pt
Since the VIL mechanical axis was not aligned with its optical axis during this experiment, alignment errors were a problem for the VIL. The VIL signal varied when the system mirrors were scanned in angle as a result of partial blocking of the transmitted beam (an example is baffle blocking of the transmitted pulse). Other signal variations can also result from changes in the gain of the avalanche photo--diode over time and/or field of view changes in the receiver. To remove these dependence, each VIL scan was normalized to a low level aerosol layer situated between 1 km and 2 km above the ground. Only backscattered signal falling within a bounded aerosol region were used in the data normalization. The region was bounded by a minimum and maximum height along with a minimum and maximum distance from the VIL. The minimum and maximum heights were determined by the size of the aerosol layer. The minimum range from the VIL was determined by the extent of the system overlap regime. A maximum range of 10 km was chosen to stay well above the minimum detectable signal for the VIL. The median backscattered signal from the bounded aerosol layer for each lidar profile was set equal to the median backscattered signal in the normalization regime. This normalization technique removed both the time and angular dependence of the cirrus clouds scans with the assumption that the aerosol layer did not change in time. If the aerosol layer did not have temporal and spatial uniformity, errors would result from the normalization process. Nonuniformities would cause variations in the cirrus cloud backscatter which are the result of data processing and not cirrus cloud variability. The aerosol layer used for the VIL data normalization was detected by the HSRL and appeared uniform in time. Additional errors can also result if too much outgoing signal was blocked before it left the transmitter. When this occurred, the radiation backscattered from the atmosphere was too small to be separated from the noise. These system limited data points were removed from the data set as described in Appendix A.