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.
Lidar can detect real cloud top for some optical thin clouds.
It can be done by examining the signal behavior above the cloud top. If signal above cloud top is below P min with a slope of about zero, the cloud top is possibly a effective top. On other hand, if signal above cloud top is much larger than P min and with a negative slope around the slope of air backscattering signal, the cloud top is a real top. Multiple-scattering will bring some problems for this analysis to some systems. The fifth step is to distinguish cloud base from virga base for mixed phase clouds.