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For this page:
INTERNAL LINKS
THE NERC MST RADAR FACILITY AT ABERYSTWYTH
WIND VECTOR DETERMINATION BY THE DOPPLER BEAM SWINGING TECHNIQUE
radial velocity geometry - fig 1 In the diagram to the left, i, j, k and r represent unit vectors in the East, North, vertical and radial directions, respectively. For a wind-profiler radar beam directed at a zenith angle of θ and at an azimuth angle of φ, the radial component of the wind vector, vR, i.e. that along the beam pointing direction, is related to the zonal, u (i.e. towards East), meriodonal, v (i.e. towards North), and vertical, w, components through the following expression:

equation for radial velocity

In order to determine the full three dimensional wind vector, observations must be made in a minimum of 3 non-coplanar beam directions.

radial velocity geometry - fig 2

A typical sequence includes observations made in the vertical direction and at an off-vertical angle (of around 17° for boundary-layer wind-profilers, or at 6° in the case of the NERC MST Radar) in two orthogonal azimuths; this is known as the Doppler Beam Swinging (DBS) technique. Since the the azimuths of the off-vertical beams are not, in general, aligned with the cardinal directions, it is useful to consider the problem in the frame of reference of a vertical plane along φ; the component of the horizontal wind vector in this plane, vH(φ), is given by:

equation for horizontal velocity in azimuth phi
radial velocity geometry - fig 3

The equation for the radial component of velocity simplifies to:

equation for radial velocity in Z-phi plane

In the case of boundary-layer wind-profilers, which typically make observations in only three beam pointing directions, the vertical velocity is determined from the Doppler shift of the vertically directed beam, and the horizontal component, in each azimuth, is determined through the relationship

equation for radial velocity in Z-phi plane

The NERC MST Radar typically makes observations at in the vertical direction and at 6° off-vertical in four different azimuths, each one separated by 90° from the next; this allows each component of velocity to be derived from three different combinations of the radial velocities. In addition to the combination described above for 3-beam observations, the radial velocities for off-vertical beams with opposite azimuths can either be added or subtracted to derive the vertical or horizontal components of velocity, respectively:

equation for radial velocity in Z-phi plane

Internal Links:
Return to the top of the page
An introduction to wind-profilers
Wind vector (direction) notation conventions
Page maintained by David Hooper
Last updated 15th August 2002