During the Development and Evolution of Coastal Stratocumulus (DECS) Experiment in the summer of 1999, five research flights were made to focus on the longitudinal variation of the marine boundary layer, especially the properties of the sloping inversion capping the stratocumulus clouds. These flights by the Twin Otter operated by the Center for Interdisciplinary Remote Piloted Aircraft Studies (CIRPAS) at the Naval Postgraduate School (NPS), made multiple slant-path soundings along a vertical cross-section from the coast of Monterey to 150 to 250 km into the East Pacific. In additions, long horizontal level legs near the surface and turbulence level legs at multiple altitudes were also made during some of the flights. The dataset provides many vertical cross-sections of the properties of sloping boundary layer and is valuable to study the horizontal variation of the cloudy boundary layer near the coast. In particular, we analyzed the characteristics of the sloping inversion capping the cloudy boundary layer, as well as the turbulence and thermodynamics within the marine boundary layer. The variation of the sea surface temperature and its influence on the boundary layer development are also discussed. The data analysis is also aided by mesoscale simulations of the cloud field using the Navy’s operational mesoscale model (COAMPS) to quantify the variation of the coastal divergence field.