The spatial structure and temporal development of marine air penetration along the northern and central California coast have been studied using simulations from Mesoscale Model 5 (MM5) and a Lagrangian random particle dispersion model for several days in June 1996. The MM5 model was evaluated using a large dataset of observations from both offshore and over the land and then used as an input for the Lagrangian random particle dispersion model. The Lagrangian random particle dispersion model has been used to visualize and quantify the strength and distribution of marine air penetration along the California coast. During a specific case study in June 1996, a typical summertime northwesterly flow along the coast was perturbed due to forcing by synoptic and mesoscale pressure gradient effects. As a result, offshore winds turned onshore and upcoast. The authors have examined the significance of this turning and its later restoration into the northwesterly flow as well as the associated changes in the structure of marine air penetration. The analysis was extended to other cases of typical as well as perturbed wind fields during June 1996. The study has revealed the significance of diurnal effects on the changes in the flow field and identified the regions of significant penetration.