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Within the progressive improvement in wave modeling we focus on the attenuation of swell waves by rain. Till now ignored, the process is shown to be relevant, especially for the correct estimate of swell. Following the practical impossibility of devoted field experiments, we exploit the global model results over a period of more than four years to extract the tiny signal associated to the attenuation by rain. A direct comparison of the ratio model by altimeter significant wave heights versus the encountered rain amount hints to a marked related dependence. A proper quantification of the related physical effect requires a multiple step procedure that we describe in detail. We check the reliability of the results, and we provide the related source function ready for implementation in operational wave models.

Plain Language Summary While wave modeling is now a well-acquired science, the correct estimate of swell, that is, the long waves crossing the ocean, particularly in the tropical zone, is still a problem for several operational wave modeling centers. There are several reasons, some understood, others still unknown. In this paper we explore a process presently ignored in wave modeling and the related interaction with wind, that is, the effect of rain on the sea surface. In this paper we hint to the possible implications and focus on one of the discussed aspects, that is, the attenuation of swell waves by rain. Discussed in paper in 1990, this has never found its way into operational modeling, also because in meteorological modeling rain is considered as a product without any mechanical feedback on the system. Using 4 years of model results, we show, first by theory then by measured evidence, that rain is in effect attenuating swell in the ocean at a potentially significant level. We provide a formula for direct implementation of this effect into the operational wave models.