Analog rain experiment: plumes of soluble particles*

Rain plays a major role in the climate system. A key ingredient of the rain process is the coupling between flow and phase change. We have set up an analog experiment, where the air in the atmosphere is replaced by water in a tank, and the phase change between rain droplets and water vapor is replaced by salt particles dissolving in water.

To mimic the rain, a certain amount of salt is injected into the water tank. The sedimentation of the particles leads to the formation of a plume. The salt particles dissolved partly during falling. Non-dissolving particles, like sand or plastic beads, are also used as a reference. This non-dissolving plume is comparable to plumes obtained by the injection of a heavy liquid.

The particle concentration is measured by light absorption and velocity field by PIV. For both cases, the particle concentration profile shows a Gaussian shape, whose width increases linearly with height. The width of salt plume is more narrow and vertical compared to non-dissolving plume. The plume particle flux is measured by integration of the local concentration along the horizontal direction. It is roughly constant for non-dissolving plumes, but decreases linearly for salt plumes due to dissolution. The hydrodynamic behaviors of the plume are investigated and modeled.