Path-average rainfall estimation with a microwave link: Theory, application, and ground validation

An accurate estimation of the drop size distribution (DSD) is essential in the retrieval of rainfall parameters such as rainfall rate, and cumulative rainfall. The DSD is characterized by a high temporal and spatial variability that affects both microwave measurements and ground validation. In order to explore and develop new techniques for the estimation of rainfall, a multi-frequency Microwave Link was deployed on Wallops Island, VA. The Link path-average measurements of attenuation provide information about the DSD that can be used to improve the accuracy of rain rate and liquid water content estimates over conventional methods, and provide a more detailed understanding of the microphysics and dynamics of rainfall.; A theoretical model based on the non-Rayleigh scattering properties of the microwave signals is used to formulate a dual-frequency inversion technique to estimate parameters of a three-parameter gamma DSD. The dual-frequency inversion technique is implemented using the Link attenuation measurements at 25 GHz, and 38 GHz. The results are evaluated in conjunction with the measurements from three impact disdrometers located directly under the microwave path. Additionally, the DSD estimates and the disdrometer measurements yield rain rates and rainfall accumulation, which are evaluated in conjunction with the measurements obtained from a network of four optical and six tipping bucket rain gauges located under the path.; The inter-comparisons between the Link estimates and the measurements obtained with the disdrometer and the network of rain gauges not only permit the validation of the inversion techniques but also provide the means the study the effects of the DSD variability on ground rainfall estimation techniques. The results from this work shows that dual-wavelength inversion technique yields rainfall estimates which are consistent with the path-average measurements from the network of disdrometers and rain gauges located beneath the microwave link path. Moreover, the dual-wavelength inversion technique provides an improvement in rainfall estimation since the path-integrated measurements of attenuation can surpass disdrometer and rain gauge systems by overcoming typical problems such as DSD variability, poor sampling, and ground surface winds effects.