Surface diurnal cycle and boundary layer structure over Rondônia during the rainy season

Abstract

An integrated data set with simultaneous observations at the surface, from tethered balloons within the boundary layer and from rawinsonde ascents, was collected during the wet season experiment of the Large‐Scale Biosphere‐Atmosphere (LBA) Experiment in Amazonia during January and February of 1999 in support of the ground validation for the Tropical Rainfall Measuring Mission (TRMM). We analyze the surface diurnal cycles of temperature, humidity, lifting condensation level, equivalent potential temperature, and the surface fluxes of sensible and latent heat, ground heat flux and net radiation, for easterly and westerly wind regimes in the lower troposphere. During the easterly wind regimes, the diurnal evolution of mixing ratio shows that the flux of water vapor through cloud base exceeds the large surface evaporation. There is a trend toward a wetter and cooler subcloud layer as the rainy season progresses. Daytime surface Bowen ratio for this pasture site is about 0.4, and falls slightly as the rainy season progresses. Typically in the afternoon, evaporatively driven downdrafts from convective rainbands transform the boundary layer. The fall of equivalent potential temperature in the boundary layer is similar for both regimes, but the boundary layer cooling by convective events during the westerly regimes is reduced, because the subcloud layer is shallower on average. Tethersonde ascents through the edges of gust fronts show that subcloud air is first cooled and moistened by rainfall evaporation before the arrival of downdraft air at the surface. These measurements provide a detailed observational basis for the validation and improvement of parameterizations for shallow and deep convection in numerical forecast models.