Bright structures represent cloud echoes. The automatic cloud base altitude estimates correlate well with the manually determined estimates. Manual estimation is performed by visually inspecting the RHI scans. Figure 13 presents the manual and automatic cloud base estimates for the 1989 FIFE experiment data. To reduce statistical fluctuations due to sampling differences, a 15-minute minimum filter is applied to the results.
Most manual estimates are greater than or equal to automatic estimates.
Therefore, the lowest clouds in the are not likely to be sampled, which causes systematic overestimation. Statistical fluctuations on the order of 50 m are expected since the vertical resolution is limited by the elevation angle step size used in the volume scan. The RHI scans for these cases were visually re-examined to explain the differences. One sample in box c is due to a dust cloud rising up to 500 m, which caused the automatic method to fail. The other measurements in box c were made when there was a cloud layer above a few fair-weather cumulus clouds.