Observations of a Rain-Formed Mixed Layer

The structure and dynamics of a rain-formed mixed layer (ML) are studied using hourly STD and profiling current meter casts. Because the fluid beneath the rain-formed ML was vertically homogeneous, the ML buoyancy g′ and horizontal velocity difference δV were easily observable as the ML depth h increased by entrainment from 7 to 18 m in a period of 8 h. The overall Richardson number of the mixed layer g′(h + d/2)/δV2 ≈ 0.7 during that period, where d is the thickness of the transition layer at the base of the ML. The entrainment rate was consistent with that of a laboratory surface half-jet (Ellison and Turner, 1959). The transition layer thickness was an appreciable fraction of the ML thickness, roughly d ≈ h/3. The density profile through the transition layer was linear and symmetric, and the overall Richardson number of the transition layer g′d/δV2 ≈ 1/4.