Spatial and Temporal Variations of the Turbulent Fluxes of Heat, Momentum, and Water Vapor Over Lake Ontario During IFYGL

Book excerpt: During the 1972 IFYGL 'alert' periods, the highly instrumented NOAA/RFF/DC-6 aircraft was used to record the time series of wind, temperature, and water vapor at heights ranging from 18 to 300 m above the surface of Lake Ontario. The aircraft was equipped with a gust probe system, a fast response thermistor, a microwave refractometer (for water vapor measurements), and a downward-pointing IR system; as well as the normal in-flight measurement of standard meteorological parameters. The time series records have been found to display a highly intermittent nature. This is especially the case for evaporation when, in the fall, Polar Continental outbreaks move across the lake. In particular, such an outbreak of cold dry air moved across the lake at 12-15 m s−1 on 9 October 1972. This resulted in the air temperature at 30 m above the lake to drop from 12 to 6 C while the evaporation rate increased to more than 1 cm day−1. This may be compared with the 0.5 cm day−1 normal evaporation observed in the tropics during BOMEX. Furthermore, IR lake surface temperatures show cold regions (~5 C) along the north shore, presumably due to strong upwelling, while the center and south shore regions of the lake were of the order of 12 to 15C. The turbulent, flux quantities of momentum, heat, and water vapor were obtained by the eddy correlation technique and their spectra were determined at several locations over the lake surface for 3-minute sampling lengths. At the aircraft speed of 92 m s−1, this represents a flight path of ~17 km for both along wind and constant fetch patterns. The spectra demonstrate the tendency for the peak value to march to higher wavelengths with increasing height.