THE NERC MST RADAR FACILITY AT ABERYSTWYTH
Wind-profiling radar systems can be designed to operate at frequencies anywhere between 40 and 1400 MHz. In practice, however, they are restricted to frequencies around 50, 400 and 1000 MHz; the principles of operation are similar in all three cases. The most common technique is known as Doppler Beam Swinging (DBS) which involves making observations in a cyclic sequence of vertical and near-vertical beam pointing directions. The 'targets', from which small fractions of the pulsed radar signals are returned, are irregularities of atmospheric refractive index, which cause back-scattering (so-called 'clear-air' returns), and hydrometeors, which give rise to Rayleigh scaterring. The scattered signal is Doppler-shifted according to the radial component of the target's velocity, i.e. that along the radar beam pointing direction. In order to derive the full three-dimensional wind vector, observations must therefore be made in a minimum of 3 non-coplanar beam pointing directions; a typical sequence includes observations made in the vertical direction and at an off-vertical angle of between 5° and 20° in two orthogonal azimuths. Profiling is achieved by sampling the radar return signals as a function of delay from the time of the transmitted pulse; the transmitted pulse length determines the range resolution.
The choice of radar operating frequency has a number of important consequences:
- Significance of the vertical velocity: at frequencies
around 50 MHz, although strong precipitation is detectable,
clear-air returns always dominate over hydrometeor returns; at
around 400 MHz hydrometeor returns dominate for anything more than
light rain; at around 1000 MHz hydrometeor returns dominate whenever
precipitation is present. Since hydrometeors only share the
horizontal components of velocity with the wind, it is not always
possible to determine the vertical velocity of the atmosphere from
observations made at frequencies around 400 and 1000 MHz; the
vertical velocities derived from the radar observations represent
the fall-speeds of the hydrometeors.
Antenna dimensions: in order to restrict the horizontal extent from which radar signals are returned, wind-profiling systems require narrow angular beam widths; 1° corresponds to 175 m at a range of 10 km. The beam width is related (inversely proportional) to both the horizontal dimensions of the antenna and the frequency of operation. For 50 MHz radars, typical antenna dimensions and (one-way half-power half-) beam-widths are of the order of 100 m and 1.5°, respectively; for 400 MHz radars they are 10 m and 3°, respectively; for 1000 MHz radars they are 2 m and 5°, respectively. Largely for reasons of cost, antenna arrays (composed of a large number of individual aerials) are used rather than steerable parabolic dishes; beam steering is achieved electronically by altering the phase of the signals fed to and from different parts of the array.
The maximum observable altitude: this decreases with increasing frequency and is typically 20 km at 50 MHz, 10 km at 400 MHz and 3 km at 1000 MHz. It is determined in part by the nature of the scattering mechanisms and in part by a combination of the transmitter power and antenna dimensions. The minimum observable altitude also decreases with increasing frequency and is typically 2 km at 50 MHz, 0.5 km at 400 MHz and 0.1 km at 1000 MHz; this is determined largely by the differences in hardware design at the different frequencies. For the above reasons, profilers operating around 50 and 400 MHz are called Stratosphere-Troposphere (ST) radars (or MST radars in the case of 50 MHz radars which are sufficiently powerful to detect the sporadic and weak returns from the Mesosphere), whereas those operating around 1000 MHz are called boundary-layer wind-profilers.
The variability of altitude coverage as a function of time: owing to the fact that hydrometeor returns become increasingly significant with increasing radar frequency, the maximum observable altitude at 1000 MHz can increase suddenly from 3 to 8 km with the onset of precipitation. Moreover, the wind-profiles can be discontinuous even below 3 km owing to the fact that radar return signals at these frequencies can contain contributions from a number of unwanted targets such as insects, birds, aircraft, trees and power lines; the latter two are detected through the beam's sidelobes and so do not need to be located along the main lobe's pointing direction. At 50 MHz the only unwanted targets are aircraft and the changes in altitude coverage at are far less dramatic. The radar return power typically decreases with increasing altitude and the signals are often close to the point of detectability within the 15 - 20 km altitude region leading to discontinuities in the wind-profiles. The radar return power typically shows a step increase at the tropopause level and the signals can also be on the point of detectability just below this, especially when the tropopause is above 11 km.
Wind profiler radar returns are parameterised by their signal powers and spectral widths (i.e. the variance of scatterer velocities about the mean) in addition to their Doppler shifts. This information can be used, under certain circumstances, to provide additional information about the atmospheric static stability (thus allowing monitoring of the altitude and sharpness of the tropopause), humidity fields and turbulence (of at least moderate intensity).