Wind Direction Meander at a Coastal Site during Onshore Flows

Abstract

About 700 cases of wind direction meander occurred in a three-year period during onshore flow at a Long Island coastal site. Most appeared to be caused by internal gravity waves but some by roll vortices. Each case was documented with respect to date, time, wind speed, wind direction and stability, and described by duration, number of waves, angular amplitude and period. Hourly wind data for the same years were examined to determine the frequency of onshore flows. Most meander cases occurred in spring and summer, at low to moderate wind speeds and with stable conditions over the ocean. Duration varied from 4 to 438 min and averaged 94 min. The number of half-waves per case varied from 4 to 63 and averaged 10. Frequency varied from 0.3 to 30 with a mean of 4 waves per hour. Mean angular amplitude ranged from 4 to 68 and averaged 14°. Maximum amplitudes as large as 125° occurred. Results were compared to σy data from oil-fog smoke diffusion experiments. Calculations show that horizontal dispersion caused by a combination of meander and turbulent diffusion can be more than 30 times greater than that caused by diffusion alone and averaged 4 times greater. However, meander with angular amplitudes greater than 3° occurred only 15% of the time during onshore flow even though the air over the ocean was stable 59% of the time. Thus, the presence of significant meander cannot be assumed for diffusion calculations, although diffusion measurements indicate that minor meanders are frequent during stable conditions.