A numerical investigation has been conducted to understand the meteorological influences on pollutant transport and diffusion due to interactions among multiple sea-breeze circulations over Cape Cod and southeastern MA. Three 48-h cases from summer 2000 were studied using the PSU/NCAR MM5 mesoscale model with four nested grids. The finest mesh had a resolution of 1.33 km with 115 X 115 points to resolve local circulations over Cape Cod, the islands of Nantucket and Martha's Vineyard, and the water bodies of Massachusetts Bay and Buzzard's Bay. Trajectories and plume dispersion rates were calculated for emissions released from four sources in Upper Cape Cod and nearby in southeastern MA. Two sources were from power-plant stacks, one from a heavily traveled stretch of highway and a fourth at Otis Air Force Base.

Statistical analysis of the meteorological fields versus NWS data showed good agreement, with very similar skill to that found in prior air-quality applications of meteorological models from various locations around the U.S. Further validation was performed by comparing the model fields to mesoscale analyses at 3-h intervals, which confirmed the model's simulation of many observed characteristics of the sea breeze and land breeze in these cases. The results indicate that, in weakly forced synoptic conditions, multiple sea breezes and island breezes develop around mid-morning over Cape Cod, southeastern MA and the islands of Nantucket and Martha's Vineyard. As temperatures over land increase through mid-day and these circulations intensify, the sea-breeze fronts move inland and often collide over Upper Cape Cod and Martha's Vineyard. However the greater land mass of southeastern MA (surrounded by water on three sides from Narragansett Bay to Boston) is large enough to develop a thermal low-pressure center during the afternoon. This thermal low is evident in the pressure observations and drives the dominant sea breeze of the region. As the thermal low approaches maximum intensity in late afternoon, the sea breeze fronts from Massachusetts Bay and Narragansett Bay move inland by 50-60 km. Under the influence of this dominant circulation, the local sea breezes over Cape Cod become distorted and re-align with the flow directed toward the thermal low. Trajectory calculations and dispersion fields from the SCIPUFF plume model show that pollutants are lofted above the top of the prevailing boundary layer by vertical motions in the colliding sea-breeze convergence zones. These features act as chimneys to remove a portion of the pollutants from the boundary layer, where they can be advected rapidly away from the region.