Figure 31 shows the probability as a function of time and altitude for quarter-hourly averaged wind measurements on July 26, 1989. The contours show the wind speed increasing from 5 ms to 8.5 ms over the period. Even using 0.2 ms contour spacing, the wind profiles are smooth and small details are visible.
The measurements have good internal consistency, except for some small disturbances with a smaller . The results are most reliable in the convective boundary layer, since there is good contrast between convective aerosol structures and clearer free air. Above the convective boundary layer there are sometimes not enough aerosol structures for correlation analysis, which appears as less reliable results. Turbulent mixing in the late afternoon frequently produces uniform aerosol distribution that leads to less reliable wind estimates. In the convective boundary layer, randomly located smaller reliabilities are generally due to the breaks in the VIL data record. This happened several times on July 26 after 14:00 CDT. These breaks are generally due to system adjustments or safety shutoff of the laser output beam when an aircraft flew into the field of view.
Figure 31: Quarter-hourly VIL wind speeds (contours), mean boundary layer (solid line), and cloud base (dash-dotted line) plotted as functions of time and altitude on July 26, 1989. The shaded areas indicate is less than 1. In general, the results are reliable from the ground to 500 m above the cloud base. There are gaps in the data record at about 14:00, 16:00, and 17:00 CDT causing to decrease. Boundary layer mixing decreases the aerosol structure contrast in the volume scans and occasionally causes less reliable estimates in the middle of the mixed layer at 13:00, 14:40, and 15:30 CDT.