Abstract:
We analyzed rainfall obtained in a network of 38 rain gauges located near the confluence of the Tapajos and Amazon rivers in the eastern Amazon Basin. We found that tipping bucket rain gauges work adequately in the Amazon rainfall regime, but careful field calibration and comparison with collocated conventional rain gauges was essential to incorporate daily totals from operational array into
... regional maps. Near-river stations miss the afternoon convective rain as expected as the river breeze promotes subsidence over the river, but paradoxically, this deficiency is more than compensated for by additional nocturnal rainfall at these locations. The 0.25-degree CMORPH passive infrared inferred rainfall data do an adequate job of describing medium scale variability in this region, but some localized breeze effects are not resolved. For inland areas away from the breezes, the nocturnal period precipitation contributes less than half of total precipitation. The large-scale rainfall increase just to the west of Santarem manifests itself locally as a 'tongue' of enhanced rain from along the wide area of open water at the Tapajos-Amazon confluence The breeze circulations associated with the Amazon River (which lies parallel to the mean flow) affects rainfall more than does the Tapajos breeze (normal to the predominant wind). The Tapajos breeze influence extends only a few kilometers inland east of the riverbank. Rainfall increases to the north of the Amazon, possibly the result of orographic effects. Dry season rainfall increases by up to 30% going away from the Amazon River, as would be expected given breeze subsidence over the river. 
