Effect of large eddies on atmospheric surface layer turbulence and the underlying wave field

To improve existing models for air-sea interaction, a better understanding of the energy transfer across the boundary layer and in particular of the c oupling of large atmospheric eddies with the air-sea interface is needed. R ecent investigations have already shown a possible coupling of large struct ures in atmospheric turbulence and surface ripples. This was done using syn thetic aperture radar (SAR) imagery of ocean surface and almost simultaneou s advanced very high resolution radiometer (AVHRR) imagery of cloud streets at a cold-air outbreak. The intent of our study is to further validate thi s hypothesis in a general case of coastal circulation. For this purpose we analyze a suite of collocated simultaneous records of airflow, radar return , and surface elevations from a coastal platform. We investigate the influe nce of large eddies (20-60 min) on the turbulent properties of the airflow in the first 2 m above the ocean surface. The analysis shows very prominent peaks in the magnitude of 12- to 16-min fluctuations which are further mod ulated in 20- to 40-min intervals. These scales and modulations are charact eristic for all variables of interest here. The detected scales and their m odulation suggest significant interaction of surface layer within the first 1-2 m with large eddies of scales of O(1) and O(10) km. The intermittent s tructure of turbulence responds by alternating contributions from bursts an d sweeps; the frequency of occurrence of bursts and sweeps also reveals the influence of large structures. The instantaneous cross correlation between the shorter scales of momentum flux and radar return, corresponding to the individual burst events, can be 4 times as strong as the overall cross cor relation.