Clear-air scattering observations: Downdraft and angels

This paper discusses two observations: a dual-frequency measurement of a do wndraft and a high temporal measurement of a dot angel. The downdraft was m easured with both an S-band (9-cm) and an X-band (3-cm) radar; it was obser ved to fall from 3-km height through a mist layer to the ground level. In t he mist layer (in the lowest 1.2-km of the atmosphere) the difference in ra dar reflectivity factor was 3 dB, which is close to the expected 0 dB; in t he downdraft it was about 23 dB; close to the 19 dB expected by the theory on coherent scatter. This demonstrates that it is possible to observe downd rafts by means of dual-frequency radar systems. In measurements where the S-band Delft Atmospheric Research Radar (DARR) po inted vertically dot angels were observed. These dot angels are also referr ed to as worms, or ghosts, and it is speculated that they originate from th e reflections of insects, birds, leaves, atmospheric plankton (anything org anic in the air) or spontaneous turbulence. These dot angels are observed w ith an unprecedented high temporal resolution of 5 milliseconds using raw r adar data. In this data we found that the dot angel had an interference pat tern. Furthermore, the unwrapped phase indicates a point scatterer which fo llows a circular trajectory. (C) 2000 Elsevier Science Ltd. All rights reserved.