Lundquist, J. Woods Hole Oceanographic Institution,
SURFACE DRAG AND MOMENTUM EXCHANGE IN HURRICANE CONDITIONS
One of the most difficult aspects in numerical weather prediction involves modeling the atmospheric boundary layer. This task is especially difficult over the ocean because the air and sea are coupled and interact dynamically. Understanding the dynamic interactions at the air-sea interface is particularly important in modeling hurricanes. Emanuel (1989, 1995) found the ratio of coefficients for enthalpy (CK) and drag (CD) to be of crucial importance in predicting hurricane intensity.
We hypothesize that bubbles, which are generated at high wind speeds by breaking waves, change the surface conditions enough to alter the exchange of momentum. We examined momentum exchange in a circular wind-wave tank under various wind speeds with and without artificially-generated bubbles. For wind speeds up to 22 m/s, bubbles lowered the drag coefficient. Above this speed, there was no longer a significant difference, possibly because at these high wind speeds, breaking waves generated bubbles in all trials regardless of whether the bubblers were on. All trials showed a leveling off of CD at these high wind speeds, which contrasts with current boundary layer theory. This discovery provides a first step towards reconciling the difference between current boundary layer theory and hurricane models.
Day: Wednesday, Feb. 3
Location: Sweeney Center