If we neglect the effect of range and attenuation, then . Most of clouds is much thicker than aerosols at the same altitude, and thin cirrus clouds only occurs at high altitude, so it is easy to preselect a value for this ratio to distinguish these two layers. The maximum of negative slope D is also good parameter to distinguish a cloud from aerosol and noise. If T and D are below their threshold, we regard the layer as aerosol layer or noise. These preselected values should change with wavelength and vertical resolution of system because optical properties of molecular, aerosols and clouds have different wavelength dependence and low vertical resolution system detects smoother cloud signal than high vertical resolution.
Noise effect depends on the manner of data acquisition.
For optical counter mode, there is two dimension integral of signal, noise usually cause a small and slow distortion in return signal. An important character of noise is its uncorrelation in time. This layer should be rejected. The fourth step is to judge the cloud top is a real top or effective top. In most of case, lidar can not penetrate the cloud layer, so the detected cloud top is an effective top.