Drop size effects on rain-generated ring-waves with a view to remote sensing applications

• Published in: International journal of remote sensing Vol. 23; no. 12; pp. 2345 - 2357
• Main Authors: Lemaire, D., Bliven, L. F., Craeye, C., Sobieski, P.
• Format: Journal Article
• Language: English
• Published: Abingdon Taylor & Francis Group 01.06.2002; Taylor and Francis
• Subjects: Earth, ocean, space; Exact sciences and technology; External geophysics; Geophysics. Techniques, methods, instrumentation and models; Meteorology; Physics of the oceans; Sea-air exchange processes; Water in the atmosphere (humidity, clouds, evaporation, precipitation); Sea surface; Water drop; Size; Terminal velocity; Roughness; Microwave; Experimental study; Droplet; Dimension spectrum; Scatterometer; Remote sensing; Rainfall rate; Rain; Physical simulation; Non linear model; Wave scattering; Surface water; Size effect; Spectral density; Kinetic energy; Radar echo
• ISSN: 0143-1161; 1366-5901
• DOI: 10.1080/01431160110107617

This paper presents an analysis of drop size effects on ring-wave spectra and radar scatterometer returns from a water surface agitated by artificial rain. For this purpose, monodisperse and polydisperse rain events were generated in the laboratory for a wide range of rain rates and various drop sizes. The water droplets reached the surface at terminal velocity. In all cases, the radar average power is well modelled by a linear function of the power spectral density at the Bragg resonant wavelength. The drop size is found to have a strong impact on the spectral shape of the ring-waves and on their total energy. A log-Gaussian model characterizes well the ring-wave spectra and empirical expressions of the spectrum parameters are given. Ring-wave energy increases with rain rate and drop size, and is found to be proportional to the kinetic energy of a single drop, indicating that one may use a model in which all drops contribute to the ring-wave energy in proportion to their squared momentum. The results from the monodisperse rain experiments are used to construct a model for natural rain. Data from the polydisperse rain experiments show that a nonlinear model which relates dissipation to the total rainfall rate provides excellent agreement with the measurements. This analysis also shows the important impact of a few large drops on the ring-wave spectrum. The model proposed can be extended to natural rains either by using measurements of the drop size distribution or by assuming a drop size distribution model that is appropriate to the study region. It is concluded that it is important to characterize ring-wave spectra as a function of rain rate and drop size distribution to develop robust radar scattering models for rain-roughened seas.