During the 2004 monsoon season, SMEX04 was conducted in Arizona and in the Sonora River basin in northern Mexico. Similarly to previous field experiments such as the Southern Great Plains series, the objective of SMEX04 is to obtain concurrent ground-based and aircraft –based soil moisture measurements in support of the development and refinement of microwave radiometers and soil moisture retrieval algorithms (SMRAs). SMEX04, however, presented unique challenges as compared to previous experiments: 1) it was conducted in a region of very complex topography, and therefore will provide a unique opportunity to test currently existing SMRAs; and 2) it was conducted in a region characterized by highly heterogeneous vegetation cover, which exhibits dramatic and fast response to rainfall forcing with ten-fold and higher increases in leaf area index during a short period of time at the onset of the North-American Monsoon. Although this experiment was designed to take advantage of the wealth of hydrometeorological data collected during NAME (North American Monsoon Experiment, and it is within NAME’s domain, it falls however outside of the coastal areas where the intensive data collection efforts during NAME took place in Mexico. To fill this gap, we conducted systematic observations of precipitation, surface energy and water budget conditions, and boundary layer conditions in Sonora during a three-week period at the onset of the monsoon. The research objective was two-fold: 1) to obtain high quality data to describe the diurnal cycle of the dynamics of the lower troposphere (winds, moisture and temperature) for constraining and validating soil moisture retrieval algorithms, including high resolution physically-based interpolation of rainfall and atmospheric water vapor observations; and 2) to characterize the evolution of the characteristics of the diurnal cycle of land-atmosphere interactions after the monsoon onset and how that relates to regional changes in vegetation dynamics. We present results of this study focusing on characterizing the diurnal cycle of precipitation and thermodynamic structure of the boundary layer, and we explore the relationship between the diurnal cycle of the boundary layer and land-atmosphere interactions as a function of changes in landscape vegetation and surface water hydrology during the onset of the monsoon.