Since the spectral radiometers were used on the Grob research platform for the first time, only days when no clouds were observed at the site of the observations were selected when deriving the aerosol properties and the columnar water vapour. This precaution allows a simplification of the complex data evaluation.
Two areas in South Australia were selected for the observations: the regions of Lake Alexandrina and of Cleve/Hincks Park (Eyre Peninsula). The map of the two regions where the field exercises took place is shown in Figure 5.1.
All run lengths are between 15 to 28 km. The run lengths give a good balance between the time necessary to fly the whole pattern and the need to obtain a representative value for one level. The radiation profiles are measured by setting the cycle time of the shadow band to 50 seconds. This corresponds to a movement of the shadow band of 0.139 /sec. The shading of the shadow band is 11.7 and the shading required to shade the diffuser only is 8.1 with a delta shading of 3.6 . Hence, the diffuser is fully shaded for 0.139*3.6=0.500 seconds. Sampling all five filters plus the blank with a rate of 13 Hz (each wavelength will be updated every 6/13= 0.461 sec) yields a scanning of all wavelengths of 0.500/0.461=1.08 times per shading circle. Since the updating of all six sample and store units occurs every 0.286 sec and is more frequent than the reading of all wavelengths with 13 Hz, the observed data are updated 1.613 times during the shading period. Flying with a speed of 40 m/s, corresponds to all six monitored wavelengths being fully shaded once every two kilometres.
Both radiometers were installed throughout the field exercise on the research aircraft. The device SPECGD2 was mounted on top of the aircraft's fuselage (see Figure 5.2a, 5.2b). This place was deemed the most suitable, since influences from the aircraft body are minimized.
The radiometer SPECG2 was mounted under the right wing of the aircraft (Figure 5.2c) so that it was pointing straight downward during the observations. With the diffuser attached to the instrument, the reflected radiation from the lower hemisphere was measured. In order to retrieve the spectral vegetation index, the diffuser was replaced by an aperture producing an opening angle of 4°. This angle is identical to that of the aircraft's infrared radiometer and a correlation between the two parameters could be performed without compensating for a differing field of view.
Lake Alexandrina is about 90 km southeast of Adelaide and is one of the important features of the lower River Murray, just before it flows into the sea.
The observations in the region of Lake Alexandrina were carried out with the Grob research aircraft on 3 April 1989 and 22 April 1989. The acronyms assigned for these field experiments are LA34 and LA224. The aims of the observations were: to obtain the vertical profiles of the bulk aerosol properties; the precipitable water profile; and the vegetation index. These flights were also used to test the performance of the radiometer system. Two different flight patterns were flown, one over Lake Alexandrina and one from Goolwa to Strathalbyn. The traverse lengths were 15 to 20 km flown at levels of 200, 1500, 3000 feet, and above the boundary layer inversion. Lake Alexandrina was selected for the derivation of the aerosol properties in order to take advantage of the constancy of the upwelling flux over water, which enables a simplified data evaluation. A triangular pattern was flown, and is shown in Figure 5.3.
The flight legs from Goolwa to Strathalbyn and back were carried out at heights of 200 ft and 6000 ft, respectively. In this region, several creeks run into the Goolwa river through dense areas of reed, providing a widely varying vegetation cover. However, malfunction of the downward facing radiometer on this occasion only allowed for the analysis of the highest flight.
On 3rd of April 1989 there was a high pressure system southeast of Australia with light winds prevalent from north to northwest at Lake Alexandrina. The synoptic situation was anticyclonic with the centre of the high pressure system over Tasmania on 22 April 1989. The winds on this observation day were light and from the northeast.
Figure 5.3 Sketch of flight pattern over Lake Alexandrina
A second field exercise was undertaken in the Cleve area on the Eyre Peninsula, where the landscape is strongly influenced by farming and only very little native vegetation remains except in Hincks Conservation Park.
The field exercise on Eyre Peninsula took place in early December 1989. Radiation observations were carried out on 4 and 5 December 1989. The acronyms identifying the flights are HP46 and HP47. Four heights of 200, 1500, 3500 and 7000 ft were flown during the HP46 mission. Tail reflections occasionally occurred depending on the heading and sun altitude. It was not possible to completely eliminate these and for this reason the 1500 ft run was rejected from the data analysis. During the HP47 experiment flights at levels of 100, 200, 1500, 3000, 6000 and 10000 ft were flown. The lengths of the traverses were 28 km. Again, tail reflection occurred, permitting only to calculate the aerosol properties at the four levels of 100, 1500, 6000 and 10000 ft. Compared to flights at Lake Alexandrina, this time identical flight routes were carried out at different heights. The cross-section of the flight paths are shown in Figure 5.4.
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