Double frequency sounding of liquid precipitation

Abstract

Measurement of microstructure characteristics of precipitation is very important for study of process of their formation and progress [1]. Use of the remote sensing methods is also quite interested especially for measurement of particle size of liquid precipitation. That is why the double frequency method for measurement of particle dimensions is quite actual. It was first proposed in the work [2] and then studied in the work [3] for case of solid aerosols. Peculiarity of liquid precipitation is dependence of their permittivity on temperature and operating wavelength. This fact essentially complicates application of double frequency method. So the goal of this paper is analysis of method facilities in the frequency band, which is traditionally used for radio meteorology (8 mm and 3 cm). The most important parameters of the liquid precipitation are water content of unit volume W (g/m³) and intensity of precipitation I (mm/h), which is written for case of monodisperse medium (all particles have the same size) as equations where ref- effective radius of particles, ρ0- particle density ≈ 1 g/cm³, N0- particle concentration 1/m³, V(ref)- particle rate of fall m/s. As it was said the complex permittivity of liquid precipitation depends on temperature and frequency of illuminating field. This dependence is well described by Debye's generalized formulas [4]. Using approximations of monodisperse medium (all particles have the same size) and incoherent scattering the SRCS of scattering volume is σ(ref=N0σp)(σp- RCS of separated particle). As it is known the Rayleigh approximation is not fulfilled for large particles of precipitation for »=8 mm. So it is necessary to use rigorous solution of diffraction problem of electromagnetic field by dielectrical sphere (Mie theory). The numerical simulation of scattering signatures was performed for single spherical drop of water to define the character of dependence of RCS on temperature and wavelength. It was shown the normalized RCS weekly depends on temperature in quite wide range of its variation [−30 °C; 30 °C], especially for larger wavelength. Because of SRCS for monodisperse medium depends on particle concentration and operating wavelength it is reasonably to enter value of differential RCS (DRCS) as ratio of SRCS measured on two wavelength λ1 and λ2, which is single-valued function of particle sizes in some range (workspace)σD=σ0(λ1,r)/σ0(λ2,r). So application of double frequency method of remote sensing permits to measurement of effective particle size of precipitation using approximation of monodisperse medium.