Diurnal cycle of rainfall over the Brazilian Amazon

The rainfall characteristics over the Brazilian Amazon were analysed using 3 yr of 3 h data from 24 rain gauges. During this period, areas in the eastern sector experienced up to 22 % of rainy hours, while those in the southern sector recorded up to 7 %. In the northern Amazon, rain tends to occur during the night, while in southern, central and central-western Amazon it occurs mainly in the afternoon. In contrast, over the eastern Amazon, rain occurs late at night and early in the morning. Harmonic analysis revealed an increase in the phase of the first harmonic phase from eastern towards central Amazon, suggesting that the rain propagates in that direction. The amplitude of the first and second harmonics revealed the existence of a relatively dry area inland about 500 km from the Atlantic coast. This area seems to be the limit for the inland penetration of systems originating at the coast. The phase of the first harmonic revealed small differences between the diurnal cycles of rainfall during the wet and dry seasons, suggesting that there is no difference between low-level circulations in either season.