Effect of rain on TOPEX-POSEIDON altimeter data

The presence of rain can significantly degrade altimeter measurements of range, backscatter coefficient (σ₀) and significant wave height (H_s). Atmospheric liquid water can attenuate the pulse by several decibels and distort the waveforms. In this paper, two cycles of Topex-Poseidon altimetry data over the North Atlantic are systematically searched for instances where the data are possibly affected by rain. Different criteria, based on σ₀ , wind speed, η_s, sea surface height (SSH), and data quality flags, especially the rain flag, were examined in order to identify the measurements affected by rain. Using the fact that pulse attenuation by rain drops is frequency dependant, criteria are proposed based on finding changes in the relationship between the Ku (13.6 GHz) and the C (5.3 GHz) bands H_s and σ₀. The criteria applied to the σ₀ measurement found over 90% of the rain events identified in one cycle, whilst the present Topex rain flag signalled less than 10%