One of the intensive observation sites of the LBA-ECO project is in and around the Tapajos National Forest (TNF), near Santarém, Pará, Brazil (2° 25’S, 54° 42’W), near the confluence of the Tapajos and Amazon rivers. The TNF is a thin strip of forested vegetation wedged between a managed mosaic of forests, pastures, agricultural fields and secondary succession to the east and the 10-25 km wide Tapajos River to the west. To the north is the braided expanse of the main Amazon River. This region experiences persistent easterlies during much of the year, a "continental trade wind" regime. These trades are punctuated at intervals by light winds associated with weakening of the large-scale E-W pressure gradient by synoptic-scale events. Breeze effects are notable in every season. During slack intervals, local circulations are seen by wind direction reversals. Breezes lead to strong gradients in precipitation and cloudiness. In the daytime, rivers are clear areas surrounded by cloudier land areas. These biases could distort the historical record toward lower precipitation and higher incident solar radiation at climate stations commonly found in river port cities. Measurements of H, LE, and FC (the CO2 flux) at the TNF may also be biased not only by precipitation anomalies but also by changes in radiative fluxes caused by enhanced cloudiness.

A small network of surface observation systems was designed to help quantify these effects near Santarém. This is complemented by a cloud ceilometer, and by hourly recording of GOES infrared and visible satellite imagery in the region. These are supplemented by the regular hourly observations at the Santarém airport and at the three flux measurement towers.

River breeze circulations dominate smaller circulations that some believe are initiated by contrasts in land-use type. Observed convective cloud base, a good measure of the mixed layer thickness, varies between 800 in the wet season to 1500 m in the dry season. Evidence of the breeze is apparent at stations within 50 km of the rivers. Wind direction reversal at the surface occurs during slack easterlies (afternoon wind speed < 1.5 m/s). Radiation anomalies depend on distance from the nearest river. The perturbation pressure gradient of the river breeze can be found through composite analysis. Nocturnal flows following local topographic gradients are seen, including the terral along the Tapajos River. Precipitation and specific humidity (q) each show two peaks diurnally. The precipitation peak in the early morning hours is consistent with previous studies of propagating squall line circulations from the coast. The q peaks occur when positive LE coexists with shallow surface layer inversions during the morning and evening transitions. Hourly composites of low cloud cover fraction from GOES images illustrate the preferred regions for initial convective activity, locations consistent with the idea of convergent river breezes. The sites of the LBA flux towers are shown located in an anomalously cloudy region during large portions of the transition and dry seasons. We anticipate that these findings will be of interest to those performing mesoscale modeling of this region.