Observations of the thermal structure of a lake using a submarine

This note describes how a submarine, the F.A, Forel carrying a vertical arr ay of high-resolution temperature sensors, was used along with conventional measurements from a lowered conductivity-temperature-depth probe (CTD) to make novel measurements of the temperature field in Lake Geneva during summ ertime conditions of stable stratification and during winter convection. Th e submarine speed was about 0.5 m s(-1). In addition to the temperatures, t he pressure, orientation, and tilt were recorded at frequencies of at least 10 Hz. Observations were made on a vertical scale of 0.1 to 2.5 m and on a horizontal scale from 0.5 m to 1 km. Examples of the data are presented. D uring the summer, evidence was found of internal waves and of extensive lay ers of low vertical temperature gradient, with vertical and horizontal scal es of 0.5 m and 0.5 km, respectively; within this gradient, the temperature changed monotonically in the horizontal. During periods favoring convectio n, in the winter, when air temperatures were about 7 degrees C below the su rface-water temperature, convectively unstable regions, typically of 5-m ho rizontal scale, were observed in the mixed layer. These appeared to be conv ective plumes. These winter measurements also included observations of a la yer of cold water that was adjacent to the sloping boundary of the lake. Th is was identified as being a plume of dense cold water with thickness on th e order of 10 m, which was driven by surface cooling, and consequent more r apid temperature decrease, in the shallow nearshore water. On meeting the t hermocline at a depth of about 100 m, this plume spread horizontally and fo rmed an intrusion some 30 m thick.