Observations of the thermal structure of a lake using a submarine

Thorpe, S. A., U. Lemmin, C. Perrinjaquet, I. Fer

Limnol. Oceanogr., 44(6), 1999, 1575-1582 | DOI: 10.4319/lo.1999.44.6.1575

ABSTRACT: This note describes how a submarine, the F.A. Forel, carrying a vertical array 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 summertime conditions of stable stratification and during winter convection. The submarine speed was about 0.5 m s-1. In addition to the temperatures, the 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. During the summer, evidence was found of internal waves and of extensive layers of low vertical temperature gradient, with vertical and horizontal scales of 0.5 m and 0.5 km, respectively; within this gradient, the temperature changed monotonically in the horizontal. During periods favoring convection, in the winter, when air temperatures were about 78C below the surface-water temperature, convectively unstable regions, typically of 5-m horizontal scale, were observed in the mixed layer. These appeared to be convective plumes. These winter measurements also included observations of a layer of cold water that was adjacent to the sloping boundary of the lake. This was identified as being a plume of dense cold water with thickness on the order of 10 m, which was driven by surface cooling, and consequent more rapid temperature decrease, in the shallow nearshore water. On meeting the thermocline at a depth of about 100 m, this plume spread horizontally and formed an intrusion some 30 m thick.

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