Idealized Simulations of the Marine Atmospheric Boundary Layer Response to Changing Coastal Land Surface Properties

The extent to which land surface properties in coastal environments influence the offshore marine atmospheric boundary layer is not well understood. In this study, we conduct a set of idealized, cloud-resolving simulations with the Navy’s Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) for a domain bisected by a north-south oriented coastline. The experiments vary coastal soil moisture, surface albedo, and surface roughness over realistic ranges within a part of the domain on the immediate coastline. We then evaluate the response of the marine atmospheric boundary layer at different distances away from the coast during the last 3 days of the 5-day simulations. In a control simulation with no mean flow and typical summer values of each land surface parameter for the east coast of the U.S., the diurnal heating of the land surface accelerates the surface winds over the ocean up to 500 km offshore during the afternoon hours. Progressively decreasing soil moisture below the control value expectedly strengthens this sea breeze circulation and increases surface latent heat fluxes over the ocean, leading to a relative moistening of the marine atmospheric boundary layer compared to the control simulation. Increasing soil moisture has a much smaller offshore impact than decreasing soil moisture, indicating an asymmetric sensitivity to coastal soil moisture. Decreasing land surface albedo has a similar amplifying effect on the sea breeze and offshore boundary layer to decreasing soil moisture, but the magnitude of the response is reduced by about 50% and the responses to increasing or decreasing surface albedo are nearly equal and opposite. Changing surface roughness over land has no discernible impact on the offshore environment. We discuss how imposing 5 m/s of offshore flow throughout the troposphere modulates these sensitivities.