Measurement of cloud height and amount is an operational requirement for the National Weather Service's (NWS) Automated Surface Observing System (ASOS). The ASOS provides critical sky condition information for airports throughout the country, and in many cases the observation is completely automated. The current requirement is to measure clouds to 3660 meters. However, production of the legacy ASOS ceilometers was discontinued in 1998, and the NWS is now in the process of procuring replacement ceilometers that will have the capability of measuring to 7625 meters. With this increased height range, it became necessary to obtain a cloud height reference measurement device for the assessment of the candidate replacement ceilometers. For this purpose the NWS purchased a Micropulse Lidar (MPL) that was commercially available from Sigma Space Corporation.

The MPL has proved to be invaluable as an observer aide in determining cloud height from a graphical display of the time history of the MPL backscatter profile. However, its internal algorithm for deriving cloud height from the backscatter profile proved to be unsatisfactory in some cases. For this reason, the NWS ASOS Product Improvement staff conducted an investigation to use a modification of the Klett inversion technique to obtain cloud height and amount from the MPL raw backscatter profile. The focus of the approach was to obtain a vertical profile of horizontal visibility to determine the presence and height of clouds overhead. The technique was refined by comparing the inversion algorithm result to human observed sky conditions that were taken at the same time to determine if the algorithm was accurately identifying clouds without falsely identifying noise or backscatter returns from sub-visible layers as clouds. The details of the algorithm and results from a variety of cloud types are described in this report.