Click on the screen image to see the radar signal associated with an ash plume generated by an explosion at Arenal volcano. The radar antenna was located 2.6 km west and downwind of the active crater C. The echo power received is shown in 8 range gates (120 m wide): the left part displays the complex digitized signal whereas the right part shows the computed Doppler velocity spectrum. The strongest signals are recorded in the the 2640-2760 m range gates where high positive velocities are observed. The main peak of each spectrum is then shifted to the left (negative velocities) because the ash plume starts to drift downwind (toward the radar). Power echoes then appear successively in closer range gates as the wind carries ash particles toward the radar.

Temporal evolution of the backscattered power and maximum velocity measured by the radar at 2.5 km W of Arenal summit during the largest explosion of the record period, on 18 February 2004. [ From Donnadieu et al., 2005].