Penetrative convection in the absence of mean shear

This paper reports the first large‐scale observations of a developing mixed layer in which only convection can be causing the deepening. The convection arises from solar heating of water beneath lake ice. Since the water is below its temperature of maximum density, heating due to the absorption of radiation passing through the ice results in a mixed layer which increases in thickness with time. Observations taken with an electronic bathythermograph and by recording thermistor chains have permitted the estimation of that fraction of kinetic energy made available by the gravitational instability, which is converted into potential energy by entrainment of stable fluid as the convecting layer advances. Twelve independent estimates vary from 0.003 to 0.113 with an average value of 0.036. This result qualitatively agrees with the laboratory work of Deardorff, Willis and Lilly (1969) who found that only a small fraction (about 0.015) was recovered in this way. However, relatively little penetration can have a significant effect on mixed layer growth; the results obtained in the present study correspond to a mixed layer thickness 17% greater than that predicted for zero penetration in the case of a linear initial density profile.