Rapid transition of boundary layer structure observed on coastal sites

During the CBLAST-low Nantucket measurements in the summer of 2003, extensive measurements of the lower atmosphere were made using a combination of rawinsonde, tethersonde, acoustic sounder (SODAR), and in situ measurements on a 20-m mast and 2-m mast as well as on the ground. This dataset, together with measurements from other CBALST-low sites and nearby buoys, are used to study the rapid transition of the low-level wind field that was observed during the intensive observational period. We will focus on analyses of one case where wind direction change was found in the early afternoon from northerly to westerly. The change of wind direction happened within 15 minutes during which the boundary layer at the Nantucket site evolved from a local convective boundary layer to a strongly stable boundary layer. Such transition was also observed on other sites of CBLAST-low (ASIT and MVCO), but not at the nearby NDBC buoys that are further offshore. On the mesoscale side, we will use the synthesized dataset from Nantucket and other sites of CBLAST and the simulations from a mesoscale mode, COAMPS, to understand the spatial extend of the wind transition and its relationship with the local sea breeze circulation. On the small-scale side, the evolution of the boundary layer, the surface fluxes, and the turbulent structure will also be presented to illustrate the near surface response to changes in the mesoscale environments. Implications of this study to measurements of the marine boundary layers on a coastal site will also be discussed.