Field study of the performance of ceilometers

The ceilometers at the Hong Kong International Airport (HKIA) have been in use for about 10 years and they are due for replacement. A field study has been conducted at HKIA to compare the performance of the existing and the new models of ceilometer. Side-by-side comparison has been arranged for about half a year, covering late summer, autumn and winter. In this paper, the preliminary comparison results would be presented. It is noted that the two ceilometer models have very similar behaviour in the determination of the base height of the first layer of clouds. As such, continuity of cloud base height records at HKIA could be assured. An experiment has also been conducted to determine the cloud cover using the time series data from the two ceilometers. By comparison with the collocated human observations, it turns out that the new model of ceilometer gives better estimate of the total cloud cover, possibly through the improvement in the detection of the medium to high clouds which are hardly detected by the older model of the ceilometer. Moreover, the new ceilometer is able to output backscatter profiles in real-time. Examples of the backscatter profiles in rain and haze would be presented.