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Simon J. Hook, Fred J. Prata and Geoffrey Schladow MS 183-501, Jet Propulsion Laboratory, Pasadena, CA 91109 USA CSIRO Atmospheric Research, PMB 1 Aspendale, Vic. 3195 Australia University of California, Davis, Department of Civil and Environmental Engineering, 116 Everson Hall, Davis, CA 95616 USA Abstract In 1999 the Terra platform was launched into earth orbit. The platform supports five instruments designed to monitor the Earth. Two of the instruments, ASTER and MODIS, can be used to measure the energy reflected and emitted by the Earth. These two instruments have very different characteristics. MODIS is designed for global studies whereas ASTER is designed for regional and local studies. MODIS produces global maps of the reflected and emitted energy from the Earth every couple of days but at relatively low spatial resolution (1 km) whereas ASTER produces local and regional maps at very high spatial resolution (15 m) with repeat coverage at a maximum of every 16 days. In order to ensure that both instruments are working correctly, validation experiments are undertaken where the satellite-measured quantity is compared to the equivalent quantity measured on the ground and propagated to the satellite. One of the sites selected for this comparison is Lake Tahoe CA/NV. Lake Tahoe is an ideal site for this comparison because it is high (small atmospheric correction) large (suitable for validating multiple instruments) homogenous (water) available year-round (does not freeze), freshwater (instrument-friendly) and has excellent logistical support. In order to undertake these comparisons the Jet Propulsion Laboratory and UC Davis have deployed four rafts that measure the bulk and skin temperature of the lake approximately every two minutes on a continuous basis. One of the rafts also includes simultaneous meteorological measurements (wind speed, wind direction, relative humidity and net radiation). These data are used to derive the kinetic temperature of the surface, which is then propagated through the atmosphere using a radiative transfer model, and convolved to the response of the sensor and compared to the satellite-measured radiance. The radiative transfer model is driven by local atmospheric measurements made at the US Coast Guard and supplemented by data from atmospheric sounding balloons. Comparisons have now been made from when the instruments were turned on in early March 2000 to present. This period covers a full annual cycle allowing the instruments to be validated over a wide temperature range from a minimum of 5 C to a maximum of 23 C. The following plot shows the predicted surface brightness temperature (derived from the ground values) versus the satellite measured brightness temperatures from the ASTER instrument.  Clearly there is excellent agreement between the ASTER instrument measured values and the predicted values indicating the instrument is working well. The ASTER instrument measures the brightness temperature in five bands and variations in these brightness temperature measurements can be used to determine the composition of the surface. Similar results were obtained for the MODIS instrument with the predicted values agreeing with the measured values to ± 0.4 C. As a result of the success of this work several other instruments are now routinely acquiring data over Lake Tahoe resulting in an excellent remote sensing dataset for Lake Tahoe and the surrounding region. The in-situ and remotely sensed data are also being used to help with a variety of other projects at the lake such as relating changes in the surface temperature of the lake to the mixing of the lake and monitoring the clarity of the lake. For more information see: http://shookweb.jpl.nasa.gov/validation |