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Abstract:
Cirrus cloud optical and structural properties were measured above southern
Wisconsin in two time segments between 18:07 and 21:20 GMT on
December 1, 1989. These clouds were measured using
the University of Wisconsin--Madison Volume Imaging Lidar (VIL), the University
of Wisconsin--Madison High Spectral Resolution Lidar (HSRL), and the VISSR
Atmospheric Sounder (VAS) situated on GOES. The VIL imaged
the cirrus cloud structure within a mesoscale volume and detected cirrus
cloud cover percentages of 81.5% and 76.8% for the two time periods.
A series of RTIs were created from the VIL cirrus scans to simulate many
vertically pointing lidars situated across the wind. The
cloud cover percentages for the individual RTIs
ranged from 54.7% to 100%. The variation of the cirrus cloud features
across the wind indicate the sampling errors associated with point measurements
when they are used to describe the mesoscale cirrus cloud structure.
The average cloud length along the wind was 130 km while the
average cloud length across the wind was 14 km. The distance between
clouds was 273 km along the wind and 24 km across the wind.
A new technique was used to calculate the cirrus cloud visible
aerosol backscatter cross sections for a
single channel elastic backscatter lidar. Cirrus clouds were viewed
simultaneously by the VIL and the HSRL. This allowed the HSRL aerosol
backscatter cross sections to be directly compared to the VIL single
channel backscattered signal. This first attempt resulted
in an adequate calibration. The calibration was extended to all the
cirrus clouds in the mesoscale volume imaged by the VIL. This enabled the VIL
backscattered signal to be converted into aerosol backscatter cross
sections at a resolution of 2 to 3 km along the wind direction and
a 60 m resolution both horizontally across the wind and in the vertical.
Usage of a constant aerosol backscatter phase function (0.0499 sr
)
enabled the calculation of extinction cross sections at each data point
in the VIL observed mesoscale volume. This allowed for the cirrus
cloud visible scattering optical
depths to be calculated throughout the imaged volume. The VIL
volume was viewed from the position of the GOES to directly compare the
VIL visible scattering optical depths to the VAS infrared absorption
optical depths. The resulting ratio of the visible scattering optical
depths to the infrared absorption optical depths was approximately 2:1.
Cirrus Cloud Optical and Morphological Variations Within a
Mesoscale Volume
Walter W. Wolf
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Antti Piironen
Thu Apr 11 08:27:54 CDT 1996