The main utilization of mobile ground-based Doppler radars is to quantify the dynamics of eruptive activity by aiming directly at the emission source. We show that they can also provide information on the initial lateral transport speed of weak ash plumes bent over by crosswind. The method is illustrated by measurements made with a transportable volcano Doppler radar (VOLDORAD) at Arenal volcano, Costa Rica. The near-source displacements of the plume are tracked through echo onsets induced by ash entering successive probed volumes in the radar beam. A constant transport velocity is commonly reached within a few seconds of the initial ash emission, as wind advection and buoyancy take over momentum. The plume azimuth and upraise angles are constrained by comparing the amplitude decrease of the radar echoes as a function of distance from the source with results from a simple geometric plume model. The three dimensional vector of the ash cloud transport speed is then reconstructed with an accuracy of a few percent. This method may have applications for volcano monitoring, for determining pyroclast fluxes, for the modeling of tephra dispersal, and for remote measurements of volcanic gas fluxes.