In situ observations of diurnal variability in rainfall over the tropical Pacific and Atlantic Oceans

In this study, the diurnal cycles in rain accumulation, intensity, and frequency are investigated for the 1997-2001 time period using measurements from self-siphoning rain gauges on moored buoys within the tropical Pacific and Atlantic Oceans. These measurements are unique in that they provide in situ, quantitative information on both the amplitude and phase of diurnal variability in tropical oceanic precipitation over an extended period of time at selected locations. Results indicate that the diurnal and semidiurnal harmonics explain a significant portion of the diurnal variance for all three rainfall parameters at the buoys. The diurnal harmonic in particular dominates the composite diurnal cycle in hourly rain accumulation, with a maximum from 0400 to 0700 local time (LT) and a minimum around 1800 LT. An early morning maximum and evening minimum are also observed in the composite diurnal cycles of rain intensity and frequency, indicating that both are contributing to the diurnal cycle in accumulation. Afternoon maxima in accumulation are also observed at several locations and are generally associated with maxima in rain intensity. While there is considerable variation in the estimates of the diurnal cycle both seasonally and regionally (especially for intensity), the results are overall consistent with previous studies of the diurnal cycle in rainfall and tropical cloudiness.