Observations of the spatial structure of internal waves in a small mid-latitude lake

Internal waves are features of stratified waters that vary in space and time. Traditionally, such features have been investigated using data accumulated by an array of self-recording sensors. There are typically one or more moorings located in a study area, with each mooring having sensors located at several different depths. While the temporal resolution of data logged by each sensor is excellent, the spatial resolution of data recorded by such a system of sensors is usually poor. Having a sensor move through the water column is one way of improving the spatial resolution of data, but such data tends to have poor temporal resolution at any given location. With the goal of obtaining data with adequate spatial and temporal resolution to characterise internal waves, a study was undertaken that combines the temporal resolution provided by a traditional moored sensor array, with the spatial resolution of data collected by sensors moving across the study area. This study was conducted at Loon Lake, which is a small (1.5 km long and 0.5 km wide), deep (similar to 50 m) lake located near Maple Ridge, British Columbia, Canada. Direct measurements of the temperature structure within the thermocline were made with moored thermistor chains, as well as with a thermistor mounted on an autonomous underwater vehicle, or AUV. Two thermistor chains, each with six thermistors within the thermocline, were moored along the major axis of the lake for six weeks. A SeaBird SBE19 CTD was mounted onboard the Underwater Research Lab's PURLII AUV. On three separate days PURLII followed a sawtooth pattern within the thermocline, and along the major axis of the lake. These data were compared and contrasted to provide a comprehensive description of the internal temperature field.