Insert graph shows aerosol fluctuations expanded by a factor of ten. Figure 12 presents an RHI with a distribution profile of the first cloud return altitudes and an estimated cloud base which is measured within the resolution of the RHI scan. Clouds are good scatterers and are clearly visible in an RHI scan. However, when inspecting cloud bases from the RHI scans, the visual difference between thermals and clouds depends on the color enhancement selected for the display window. For example, some clouds in the figure seem to extend below the detected cloud base, since the chosen color enhancement shows backscatter from clouds and aerosol plumes with similar gray shades.
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.