A Study of the Raindrop Size Distribution and its effect on Microwave Attenuation

Abstract

Current frequency allocations below Ku- band are becoming increasingly congested. The problem continues to grow as the use of telecommunications becomes more pop- ular. In order to compensate for increased in demand, telecommunication operating frequencies have to be raised, yielding larger channel capacity. However, raising the operating frequency has the adverse result of intensifying the attenuation effects of the troposphere. At high frequencies, such as V-band, rain is known to cause the most severe attenuation. It has been shown that model data for numerical weather prediction, combined with propagation prediction models, can be used to forecast telecommunication link atten- uation. The forecast of attenuation can be used to improve the effectiveness of fade mitigation techniques. A key problem in determining rain attenuation from rainfall rate is the considerable variability of the raindrop size distribution (DSD). Based on the analysis of disdrometer and numerical weather prediction model data, the aim of this work is to constrain the parameters of the DSD and ultimately generate proba- bilistic forecasts of attenuation. Using disdrometer analysis, a relationship between mean raindrop size and rainfall rate is established, and a link between drop concentration and rainfall rate is also demon- strated. The DSD is shown to vary with season and shows some small relationships with meteorological parameters. It is concluded that, despite some relation of the DSD with rainfall rate and meteorological parameters, there are too many variables to conclusively recreate the DSD. Analysis of the attenuation-rainfall rate relationship illustrates that variability due to the DSD increases with frequency above 40GHz. At 30-40GHz the DSD is shown to have little impact on the attenuation-rainfall rate relationship. Finally, this thesis examines the horizontal structure of rain cells in the UK, demonstrating the suitability of the EXCELL model and illustrating that rain cells are more elliptical in the UK.

Date of Award	1 Jun 2011
Original language	English
Awarding Institution	University of Bath
Supervisor	Robert Watson (Supervisor)