THE USE OF DUAL-CHANNEL CIRCULAR-POLARIZATION RADAR OBSERVATIONS FOR REMOTELY SENSING STORM ELECTRIFICATION

Observations of thunderstorms with a dual channel circular-polarizatio n radar have provided dramatic indications of the buildup of the elect ric field inside the storms and of the sudden collapse of the field at the time of lightning. The indications are obtained by coherently cor relating the simultaneous returns in the right- and left-hand circular polarization channels of the radar, and follow up on the pioneering o bservations of this type by Hendry and McCormick (1976). The correlati on is estimated and displayed in real time and the results enable one to predict when a storm has the potential for producing a lightning di scharge, and often to anticipate the occurrence of individual discharg es. The observations detect the presence of electrically aligned parti cles, believed to be small ice crystals, which are aligned by the elec trostatic field of the storm. The aligned particles cause the radar si gnal to become progressively depolarized as it propagates through an a lignment region, giving rise to correlated right- and left-circular po larization echoes. The alignment direction can be determined from the phase of the correlation and is found to be predominantly vertical, in dicating a similar electric field orientation. Weaker horizontal align ment is often Observed immediately following lightning discharges, con sistent with the idea that the aligned particles are ice platelets whi ch fall with horizontal orientation due to aerodynamic forces. The obs ervations have been found to reveal the onset of strong electrificatio n in developing storms and to indicate when decaying storms no longer have the potential to produce lightning. By compensating for signal-to -noise effects, the variation of the depolarization with range can be determined. This provides detailed pictures of the alignment regions w hich could be used as tracers of ice crystal populations in storms. Th e pictures also show the spatial variation of the alignment directions , raising the possibility of remotely mapping the storm electric field structure. Finally, the depolarization rate results readily enable on e to distinguish between liquid and solid precipitation in the storms.