Rain attenuation statistics from rain cell diameters and heights

The prediction of satellite link attenuation is generally based on the point rainfall rate for 0.01 per cent of the year. The physical quantity used to determine attenuation is rain rate, whereas the 0.01 per cent point is a more complex parameter, not obviously related to the dynamics of rain structure. The dynamics of the horizontal and vertical structure of rain are directly related to rain rate, rainfall volume, local geology and climate. If attenuation is to be predicted from rain rate it would be better to use a single parameter that remains substantially the same in most environments. This is not the case with the 0.01 per cent point. It is argued in this paper that a breakpoint in the rain-rate exceedances occurs close to 105 mm/h. Rain can be divided at the breakpoint into two broad classes: below this point it is more uniformly distributed over the area of rainfall; above, it has an increasingly more complex horizontal varaiability, with intense rain columns embedded in a background of less intense rain. By replacing the climatic zone dependent 0.01 per cent point rain rate with the fixed rain rate of 105 mm/h, a model for attenuation calculation can be derived that relates the mechanics of the physical processes to rain rate and elevation angle. The model has been developed and tested against the ITU-R model using the full measured rain-rate exceedances for each site in the ITU-R data bank. Attenuation exceedances have been shown to depend on the shape of the rain-rate exceedance curve, whereas the ITU-R model generates the same curve shape for all sites, because it uses only one rain rate, R(0.01). The new model performs at least as well as the ITU-R model for temperate climates and considerably better for tropical climates. For sites with no measured rain rate it is recommended that a generalized rain-rate exceedance curve be used, especially for tropical regions. Copyright (C) 2001 John Wiley & Sons, Ltd.