The cirrus cloud threshold determined in the previous section allows
the cirrus cloud structure to be examined. To calculate the cirrus cloud cover
percentage within the mesoscale volume, RTI (Range Time Indicator) profiles
were created at 100 m intervals
along the VIL cross wind scan up to a distance of 30 km from the VIL. A RTI
is a plot of the backscattered signal where time is the x-axis and altitude is
the y-axis. A total of 601 RTIs were created for each time period.
Consecutive points in each RTI were separated
by 85.5 seconds, the time between consecutive cross wind scans.
The cross wind scan time separation was converted into a distance
by using the average wind speed measured by the rawinsonde at the
cirrus cloud heights. An average wind speed of 35 ms
resulted in a distance between scans of approximately 3 km.
The RTIs simulated 601 vertically pointing lidar systems
situated across the wind at 100 m intervals. The cloud cover
percentage from each RTI
shows the cloud cover that would be observed by a vertically
pointing lidar system at that position. To determine the presence of
cirrus clouds, the VIL backscattered signal between the heights of
6 and 11 km in each simulated vertical profile was compared to the cirrus
cloud threshold value. If any of the data points between the given
heights for each vertical profile (for each simulated RTI) had a
value greater than the cirrus cloud threshold value, then
that RTI point contained cirrus clouds. Otherwise the point was labeled clear.
The cloud cover percentage calculated from the simulated RTIs was used to determine the spatial variability of cirrus clouds within a mesoscale volume. A direct comparison of average cloud cover seen by the VIL and the cloud cover detected by individual VIL RTIs shows the difference between point and area measurements of the cirrus cloud structure. Cirrus cloud-nocloud structure functions was calculated to determine the horizontal dimensions of the cirrus clouds and the distance separating the individual cloud structures. A cross wind structure function was used to determine the separation between and length of the cirrus clouds across the wind. An along wind structure function was used to determine the length of the cirrus clouds and the distances separating these clouds along the wind.
A three hour time period on December 1, 1989 was investigated. As described earlier, this three hour data set was split into two sections which were chosen as a result of the continuity of the VIL data. The first time period extended from 18:08 to 19:24 GMT and the second from 19:29 to 20:21 GMT on December 1, 1989. The earlier period contained a large amount of scattered cirrus clouds while the later period had some scattered cirrus clouds early but at 20:30 GMT a widespread precipitating cirrus cloud deck was advected into the region.