Modulation of convection in the southwestern Amazon basin by extratropical stationary fronts

Regimes of lower tropospheric northwesterly wind observed in southwestern Amazônia during the Tropical Rainfall Measuring Mission Large‐Scale Biosphere Atmosphere (TRMM‐LBA) field campaign were shown to be local manifestations of stationary frontal systems that extended into the deep Tropics along the South Atlantic Convergence Zone (“SACZ” regime). Frontal systems were transient and were confined to the subtropics during the interim periods of easterly and weak westerly flow (“non‐SACZ” regime). Observations from surface‐based radar in TRMM‐LBA suggested that mesoscale convective systems in the SACZ regime were significantly larger in areal coverage, with weaker rainfall intensity and weaker vertical development of the convective cells. The diurnal variation of rain intensity and rain areal coverage generally showed afternoon maxima for both regimes, but with important differences suggesting explosive convective cell growth in the non‐SACZ regime and the dominance of nocturnal stratiform rain processes in the SACZ regime. These results had implications for the parameterization (in general circulation models) of subgrid‐scale convective processes in Amazônia in terms of the large‐scale flow and for improving precipitation and latent heating retrieval from spaceborne platforms such as the TRMM satellite.