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Abstract
Both the laser ceilometer and the ground-based microwave radiometer equipped with infrared cloud radiometer provide essentially point measurements of the cloud base height continuously. By using a sophisticated algorithm, it is possible to determine the total cloud amount based on the continuous record of cloud base height. In the present study, the operational algorithm of Royal Netherlands Meteorological Institute (KNMI) is considered and the total cloud amount so determined is compared with co-located actual observations by human observers. The laser ceilometer, having some difficulties in detecting the thin cirrus clouds, could underestimate the cloud amount when extensive cirrus clouds cover the observation area. On the contrary, the microwave radiometer may be more sensitive to high clouds, though it may not be able to distinguish between very thin cirrus and totally clear sky. This paper demonstrates that, by choosing a suitable threshold for the cloud base height determined by the radiometer, the total cloud amount estimated by the radiometer is much more consistent with human observations compared with the cloud amount provided by the laser ceilometer. This opens up a potential application of using the microwave radiometer in automatic observation of the total cloud amount.