Pilot Radiation Observation Experiment

Project Description
The Division of Atmospheric Research took part in ARM's TOGA COARE
(Tropical Ocean Global Atmosphere/Coupled Ocean Atmosphere Response
Experiment.) PROBE experiment at Kavieng, New Guinea (2.5S, 150.8W) in
January - February, 1993. This afforded an opportunity to use the new
radiometer alongside the CSIRO Mark II radiometer in a direct
comparison. The Division's 0.532 m lidar was also used, and the data
obtained on cirrus clouds, as well as some altocumulus, will be
analyzed with the LIRAD method. The PROBE will also provide excellent
radiosonde data every six hours, together with continuous microwave
data of water vapour column and cloud liquid water column observations
from the National Oceanic and Atmospheric Administrations's (NOAA)
Wave Propagation Laboratory (WPL). The water vapour column data will
be invaluable in allowing for any variations in water vapour radiance
and transmittance at times between radiosonde observations.

A preliminary analysis of the data indicates the variability
of the cirrus and its considerable geometrical depth at times and also
the persistence of the cirrus cover. Particularly interesting was an
apparent diurnal variation in both the cirrus cover and the optical
depth with a maximum at about midday.

The ARM filter radiometer was run for about 70% of the time on the
8.62 m filter; however, for some periods, the radiometer was
run with the 10.86 m filter enabling a direct comparison with
the Mark II radiometer which used a 10.84 m filter. As the
input radiance is chopped against a 40C blackbody, the zero
radiance when viewing liquid nitrogen actually gives a large
negative signal; whereas, the zero voltage occurs when the
input radiance is from a 40C blackbody. The responses of the
two radiometers to various clouds are quite evident. The water
vapour radiance is large, which is typical for the
tropics. Periodically, there are either cirrus radiances or
larger cumulus radiances superposed. Also evident is the
superior behaviour of the new ARM radiometer. The two
radiometer apertures were equal in the comparison; however,
the ARM and Mark II time-constants were 1 second and 5
seconds, respectively. By looking at the signal and noise
levels during the calibration episodes in! more detail, we
calculate the minimum detectable radiances (MDR) of the two
instruments as 4.9 x hz-1/2(ARM) and 6.8 X hz-1/2(CSIRO Mark
II).

Contact:

C. Martin Platt (Lead Scientist)
mplatt@net2000.com.au

For more information,
link to http://www.arm.gov/docs/iops/past/afteriop_probe1993.html

[Summary provided by ARM]