The relationship between Advanced Microwave Sounder Unit (AMSU) observations and rain-rate, including beam-filling effects is investigated through analysis of an archive of collocated AMSU, AVHRR (Advanced Very High Resolution Radiometer), and Doppler weather radar data which preserves information at sub-AMSU instantaneous field of view (ifov) scale. In particular, AVHRR data and derived products (cloud type, cloud fraction etc.), together with the radar data (rain-fraction, bright band height etc.) are used to characterise the rain processes within each AMSU ifov. AMSU-B ifovs that contain no precipitation are then used to derive a relationship between the 89 and 150 GHz data in the absence of scattering, and a scattering index based rain-rate retrieval algorithm developed from those ifovs with complete rain coverage. By combining the AMSU data with the AVHRR derived within-ifov cloud data, it is possible to further refine the accuracy of the scattering index method, leading to rain-rate retrievals with R² > 0.69. The details of the method and validation results will be presented, demonstrating the capability of the combined microwave, infrared, and visible rain-rate algorithm to produce spatially accurate rainfall estimates of value in nowcasting, rain-process studies, and the identification of radiative contaminants in the observations used in numerical weather prediction models (e.g. the New Zealand Limited Area Model, NZLAM-VAR).