Funds are provided to support a series of laboratory experiments the objective of which is to explore the physics of the water surface response to rainfall and the relationship between the surface response and radar backscatter. The surface response includes splashes and ring waves generated by the spatial and temporal distributions of drop impacts in the rain zones. Experiments with various rain rates, rain drop diameters and spatial distributions of rain on the water surface will be carried out in a wind-wave tank (12.8 m long, 1.15 m wide with a water depth of 0.91 m) with and without wind and surfactants. Experiments with salt water will be performed in a smaller tank. Measurements will include surface profile histories at the center plane of the rain field using a cinematic laser induced fluorescence technique, rain and secondary drop size and trajectories using a cinematic shadowgraph technique. Additional measurements of the turbulent flow fields several centimeters below the mean free surface will be made using Particle Image Velocimetry (PIV). Measurements of radar backscattering from the water surface in and around rain zones will be made with a pulsed ultra-wide band radar system. In cases with surfactants, water surface properties including surface pressure isotherms and surface viscoelasticity will be measured using a Langmuir trough/longitudinal wave propagation device and a maximum bubble pressure measuring device will be used to determine the in-situ diffusion constants and surface tension.
Air-sea transfer of mass, momentum, energy and heat are important components of the dynamical system controlling the local and global climate. Since rainfall over the ocean makes an important contribution to these transfer processes, the results of this project will inform understanding of climate variability. The radar measurements will provide understanding of the information content of remotely sensed measurements of the sea surface using microwaves.
