Examination of the cirrus cloud structure has shown the advantages of a volume scanning system and the difficulties encountered by a vertically pointing instrument when attempting to describe the cirrus cloud spatial structure across a mesoscale volume. As shown by the simple radiative transfer calculations, the cirrus cloud optical properties play an important role in the radiation balance within the Earth-Atmosphere system. Like the cirrus cloud spatial structure, the cirrus cloud optical properties at visible and infrared wavelengths may vary significantly across a mesoscale volume. The visible optical depth influences the amount of solar radiation which passes through the cirrus clouds and reaches the lower levels of the Earth's atmosphere. The infrared optical depth moderates the amount of infrared radiation lost to space through these clouds. To understand the effects of cirrus clouds on the Earth's radiative budget, the cirrus cloud optical properties have to be known.
Previous attempts to determine the cirrus cloud visible optical depth, to compare to the cirrus cloud infrared optical depth, have been made using satellite based radiometers and ground based lidars. The calculations using satellite data could not independently determine the cirrus cloud visible optical properties since additional measurements of the cloud coverage and cloud types within each pixel were needed. These calculations also needed measurements of the reflectivity of the Earth's surface at the visible wavelengths for each pixel. Errors also occurred in the visible optical depth calculations due to poor calibrations of the visible channels. The visible channels were calibrated at the ground and the calibration has degraded over time. Ground based lidars have also been used to determine the cirrus cloud visible optical properties. The measurements by a vertically pointing lidar are limited by the instrument viewing capability. These lidars only observe the cirrus clouds advected over their position producing biased results due to the cirrus cloud spatial variability and the possible preferential alignment of the cirrus clouds. A volume scanning ground based lidar needs to view a large enough volume of the atmosphere to remove pixel alignment errors associated with the direct comparison of the cirrus clouds viewed by the scanning lidar and the satellite radiometer (described in Section 5).
A new technique to calculate the cirrus cloud visible optical properties throughout a mesoscale volume is described in the following section. This method uses coincident cirrus cloud measurements from two ground based lidar systems, the VIL and the HSRL. The cirrus cloud visible aerosol backscatter cross sections calculated from the HSRL data are used to calibrate the VIL backscattered signal. The VIL cirrus cloud scans will then be used to extend the calibration to a mesoscale volume. This new calibration method, which uses data from two lidar systems, was attempted without previous knowledge of the errors associated with this calibration process.