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3. Improvement of the geometric features determination

Initially, the geometrical feature of cirrus clouds measured with lidar at CLS, were determined using a manual algorithm (Estevan et al., 1998). As in any manual method, the human factor played a role, contributing to the uncertainties of measurement process. Recently, it has been developed in an software for making such processing with standard criteria and eliminating the subjectivity of the former one.

The new processing algorithm based in the determination of minimum and maximum, using differential method analysis of the number of photons signal. Begins with the elimination of the noise in the number of photons vs. Altitude curve, using moving average . Then series minimum & maximum are determined, and only the pairs of consecutive minimum & maximum with differences higher than 50 photons are selected as probable cirrus geometric base and retrodispersion maximum. The criteria of the minimum difference of 50 photons was determined from manual analysis of the dataset.

The further analysis take in consideration that the difference in the original profile between the probable minimum and maximum is at least twice the difference between the number of photons at the maximum's altitude in the original dataset and the one filtered. After this point the minimum value which comply with the former criteria are selected as base height of the cirrus clouds.

Then we process to determine the top height In the original data we found the altitude above the minimum, which determined as base height, which number of photons is equal or less than the number of photons in the altitude of the base. This is the so called "apparent top", defined by (Pal et al., 1992).

This procedure is repeated for all the minimum & maximum selected that are higher than top height, figure 1 shows the flowchart of the algorithm.

Figure 1. Flowchart of the algorithms

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