Statistical rainstorm models: Their theoretical and physical foundations

Precipitation has become a serious source of attenuation as higher frequencies are being employed for microwave communications. System performance is strongly influenced by the quantity and character of precipitation that occurs over the links of the system. Rain appears to be the precipitation form that accounts for most of the serious attenuation occurrences. It is also for rain that the Scattering theory is most complete. Rain attenuation can be accurately predicted if the drop-size distribution along the propagation path is known. The drop spectrum determines as well the rainfall rate, radar reflectivity, and microwave emission of rain. Consequently, it is possible to make estimates of rain attenuation through indirect measurements by raingauge, radar, and radiometer. Recent experiments have confirmed that these estimates are sufficiently accurate for practical purposes. From propagation experiments and studies of the fine-scale structure of rain, data are becoming available on the horizontal extent of heavy rain areas and the way this structure influences system performance. These data have been used to formulate statistical raincell models that permit prediction of the performance of single-path and path-diversity systems. The current status of raincell models is reviewed and suggestions for future research are offered.