Spray Stress Revisited

Abstract

In winds approaching hurricane strength, spray droplets proliferate. Once created, these droplets accelerate to the local wind speed in 1 s or less and thereby extract momentum from the wind. Because these droplets have substantial mass, they eventually plunge back into the ocean, delivering their horizontal momentum to the surface in the form of a spray stress. Inadequate information on the production rate and size distribution of spray droplets, however, hampered previous attempts to estimate the magnitude of this spray-mediated momentum exchange. This paper therefore uses recent estimates of the spray generation function to reconsider spray's ability to alter air–sea momentum exchange. Conservation of momentum requires that spray cannot enhance the air– sea stress beyond what the large-scale flow dictates. However, spray can redistribute stress in the near-surface atmosphere since the wind must slow if the spray droplets accelerate. For a wind of 30 m s⁻¹, spray supports about 10% of the surface stress; for a wind of about 60 m s⁻¹, spray supports all of the surface stress. The paper goes on to show how this partitioning affects the near-surface wind speed profile. Last, the paper reviews evidence that suggests the sea surface undergoes a transition in its aerodynamic behavior in the wind speed range 30–40 m s⁻¹. The fact that whitecap coverage extrapolates to 100% in this range may be one cause. Also in this range, the “rain” of spray droplets back onto the sea surface creates a mass flux with a magnitude that has been shown to damp the short waves that sustain most of the atmospheric drag on the sea surface. As a consequence, spray may play a key role in a negative feedback loop that limits air–sea momentum transfer.