Drop by drop backscattered signal of a 50 × 50 × 50 m3 volume: A numerical experiment

Abstract

The goal of this paper is to analyse the influence of individual drop positions on backscattered radar signal. This is achieved through a numerical experiment: a 3D rain drop field generator is developed and implemented over a volume of 50 x 50 x 50 m 3 , and then the sum of the electromagnetic waves backscattered by its hydrometeors is computed. Finally the temporal evolution over 1 second is modelled with simplistic assumptions. For the rainfall generator, the Liquid Water Content (LWC) distribution is represented with the help of a multiplicative cascade down to 0.5 m, below which it is considered as homogeneous. Within each 0.5 x 0.5 x 0.5 m 3 patch, liquid water is distributed into drops, located randomly uniformly according to a pre-defined Drop Size Distribution (DSD). Such configuration is compared with the one consisting of the same drops being uniformly distributed over the entire 50 x 50 x 50 m 3 volume. Due to the fact that the radar wave length is much smaller than the size of a rainfall "patch", it appears that, in agreement with the theory, we retrieve an exponential distribution for potential measures on horizontal reflectivity. Much thinner dispersion is noticed for differential reflectivity. We show that a simple ballistic assumption for drop velocities does not enable to reproduce radar observations, and turbulence should be taken into account. Finally the sensitivity of these outputs to the various model parameters is quantified.

Combined high-resolution rainfall and wind data collected for 3 months on a wind farm 110 km southeast of Paris (France)

“…Improving our understanding of rainfall requires an in-depth understanding of its relationship to wind turbulence across scales. This could notably lead to the development of a 3D+1 model for the drops' location, which could be used to overcome the simplistic assumption of homogeneous distribution within a radar gate (see Gires et al, 2016, and references therein for an initial discussion on the topic) or to improve wind drift correction scheme for radar algorithms. Such developments will improve precipitation estimation with the help of radars.…”