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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