Observations of a Rain-Formed Mixed Layer

Abstract

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 dV 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))dV**2 ~ 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. he density profile through the transition layer was linear and symmetric, and the overall Richardson umber of the transition la er g'd/dV**2 ~ l/4.