Methodology to simulate rain and attenuation fields spatially correlated at mid and large scale

Abstract

This paper is related to the modelling of rain fields and attenuation fields at small (size of a rain cell), mid (∼150 km²) and large scale (∼1000 km²). The methodology lies on a cellular description of rain fields by the HYCELL model. The latter allows not only to describe the rain cell horizontal structure but also to generate two dimensional rain rate fields at mid scale (∼150 km², terrestrial network, satellite telecommunication beam) which account for the local Cumulative Distribution Function given as an input parameter. Nevertheless, the mid-scale fields generated that way are not spatially correlated. To overcome this limitation, using radar observations at mid-scale, the rain field internal organisation is analysed as a function of the rain intensity (intercellular distances, nearest neighbour distance, inter aggregate distance at mid-scale). This observational study allows to model the rain cell spatial location within a mid-scale area by a doubly aggregative isotropic random walk. Coupled with the HYCELL modelling of rain field at mid-scale, the random walk allows to generate rain fields spatially correlated at mid-scale, while accounting for the local climatology characteristic to the simulation area.

The next step of the paper is related to the modelling of rain field at large scale (∼1000 km², French national territory for example). In this context, low-pressure systems are modelled in the Fourier plane, assuming an anisotropic covariance function. The large scale field obtained that way is then split into mid-scale areas over which we proceed to the generation of (mid-scale) rain fields according to the HYCELL methodology presented above. The rain field is then spatially correlated at mid and large scale. It accounts for the climatological characteristics over each of the mid-scale areas which compose it. Assuming a telecommunication link with OLYMPUS (19°W), the rain fields simulated at large scale over the French national territory are then turned into attenuation fields. The statistical properties of the resulting rain fields and attenuation fields are then compared to radar observations and ITU-R recommendations. It is concluded that this large scale model is a new tool which deserves to be considered by system designers to compute propagation parameters. In this context, typical two-dimensional rain rate fields and attenuation fields over an area corresponding to the size of a country, a satellite beam, a Local Multipoint Distribution Services (LMDS) network or a radar coverage can be simulated to evaluate diversity gain, terrestrial or slant path attenuation for different azimuth directions while accounting for the local meteorological characteristics.