vectors, À sin h cos / sin d cos h cos d, gives the cosine of the angle, H, between them. If the plane is that of the aspect-sensitive layers above the radar, having been tilted from ...
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Citations
... The angular variation of the echo strength has been attributed to either diffuse reflection from stable temperature sheet structure or the presence of corrugated sheets or anisotropic turbulence (Das et al., 2014). Azimuth angle variation has been attributed to the presence of tilting layers which are generated due to gravity waves Tsuda, Gordon, & Saito, 1997) or mountain waves (Worthington, 1999). Studies in these tilting layers have found small-scale Kelvin-Helmholtz instability (KHI) to be responsible for the redistribution of scatterers into a tilted layer (Das et al., 2008(Das et al., , 2016Ghosh et al., 2004;Worthington & Thomas, 1997). ...
This paper presents the first ever observations on aspect‐sensitive characteristics of 205 MHz stratosphere–troposphere (ST) radar located at a tropical station Cochin (10.04°N, 76.3°E) using volume scanning. The most significant and new observation is that the signal‐to‐noise ratio in zenith and off‐zenith beams are nearly equal in some height region, indicating the presence of isotropic turbulence. Signal strength decreases by 0.75 dB per degree from 0 to 10 degree off‐zenith, 0.9 dB per degree from 10 to 20 degree off‐zenith and 0.3 dB per degree beyond 20 degree off‐zenith. Different causative mechanisms are discussed on the basis of various estimated parameters associated with aspect sensitivity. Maximum aspect sensitivity is observed between 12 and 17 km, indicating the presence of dynamic instability arising due to strong wind shear and atmospheric stability. When both the square of wind shear and stability parameters are above 0.25 × 10⁻³ s⁻², the scatterers become mostly isotropic. The study also shows a power difference in the symmetric beams as well as azimuth angle dependency. Analysis suggests that this asymmetry is due to the tilting of layers by the action of atmospheric gravity waves generated through Kelvin‐Helmholtz instability. The present configuration of radar can provide a better understanding of three‐dimensional structures of turbulence and instabilities.
... However, even at night the azimuth distribution of mountain-wave horizontal wavevector (i.e. horizontally perpendicular to phase lines or cloud bands, pointing upwind) is centred between the azimuths of the surface and free tropospheric wind, for fairly isotropic mountains (Worthington 1999aWorthington , b, 2006), needing some further explanation. One possibility is that the turbulent non-convective surface layer acts as an 'effective mountain' a few hundred metres higher than the actual mountains. ...
Mountain waves can be assumed to be forced directly by mountains, neglecting the boundary layer (type 1) or indirectly by mountain boundary-layer turbulence and convection (type 2). This study investigates if the two types of mountain wave can be identified objectively. Data are from the Aberystwyth meso-strato-troposphere (MST) radar, and advanced very high resolution radiometer. Mountains around the MST radar are fairly low and isotropic, except for the Cadair Idris ridges. Waves downwind of these mountains, previously simulated by a type 1 numerical model, appear to be type 1 in reality. However, another case study shows interacting convective rolls and mountain waves (type 2) even above Cadair Idris, as also observed occasionally at high, ridge-like Cross Fell. Over seven years of MST radar data on wave alignment, compared to surface wind, show type 2 waves for most wind directions. However, there is a deviation for near-northerly winds consistent with the Cadair Idris ridges producing type 1 waves. For other wind directions, wave alignment is examined as a function of boundary-layer temperature gradient and surface wind speed, showing little dependence on either, which is similar to the Ekman rotation’s lack of dependence. It is suggested that not only Ekman rotation but also type 2 mountain waves involve the ubiquitous turbulent surface layer, which acts as an effective mountain a few hundred metres higher than the actual mountain, even in the absence of convection, and commonly causes type 2 mountain waves.
... Variations of jwj with percentage precipitation inFig. 3 are assumed to refer to mountain waves and not other gravity waves.Figure 4 , therefore, checks another wave characteristic , horizontal wavevector azimuth measured by MST radar (Worthington 1999a); wave azimuth near 90° to the wind would be inconsistent with mountain waves because of critical layer absorption. Measurements are for 5–15 km height and 30 min time intervals with jwj [ 0:05 ms À1 and azimuth error \25°. ...
Meso-Strato-Troposphere and weather radars are used to show the effect of saturated air near the ground on mountain waves
in the troposphere and lower stratosphere, at 52.4°N, 4.0°W. Mountain waves are observed above scattered precipitation; however,
long-term observations confirm here that wave amplitude is reduced above extensive precipitation, as predicted from numerical
models. Ceilometer measurements of average cloud base near the mountain tops suggest that saturated air could be reducing
the generation of mountain waves, in addition to trapping or absorbing waves.
... The observed structure matches with the vertical wind profile model reported in Worthington et al. (2001) (Fig. 1c) and can be explained by model 5 of his paper. This model considers a real residual mountain wave component since the phase of mountain waves above the radar is not completely random (Worthington, 1999 ). The Indian MST radar, located at Gadanki, is surrounded by hills with a maximum height of 500–1000 m above the mean sea level. ...
MST radars are capable of measuring vertical motion along a vertically directed beam. We present 8 years (1995–2003) averaged profile of vertical velocity in the troposphere and the lower stratosphere over Gadanki (13.5° N, 79.2° E), a tropical station. A downward mid-tropospheric w is observed with a reversal of sign around 10 km and a further reversal can also be seen at ~17 km. A significant diurnal and semidiurnal variation in vertical wind is observed for all heights with subsidence during the evening hours. Seasonal variability of vertical wind is also found to be quite appreciable. Vertical velocities have been derived using symmetric pairs of off-vertical beams and a comparison has been made with direct vertical beam measurements. Vertical components estimated from E-W and N-S radial velocities do not match and are also found to have discrepancy with direct measurements. Plausible causes of the discrepancy have been investigated with the help of some case studies. Vertical shear in horizontal wind, gradients in horizontal velocities and echo power imbalance may be some of the factors responsible for the observed discrepancy.
... Either of these is possible under conditions of short-period gravity wave activity. Although the typical horizontal wavelengths of mountain waves, 10–30 km (e.g. Worthington, 1999), are much larger the separation between the radar observation volumes, those of convectively-generated gravity waves can be in the range 2.5–8.5 km (e.g. Hauf, 1993). ...
... This can be caused by the tilting of isentropes by wave activity (e.g. Worthington and Thomas, 1996b; Worthington, 1999) or by Kelvin-Helmholtz instabilities (e.g. Muschinski, 1996). ...
This paper describes a new signal processing scheme for the 46.5 MHz Doppler Beam Swinging wind-profiling radar at Aberystwyth, in the UK. Although the techniques used are similar to those already described in literature - i.e. the identification of multiple signal components within each spectrum and the use of radial- and time-continuity algorithms for quality-control purposes - it is shown that they must be adapted for the specific meteorological environment above Aberystwyth. In particular they need to take into account the three primary causes of unwanted signals: ground clutter, interference, and Rayleigh scatter from hydrometeors under stratiform precipitation conditions. Attention is also paid to the fact that short-period gravity-wave activity can lead to an invalidation of the fundamental assumption of the wind field remaining stationary over the temporal and spatial scales encompassed by a cycle of observation. Methods of identifying and accounting for such conditions are described. The random measurement error associated with horizontal wind components is estimated to be 3.0 - 4.0 m s-1 for single cycle data. This reduces to 2.0 - 3.0 m s-1 for data averaged over 30 min. The random measurement error associated with vertical wind components is estimated to be 0.2 - 0.3 m s-1. This cannot be reduced by time-averaging as significant natural variability is expected over intervals of just a few minutes under conditions of short-period gravity-wave activity.
... 43, RS4020, doi:10.) and downward in the deeper stratosphere—have also been verified [Worthington, 1999; Worthington et al., 2001]. Even a reversal of mean vertical velocities in the middle troposphere was once observed in the tropical zone [Jagannadha Rao et al., 2002]. ...
Clear-air VHF/UHF radars worldwide have observed that long-term mean
vertical winds are downward in the middle troposphere and usually
slightly upward above the jet-stream wind maximum. Kelvin-Helmholtz
instability (KHI), which can tilt quasi-specular layers in opposite
orientations above and below the jet-stream wind maximum, has been
postulated to be one important contributing factor to the radar-measured
mean vertical wind (?r). This factor is examined here using
simultaneous radar interferometric observations of echo centers and
layer structures. The altitude of layer structure and the incident angle
of echo center were estimated, respectively, with multiple-frequency and
multiple-receiver techniques. Radar data were collected with the
Japanese MU radar, between 3 km and 22.2 km and over 33 h. The
observations of ?r showed downward tendency in the middle
troposphere, with a maximum of ˜10 cm/s at the height of ˜8
km. However the reversal height of ?r was at ˜15 km,
which is higher than the jet-stream wind maximum observed (˜12
km). Positive correlations between the vertical velocities
(wr) and the incident angles of echo centers were found in
the region of downward ?r, and moreover, the mean vertical
velocities derived from the incident angles of echo centers below
˜10 km were close to ?r. Statistical distributions of
layer slopes, incident angles of echo centers, and echo power imbalance
between two symmetrically oblique radar beams provide evidence of
asymmetrically tilted layer structures in the region of downward
?r, suggesting that wind-shear tilted/KHI layers contributed
a significant part of ?r in the middle troposphere.
... Echo power of VHF atmospheric radar is often increased in beams pointing near zenith ( " aspect sensitivity " ), because of thin stable layers or anisotropic turbulence. The diffuse glinting pattern of VHF echo power can also be tilted from zenith, when atmospheric layers are tilted from horizontal (Gage et al. 1981; Tsuda et al. 1997; Worthington 1999a), for instance by mountain waves. Since VHF aspect-sensitivity is caused by atmospheric stability layered on vertical scale ∼3 m, which is typically less than ∼0.2% of mountain-wave vertical wavelengths, thin glinting layers could act as tracer of air flow through a wave pattern, without modifying the waves. ...
... has been assumed a tracer, showing streamlines of air flow tilted from horizontal in mountain waves (Starr and Browning 1972 ). However, it is unclear if thin stable layers similarly follow the streamlines of mountain-wave airflow, which is an assumption for measurements of mountain-wave alignment and launching height (Worthington 1999aWorthington ,b, 2002). Vertical shear of horizontal wind also can cause skewing of the echo power pattern (Worthington and Thomas 1996; Yamamoto et al. 2003; Hirono et al. 2004). ...
... Vertical shear of horizontal wind also can cause skewing of the echo power pattern (Worthington and Thomas 1996; Yamamoto et al. 2003; Hirono et al. 2004). This paper uses a simple DBS (Doppler beam swinging) method of volume-imaging, to measure the tilting of VHF glinting patterns in three dimensions, and check if thin stable layers remain parallel to air flow in mountain waves (Worthington 1999aWorthington ,b, 2002).Figure 1 shows the MU radar in a 64-beam DBS mode; 63 beams are plotted since there is a duplicate vertical. The beam transmitted by the entire radar array switches direction every 400 µ s to volume-image the atmosphere, with range resolution 150 m and transmitted beamwidth 3.6 @BULLET . ...
Thin stable atmospheric layers cause VHF radars to receive increased echo power from near zenith. Layers can be tilted from horizontal, for instance by gravity waves, and the direction of VHF "glinting" is measurable by spatial domain interferometry or many-beam Doppler beam swinging (DBS). This paper uses the Middle and Upper atmosphere (MU) radar, Shigaraki, Japan as a volume-imaging radar with 64-beam DBS, to show tilting of layers and air flow in mountain waves. Tilt of aspect-sensitive echo power from horizontal is nearly parallel to air flow, as assumed in earlier measurements of mountain-wave alignment. Vertical-wind measurements are self-consistent from different beam zenith angles, despite the combined effects of aspect sensitivity and horizontal-wind gradients.
... Alignment of mountain waves can be measured, using the effect of tilted, thin stable atmospheric layers on VHF echo power (Worthington, 1999a, b). Mountain wave-vector alignment in Figure 9b is centred around 290–310 • , which is clockwise from the surface wind direction, and the lines of cloud-street temperature variation in Figure 3b are similarly rotated clockwise from 90 • to the surface-wind and cloud-street alignment. ...
Mountain waves are observed in the free atmosphere, even when the mountainboundary layer (the source region of these waves) is neutral or convectivelyunstable, and filled with convective rolls, revealed by cloud streets. This paperinvestigates if mountain waves are caused not simply by air flow over mountainridges, but also by flow over boundary-layer convective activity, similar toconvection waves above plains and oceans. Disturbance of stable air flow abovemountains by convective activity, mechanical and convective turbulence and rotors,near the ground, could move the effective mountain-wave launching height to higherthan the mountain peaks.
... Muschinski, 1996; Worthington and Thomas, 1997). These tilt effects can be suppressed or at least reduced by averaging over a long period (several hours), since the effective beam tends to be centered on the zenith, as shown by Worthington et al. (1999a,b, 2000a). However, if the tilts are a consequence of orographic effects, biases can occur also for long-term averages (Worthington et al., 2001). ...
... The isotropic level is thought to be detected for a zenith angle larger than ∼10–15 • (e.g. Green and Gage, 1980; Röttger et al., 1981; Tsuda et al., 1986; Hocking et al., 1990). However, some recent results obtained with a Beam-Scanning analysis (see appendix) emphasized that residual aspect sensitivity can occur at larger zenith angles with an azimuthal dependence up to 20 • (Tsuda et al., 1997a), or even 30 • in extreme cases (Worthington et al., 1999a,b). These results seem to indicate that a very large zenith angle should be used in order to completely suppress the aspect sensitivity effects at lower VHF. ...
... The maximum of echo power is observed overhead. In some azimuths, the isotropic level is not observed before 30 • off the zenith (after Worthington et al., 1999a). et al. (1999a,b, 2000a) presented horizontal distribution patterns of echo power at tropospheric heights using the Beam- Scanning technique with 320 directions. ...
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)
... Tsuda et al., 1997b). This may be caused by gravity waves tilting the aspect-sensitive scatterers, ®rst proposed by Gage et al. (1981), and further evidence of tilting caused by mountain waves is presented by Worthington (1999). VHF echo power patterns are also found to be skewed in regions of wind shear, as observed using the MST radar at Aberystwyth (Worthington andThomas, 1996 (Sect. ...
... This may be caused by gravity waves tilting the aspect-sensitive scatterers, ®rst proposed by Gage et al. (1981), and further evidence of tilting caused by mountain waves is presented by Worthington (1999). VHF echo power patterns are also found to be skewed in regions of wind shear, as observed using the MST radar at Aberystwyth (Worthington andThomas, 1996 (Sect. 4.2, 5), 1997) and the MU radar (Palmer et al., 1998;Worthington et al., 1999). ...
Using the MU radar at Shigaraki, Japan
(34.85°N, 136.10°E), we measure the power distribution pattern of VHF radar
echoes from the mid-troposphere. The large number of radar beam-pointing
directions (320) allows the mapping of echo power from 0° to 40° from zenith,
and also the dependence on azimuth, which has not been achieved before at VHF
wavelengths. The results show how vertical shear of the horizontal wind is
associated with a definite skewing of the VHF echo power distribution, for beam
angles as far as 30° or more from zenith, so that aspect sensitivity cannot be
assumed negligible at any beam-pointing angle that most existing VHF radars are
able to use. Consequently, the use of VHF echo power to calculate intensity of
atmospheric turbulence, which assumes only isotropic backscatter at large beam
zenith angles, will sometimes not be valid.Key words. Meteorology and atmospheric dynamics (middle
atmosphere dynamics; turbulence; instruments and techniques)
