The Plan Shear Indicator (PSI) is a new mode for display of meteorological Doppler radar information which may prove to be valuable in identification of hazardous winds and turbulence in storms. It provides real-time location and convenient highlighting of regions within precipitation echoes of abnormally large shear and spectral broadening of the Doppler velocity. The real-time capability of the PSI and its high data rate are achieved at the cost of mediocre resolution of ranges and velocities, and the ability to display only the shear and not the absolute value of wind components along the Doppler beam.
The PSI display for a scanning Doppler radar utilizes an ordinary PPI scope intensity-modulated by a coherent memory filter. The resultant pattern is a series of concentric arcs, each one located on the scope at its appropriate range plus an incremental displacement which depends on the radial component of velocity at that range. Radial shear is indicated by gaps or bunching of the arcs, while tangential shear, as, for example, a vortex, is indicated by wrinkles in the arcs. Turbulence on a scale smaller than measurable wind shear would broaden the Doppler spectrum and would be revealed on the PSI by an increase in line widths of the arcs. The instrument has been used during an outbreak of severe thunderstorms in New England, and appeared to be successful in identification of storms which produced damaging winds and hail.