Spatial and temporal rainfall variability near the Amazon-Tapajos confluence

Do the influences of river breezes or other mesoscale effects lead to a systematic river proximity bias in Amazon rainfall data? We analyzed rainfall for a network of 38 rain gauges located near the confluence of the Tapajos and Amazon rivers in the eastern Amazon Basin. Tipping bucket rain gauges worked adequately in the Amazon rainfall regime, but careful field calibration and comparison with collocated conventional rain gauges were essential to incorporate daily totals from our array into regional maps. Stations very near the large rivers miss the afternoon convective rain, as expected if a river breeze promotes subsidence over the river, but paradoxically, this deficiency is more than compensated by additional nocturnal rainfall at these locations. The NOAA Climate Prediction Center (CPC) Morphing technique (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 at 0.25 degrees resolution. For areas inland from the rivers, nocturnal rainfall contributes less than half of total precipitation. A 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 Amazon River breeze circulation affects rainfall more than does the Tapajos breeze, which moves contrary to the predominant wind. East of the riverbank, the effects of the Tapajos breeze extend only a few kilometers inland. Rainfall increases to the north of the Amazon, possibly the result of uplift over elevated terrain. Dry season rainfall increases by up to 30% going away from the Amazon River, as would be expected given breeze-induced subsidence over the river.