Rain rate and attenuation statistics along paths in a tropical coastal area from radar data

Abstract

A data set on rain rate distribution was collected by radar from a coastal site located at tropical latitude in West Africa where the rain field is strongly heterogeneous. It is used to compute rain rate (R) and microwave attenuation (A) statistics integrated along four paths located onshore, offshore, and perpendicularly to the coast, that is, north and south of the radar. These statistics are analyzed in order to see if coastal effects on rain fields induce significant differences between the conditions of microwave propagation along the four paths. R and A averaged along the paths are found to be mixed lognormal distributions with parameters μ and σ (mean and standard deviation, respectively) almost constant for the four paths, which shows that locally, the rain field is approximately ergodic. The probability of rain occurrence is found to be lower for the paths located offshore and north with respect to onshore and south. These differences are linked to the presence of positive gradients of rainfall occurrences between sea and land areas and between north and south areas. The magnitude of these differences is about 100%. The statistical variation coefficient, that is, CV = μ/σ, is found to be close to 2.24, a value observed in many other sites. For the attenuation, CV is found to be dependent on the exponential coefficient of the relation between the specific attenuation and the rain rate. For attenuation, the parameters of the probability density function are given for nine frequencies between 3 and 94 GHz.