Thursday, 13 February 2003
Characterizing bubble and surface wave processes during the Rough Evaporation Duct Experimement
Electromagnetic and electro-optical propagation near the oceanís surface is dependent on both the atmospheric conditions and the interaction of the propagated signal with the ocean waves. Factors which impact the nature of the propagation of these signals include the profiles of mean and turbulent fluctuations of humidity, temperature, wind speed, ocean surface waves, and aerosols. While the average and fluctuating atmospheric conditions determine the ducting, scintillation, and attenuation of the EM/EO signals, the importance of the ocean waves are two-fold since they provide the bottom boundary condition which scatter the signal as well as modulate the marine atmospheric boundary layer. In addition, marine aerosols generated by both active wave breaking / whitecapping and bubbles bursting at the sea surface will also impact visible and infrared propagation by the aerosolís ability to modify the extinction of the signal. The additional air-sea heat fluxes which result from evaporation of the water component of the aerosols will also impact the near-surface EM propagation duct. It is anticipated that critical to understanding the source function of the aerosols will be an understanding of the spatial and temporal statistics of both the flux of bubbles across the sea-surface and the moments of the kinematics of active whitecapping. Despite the close coupling between these two aerosol generation mechanisms, we make the distinction between the two sources since the two processes will contribute to different size regimes in the total aerosol size distribution. While it is expected that bubbles bursting at the oceanís surface are the dominant source for the smaller particles, the larger aerosols will be introduced to the atmosphere by the active breaking region of the whitecap. The larger aerosol sizes generated by this mechanism will most likely overlap in size ranges with the sizes generated by the bubbles and will extend up through the sea spray regime. Advancements in measurement technology now allow us to measure these processes independently and couple them to the overlying wind and wave field. As part of the Rough Evaporation Duct Experiment, both airborne and in-situ measurement systems were deployed by Scripps Institution of Oceanography to provide measurements of the subsurface bubble field, whitecap kinematics, the surface gravity wave field, and the turbulent dissipation of kinetic energy in the wave boundary layer. A description of the in-water measurement methodology and results from this program are presented with a focus on describing the temporal and spatial variability of the bubbles generated by wave breaking and their dependence on the overlying synoptic wind and wave conditions. Trends in the both the atmospheric aerosol concentration and EM transmission loss will also be discussed in the context of the air-sea interaction processes measured. Readers are referred to a separate paper by Melville, Matusov, & Terrill for results stemming from the airborne measurement program designed to quantify the kinematics of surface wave breaking.