3D trajectories and velocities of rainfall drops in a multifractal turbulent wind field

Abstract. Weather radars measure rainfall in altitude, whereas hydro-meteorologists are mainly interested at ground level. During their fall, drops advected by the wind, which affects location of measured field. The governing equation a rain drop's motion relates acceleration to forces gravity and buoyancy along with drag force. It depends non-linearly on instantaneous relative velocity between drop local yields complex behaviour. Here, force is expressed standard way help coefficient as function Reynolds number. Corrections accounting for oblateness greater than 1–2 mm suggested validated through comparison retrieved “terminal fall velocity” (i.e. without wind) commonly used relationships literature. An explicit numerical scheme then implemented solve this 3+1D turbulent wind field, hence analyse temporal evolution velocities trajectories during fall. appears that multifractal features input simply transferred an additional fractional integration whose level size, slight time shift. Using actual high-resolution 3D sonic anemometer scale invariant approach simulate realistic fluctuations space, various sizes falling form 1500 m studied. For strong event, located within radar gate altitude 5 min spread over area size few kilometres. given diameter found cover pixels. Consequences measurements hydro-meteorological extremes needed improve resilience urban areas discussed.