Abstract ID: 368

River proximity bias in Amazon climate data: a decade of observations near Santarém

We analyzed rainfall for a network of weather stations located near the confluence of the Tapajós and Amazon rivers in the eastern Amazon Basin. 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. Inland from the rivers, nocturnal rainfall contributes less than half of total precipitation. A large-scale rainfall increase just to the west of Santarém manifests itself locally as a ‘tongue’ of enhanced rain from along the wide area of open water at the Tapajós-Amazon confluence. 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. The interval between rainfall events decreases markedly close to the rivers during the dry season. Radiation, temperature & winds. Median surface wind and the orientation of frequent cloud streets attest to the NE to E trade winds that course over the Tapajós-Amazon confluence. Composite analysis adequately describes the mesoscale pressure gradient that forces the breezes and the median gradient is comparable to that observed for particular cases previously simulated. Actual reversal of the easterly provoked by the river breeze, a classic breeze signature, only occurs during weak easterly episodes (U <=1.5 m/s near the surface), which are associated with weakening of the large-scale E-W pressure gradient by cold air intrusions (friagens) into the central Amazon basin. Distinctly different median hourly incident solar radiation appear at stations very near the rivers compared to those only a few km inland. Hourly extreme values of incident solar radiation are consistent with limited sensor degradation over the measurement period. These illustrate that the clearest skies occur during the rainy season between rains. Incident solar radiative flux at tower sites in the FLONA (Tapajós Nat’l. Forest) is only slightly lower than that in nearby cleared areas. We had difficulty identifying a temperature or pressure gradient signal that would indicate the presence of a ‘vegetation breeze’ between the cleared and forested regions.

Session:  Feedbacks to Climate - Land cover, surface hydrology, and atmospheric feedbacks. (A)

Presentation Type:  Oral