Numerical investigation of intense rainfall effects on coherent and incoherent slant-path propagation at K band and above

A model investigation is carried out to analyze the impact of intense rainfall on slant-path microwave propagation, using a rainfall microphysical model. The effects are evaluated both for path attenuation, undergone by coherent radiation, and for multiple scattering phenomena, originating incoherent radiation along the path. Atmospheric spatial inhomogeneity is taken into account by considering a precipitating-cloud three-dimensional distribution, obtained from the numerical outputs of a microphysical cloud-resolving model. The electromagnetic propagation model is formulated by means of the radiative transfer theory, rigorously defining the forward coherent multiple scattering effect within this framework. The propagation model is applied both to simplified rain slabs and to vertically and horizontally inhomogeneous raining cloud structures in order to compare the impact of atmospheric models on coherent and incoherent propagation. Beacon frequencies between 20–50 GHz are considered together with elevation angles between 20 – 40 and surface rain rates from 1 to 100 mm/h. Appropriate sensitivity analysis parameters are defined to present and discuss the numerical results. The main conclusion of this numerical study is that the impact of the convective rainfall structure can be significant both in determining total attenuation and to quantify multiple scattering contribution to the received power. For intense rainfall, the use of a rain slab model can overestimate coherent attenuation and, at the same time, underestimate incoherent intensity. The analysis of realistic raining clouds structures reveals the significance of modeling the volumetric albedo of precipitating ice, particularly at V-band. Total path attenuation can strongly depend on the pointing direction of the receiving antenna due to the intrinsic variability of the precipitating cloud composition along the slant path. Coupling cloud-resolving models with radiative transfer schemes may be foreseen as a new approach to develop statistical prediction methods at Ka-band and above in a way analogous to that pursued by using weather-radar volume data